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ALICE pins down hypermatter properties

Tue, 20/09/2022 - 17:51
ALICE pins down hypermatter properties

The international ALICE collaboration at the Large Hadron Collider (LHC) has just released the most precise measurements to date of two properties of a hypernucleus that may exist in the cores of neutron stars.

Atomic nuclei and their antimatter counterparts, known as antinuclei, are frequently produced at the LHC in high-energy collisions between heavy ions or protons. On a less frequent but still regular basis, unstable nuclei called hypernuclei are also formed. In contrast to normal nuclei, which comprise just protons and neutrons (that is, nucleons), hypernuclei are also made up of hyperons – unstable particles containing quarks of the strange type.

Almost 70 years since they were first observed in cosmic rays, hypernuclei continue to fascinate physicists because they are rarely produced in the natural world and, although they are traditionally made and studied in low-energy nuclear-physics experiments, it’s extremely challenging to measure their properties.

At the LHC, hypernuclei are created in significant quantities in heavy-ion collisions, but the only hypernucleus observed at the collider so far is the lightest hypernucleus, the hypertriton, which is composed of a proton, a neutron and a Lambda – a hyperon containing one strange quark.

In their new study, the ALICE team examined a sample of about one thousand hypertritons produced in lead–lead collisions that occurred in the LHC during its second run. Once formed in these collisions, the hypertritons fly for a few centimetres inside the ALICE experiment before decaying into two particles, a helium-3 nucleus and a charged pion, which the ALICE detectors can catch and identify. The ALICE team investigated these daughter particles and the tracks they leave in the detectors.

By analysing this sample of hypertritons, one of the largest available for these “strange” nuclei, the ALICE researchers were able to obtain the most precise measurements yet of two of the hypertriton’s properties: its lifetime (how long it takes to decay) and the energy required to separate its hyperon, the Lambda, from the remaining constituents.

These two properties are fundamental to understanding the internal structure of this hypernucleus and, as a consequence, the nature of the strong force that binds nucleons and hyperons together. The study of this force is not only interesting in its own right but can also offer valuable insight into the particle interactions that may take place in the inner cores of neutron stars. These cores, which are very dense, are predicted to favour the creation of hyperons over purely nucleonic matter.

Measurements of the hypertriton’s lifetime performed with different techniques over time, including ALICE’s new measurement (red). The horizontal lines and boxes denote the statistical and systematic uncertainties, respectively. The dashed-dotted lines represent different theoretical predictions. (Image: ALICE collaboration)

 

The new ALICE measurements indicate that the interaction between the hypertriton’s hyperon and its two nucleons is extremely weak: the Lambda separation energy is just a few tens of kiloelectronvolts, similar to the energy of X-rays used in medical imaging, and the hypertriton’s lifetime is compatible with that of the free Lambda.

In addition, since matter and antimatter are produced in nearly equal amounts at the LHC, the ALICE collaboration was also able to study antihypertritons and determine their lifetime. The team found that, within the experimental uncertainty of the measurements, antihypertriton and hypertriton have the same lifetime. Finding even a slight difference between the two lifetimes could signal the breaking of a fundamental symmetry of nature, CPT symmetry.

With data from the third run of the LHC, which started in earnest this July, ALICE will not only further investigate the properties of the hypetriton but will also extend its studies to include heavier hypernuclei.

ndinmore Tue, 09/20/2022 - 16:51 Byline ALICE collaboration Publication Date Tue, 09/20/2022 - 17:00

Higgs10: inventing the future of Higgs research

Fri, 19/08/2022 - 10:32
3D dipole integration showing several parts with an interconnection open. Integration 3D du dipole et interconnection ouverte. (Image: CERN) Higgs10: inventing the future of Higgs research

By: Matthew McCullough

10 August, 2022 · Voir en français

In the final part of the Higgs10 series, history teaches us that those who explore relentlessly and fearlessly are often the ones rewarded with the greatest prize of all: the truth.

 

In 1975, three CERN theorists, John Ellis, Mary K. Gaillard and Dimitri Nanopoulos, undertook the first comprehensive study of the collider phenomenology of the Higgs boson. Almost 40 years later, it was discovered at the LHC. Now, ten years on, might we have such long-term foresight in anticipating the varied paths that future Higgs research may follow?

On 4 July 2022, enjoying the many beautiful presentations at the Higgs@10 symposium, a phrase kept ringing in my ears: “Compatible with Standard Model (SM) predictions”. Alarm bells were ringing. Really? Are we sure? Whether or not the Higgs is SM-like is a question that will shape the experimental future of Higgs research.

We may quantify an answer through the language of effective field theory, which is a mathematical manifestation of the notion that the most effective way to describe an object depends on the length scale you’re viewing it from. To astronauts, Earth is very effectively described as a smooth sphere. For summer students hiking to Le Reculet, it is not. So, too, of the quantum world. Far from a neutral atom, it effectively appears as a point-like particle with some leftover multipolar interactions with photons. At shorter distances, getting in amongst the electrons, this description fails entirely.

Ditto the Higgs. Whatever’s going on in there, at energies near enough to mh, it is effectively described as a point particle with a handful of additional “operators”, which are essentially new particle interactions that aren’t contained in the SM (don’t feature on that mug or T-shirt) but do involve SM particles. By eye, the astronaut may be able to make out some features on Earth and surmise that there may be mountains, but they couldn’t actually estimate the students’ elevation gain. Similarly, the non-SM Higgs operators can capture the long-distance leftover effects of the microscopic innards of the Higgs, but not reveal their full glory in detail. If all of these extra operators vanish, the Higgs is SM-like. Let’s consider two hand-picked examples and investigate just how SM-like the Higgs is...

How “fuzzy” is it?

Is it point-like down to the smallest distance scales or is it, like the pion, made up of other as-yet-unidentified new particles? In the latter case, much as for the pions and their constituent quarks and gluons, directly observing the new stuff would require going to higher energies. Alternatively, it could be point-like but probing it closely may reveal the telltale clues of a cloud of new particles that it interacts with. For your interest, the operator that can capture these properties is written (∂μ|H|2)2. If it vanishes, the Higgs is entirely point-like. If not, it’s fuzzier than expected. How fuzzy is it? Present LHC Higgs coupling measurements suggest it is effectively point-like down to a length scale merely a factor three below the electroweak scale. It could still be very fuzzy indeed! As fuzzy as a pion. If so, hardly an SM-like Higgs! We must do better and, through much more precise coupling measurements at the 0.2% level, a future Higgs factory like the FCC-ee could determine if the Higgs is point-like as far down as the 6% level.

Does the Higgs find itself attractive?

Yes, according to the SM. New particles means new forces and so it follows that if the Higgs boson interacts with new heavy particles they will generate a new force between the Higgs and itself. The operator effectively capturing this is |H|6 and it literally shapes the way in which the Higgs field gave mass to particles during the very nascence of our universe! So, how SM-like is the Higgs self-attraction? With present experimental constraints, we know the Higgs self-attraction could be 530% stronger than the SM value (not merely self-attraction, more like outright vanity) or even −140% less (self-repulsive, more like). Hardly SM-like in either case! To have any idea of whether the self-attraction is SM-like, we must do a lot better. A future facility, such as the FCC-hh, CLIC or a muon collider, could probe the self-attraction at the much more precise 5% level.

Patience is a virtue; complacency is not

It is far too early to call time at the bar for the Higgs boson. Who knows, we may even be served with something completely unexpected, like a new window into the dark sector of the universe. Truly exploring all facets of the nature of the Higgs boson, understanding whether or not it is SM-like, will take time (measured in decades) and a lot of hard work. But it can and should be done. This is the experimental future of Higgs research that we look forward to.

All that said, it’s no secret that many theorists expected the Higgs to be much less SM-like than it appears to be already. Heads duly scratched, a theoretical coup d’état is now silently under way. There were good reasons to expect something different: chiefly the hierarchy problem. This problem is not simply aesthetic. The SM breaks down at high energies, ultimately making pathological predictions, thus it can only be a long-distance effective field theory description of something else more fundamental. If, as was the case for pions, the Higgs mass is determined by the more fundamental parameters, then for the Higgs there is no mechanism to keep it lighter than the mass scale of the new particles in that theory. Yet colliders tell us there is a gap between the mass of the Higgs and that of those new particles. In the past, this motivated the discovery and development of new mechanisms to explain a light Higgs, such as the venerated low-scale supersymmetry, thus far a no-show at the LHC physics party, with its attendant non-SM-like Higgs.

Rudely awoken by the deluge of exclusion plots, coffee reluctantly smelled, theorists have, in recent years, put forward what could well transpire to be revolutionary theoretical developments. The hierarchy problem hasn’t gone away and neither has the data, so the other foundational assumptions covertly injected into the old theories, often linked to symmetry or aesthetic principles such as simplicity or minimality, have been interrogated and found wanting. In response, intrepid new classes of theories have been developed that can address the hierarchy problem whilst being consistent with all those bothersome exclusion plots. They range from relatively modest conceptual tweaks of existing structures, to the abandonment of aesthetic principles, and then all the way out the other side to attempts to link the Higgs mass to the origins of the universe, cosmology, the nature of the Big Bang and, at an extreme, speculations about possible links between the Higgs mass and the existence of life itself. You name it, we’re boldly going.

It’s no fait accompli

None of these ideas are as intoxicating as supersymmetry or as stupefying as extra dimensions, each leaving those who study them with more of a “watch this space” feeling than the “eureka” that Archimedes enjoyed. Variously, they’re not radical enough, too radical or simply not to taste. No Goldilocks moment just yet. However, in my view these issues are cause for hope. In similar moments in the past, we have been essentially on the right track, having to wait a little longer than expected for the confirming experimental data (top quark). At other times, the right ideas have been too radical for most to stomach in one sitting (quantum mechanics). Yet for others the correct approaches languished in relative obscurity far too long, simply for not being à la mode (quantum field theory). Look up the citation records of the original Brout-Englert, Higgs, Guralnik-Hagen-Kibble papers or Weinberg’s “A Model of Leptons”, all foundational to the physics of the Higgs boson, and you’ll see they are important cases in point that we would do well to remember. Nature made no promises that understanding the origins of the Higgs should have been easy, nor should it be in the future, but history teaches that those who explore relentlessly and fearlessly are often the ones rewarded with the greatest prize of all: the truth.

Where will all this go in coming years?

Will we be tenacious enough to build the accelerator, the detectors and the village it will take to measure the Higgs self-attraction or discover the fuzziness of the Higgs? Will some plucky theorists unlock the door to the fundamental theory beyond the SM? Will future phenomenologists lay the first foundational stones on the path to discovering it?

As Dennis Gabor, the inventor of holography, put it: “The future cannot be predicted, but futures can be invented.”
We’re working on it.

History teaches that those who explore relentlessly and fearlessly are often the ones rewarded with the greatest prize of all: the truth.

Higgs10: inventing the future of Higgs research

Wed, 10/08/2022 - 10:04
Higgs10: inventing the future of Higgs research

In 1975, three CERN theorists, John Ellis, Mary K. Gaillard and Dimitri Nanopoulos, undertook the first comprehensive study of the collider phenomenology of the Higgs boson. Almost 40 years later, it was discovered at the LHC. Now, ten years on, might we have such long-term foresight in anticipating the varied paths that future Higgs research may follow?

On 4 July 2022, enjoying the many beautiful presentations at the Higgs@10 symposium, a phrase kept ringing in my ears: “Compatible with Standard Model (SM) predictions”. Alarm bells were ringing. Really? Are we sure? Whether or not the Higgs is SM-like is a question that will shape the experimental future of Higgs research.

We may quantify an answer through the language of effective field theory, which is a mathematical manifestation of the notion that the most effective way to describe an object depends on the length scale you’re viewing it from. To astronauts, Earth is very effectively described as a smooth sphere. For summer students hiking to Le Reculet, it is not. So, too, of the quantum world. Far from a neutral atom, it effectively appears as a point-like particle with some leftover multipolar interactions with photons. At shorter distances, getting in amongst the electrons, this description fails entirely.

Ditto the Higgs. Whatever’s going on in there, at energies near enough to mh, it is effectively described as a point particle with a handful of additional “operators”, which are essentially new particle interactions that aren’t contained in the SM (don’t feature on that mug or T-shirt) but do involve SM particles. By eye, the astronaut may be able to make out some features on Earth and surmise that there may be mountains, but they couldn’t actually estimate the students’ elevation gain. Similarly, the non-SM Higgs operators can capture the long-distance leftover effects of the microscopic innards of the Higgs, but not reveal their full glory in detail. If all of these extra operators vanish, the Higgs is SM-like. Let’s consider two hand-picked examples and investigate just how SM-like the Higgs is...

How “fuzzy” is it? Is it point-like down to the smallest distance scales or is it, like the pion, made up of other as-yet-unidentified new particles? In the latter case, much as for the pions and their constituent quarks and gluons, directly observing the new stuff would require going to higher energies. Alternatively, it could be point-like but probing it closely may reveal the telltale clues of a cloud of new particles that it interacts with. For your interest, the operator that can capture these properties is written (∂μ|H|2)2. If it vanishes, the Higgs is entirely point-like. If not, it’s fuzzier than expected. How fuzzy is it? Present LHC Higgs coupling measurements suggest it is effectively point-like down to a length scale merely a factor three below the electroweak scale. It could still be very fuzzy indeed! As fuzzy as a pion. If so, hardly an SM-like Higgs! We must do better and, through much more precise coupling measurements at the 0.2% level, a future Higgs factory like the FCC-ee could determine if the Higgs is point-like as far down as the 6% level.

Does the Higgs find itself attractive? Yes, according to the SM. New particles means new forces and so it follows that if the Higgs boson interacts with new heavy particles they will generate a new force between the Higgs and itself. The operator effectively capturing this is |H|6 and it literally shapes the way in which the Higgs field gave mass to particles during the very nascence of our universe! So, how SM-like is the Higgs self-attraction? With present experimental constraints, we know the Higgs self-attraction could be 530% stronger than the SM value (not merely self-attraction, more like outright vanity) or even −140% less (self-repulsive, more like). Hardly SM-like in either case! To have any idea of whether the self-attraction is SM-like, we must do a lot better. A future facility, such as the FCC-hh, CLIC or a muon collider, could probe the self-attraction at the much more precise 5% level.

Patience is a virtue; complacency is not. It is far too early to call time at the bar for the Higgs boson. Who knows, we may even be served with something completely unexpected, like a new window into the dark sector of the universe. Truly exploring all facets of the nature of the Higgs boson, understanding whether or not it is SM-like, will take time (measured in decades) and a lot of hard work. But it can and should be done. This is the experimental future of Higgs research that we look forward to.

All that said, it’s no secret that many theorists expected the Higgs to be much less SM-like than it appears to be already. Heads duly scratched, a theoretical coup d’état is now silently under way. There were good reasons to expect something different: chiefly the hierarchy problem. This problem is not simply aesthetic. The SM breaks down at high energies, ultimately making pathological predictions, thus it can only be a long-distance effective field theory description of something else more fundamental. If, as was the case for pions, the Higgs mass is determined by the more fundamental parameters, then for the Higgs there is no mechanism to keep it lighter than the mass scale of the new particles in that theory. Yet colliders tell us there is a gap between the mass of the Higgs and that of those new particles. In the past, this motivated the discovery and development of new mechanisms to explain a light Higgs, such as the venerated low-scale supersymmetry, thus far a no-show at the LHC physics party, with its attendant non-SM-like Higgs.

Rudely awoken by the deluge of exclusion plots, coffee reluctantly smelled, theorists have, in recent years, put forward what could well transpire to be revolutionary theoretical developments. The hierarchy problem hasn’t gone away and neither has the data, so the other foundational assumptions covertly injected into the old theories, often linked to symmetry or aesthetic principles such as simplicity or minimality, have been interrogated and found wanting. In response, intrepid new classes of theories have been developed that can address the hierarchy problem whilst being consistent with all those bothersome exclusion plots. They range from relatively modest conceptual tweaks of existing structures, to the abandonment of aesthetic principles, and then all the way out the other side to attempts to link the Higgs mass to the origins of the universe, cosmology, the nature of the Big Bang and, at an extreme, speculations about possible links between the Higgs mass and the existence of life itself. You name it, we’re boldly going.

It’s no fait accompli. None of these ideas are as intoxicating as supersymmetry or as stupefying as extra dimensions, each leaving those who study them with more of a “watch this space” feeling than the “eureka” that Archimedes enjoyed. Variously, they’re not radical enough, too radical or simply not to taste. No Goldilocks moment just yet. However, in my view these issues are cause for hope. In similar moments in the past, we have been essentially on the right track, having to wait a little longer than expected for the confirming experimental data (top quark). At other times, the right ideas have been too radical for most to stomach in one sitting (quantum mechanics). Yet for others the correct approaches languished in relative obscurity far too long, simply for not being à la mode (quantum field theory). Look up the citation records of the original Brout-Englert, Higgs, Guralnik-Hagen-Kibble papers or Weinberg’s “A Model of Leptons”, all foundational to the physics of the Higgs boson, and you’ll see they are important cases in point that we would do well to remember. Nature made no promises that understanding the origins of the Higgs should have been easy, nor should it be in the future, but history teaches that those who explore relentlessly and fearlessly are often the ones rewarded with the greatest prize of all: the truth.

Where will all this go in coming years? Will we be tenacious enough to build the accelerator, the detectors and the village it will take to measure the Higgs self-attraction or discover the fuzziness of the Higgs? Will some plucky theorists unlock the door to the fundamental theory beyond the SM? Will future phenomenologists lay the first foundational stones on the path to discovering it?

As Dennis Gabor, the inventor of holography, put it: “The future cannot be predicted, but futures can be invented.” We’re working on it.

thortala Wed, 08/10/2022 - 09:04 Byline Matthew McCullough Publication Date Wed, 08/10/2022 - 08:39

Mapping out the future of high-energy physics in the United States

Tue, 09/08/2022 - 13:35
Mapping out the future of high-energy physics in the United States

On 17 July 2022, hundreds of US high-energy physicists, along with colleagues from all over the world, gathered at the University of Washington in Seattle for a ten-day meeting to take one of the final steps in the latest US high-energy physics (HEP) community planning exercise. These exercises, hosted by the Division of Particles and Fields of the American Physical Society (APS), take place every seven to ten years. Their goal is to identify the most important questions in HEP for the next two decades and the tools and infrastructure required to address them. The process and the final meeting go by the name “Snowmass”, harking back to the early editions, starting in 1982, which concluded with a community summer study in Snowmass, Colorado. The Seattle meeting, originally scheduled to take place in July 2021, was delayed by one year because of the COVID-19 pandemic.

Snowmass is a “science study” in which all scientifically credible ideas and proposals are welcome. It does not concern itself with costs or budget constraints. However, the projects and proposals that it develops provide input to a follow-up Particle Physics Project Prioritization Panel, known as P5, which makes recommendations to the US Department of Energy and National Science Foundation on which HEP projects to undertake, based on various funding scenarios.

The Snowmass study was organised into ten working groups, or “frontiers”: accelerator, cosmic, community engagement, computing, energy, instrumentation, neutrino, rare processes and precision measurements, theory, and underground facilities and infrastructure. An organisation of early-career physicists helped bring the issues of young people into the community study. More than 2000 physicists from all over the world contributed to over 500 white papers that are now being distilled into frontier reports. The reports, together with an overall summary and all the white papers, will then be provided to P5.

The Seattle meeting, which ended on 26 July, was a hybrid event, with approximately 700 people attending all or part of the meeting in person and another 650 people participating remotely. It provided a final opportunity for the frontiers to discuss their visions internally, to ensure that they were aligned with those of the other frontiers and to resolve any issues before writing their final reports. The daily programme typically started at 8.00 a.m. and ran until 7.00 p.m. Days 2–8 were packed with parallel sessions in the mornings and three 90-minute plenary sessions in the afternoons. Days 1, 9 and 10 consisted entirely of plenary sessions that included special presentations of plans and planning processes by many leaders of US and international institutes and laboratories, including Fabiola Gianotti (CERN), Masanori Yamauchi (KEK, Japan), Yifang Wang (IHEP, China), and Lia Merminga (Fermilab, US).

Many scientists from the CERN community contributed their ideas to the Snowmass process and several attended the Seattle meeting. Both Director-General Gianotti and Fermilab Director Merminga acknowledged the importance of US–CERN collaboration on the LHC and the HL-LHC, the LHC Accelerator Upgrade Project and DUNE at LBNF, and expressed an openness to discuss collaboration on future projects, including future colliders. 

On the final day, Priscilla Cushman of the University of Minnesota put it all in perspective with her inspiring “Community Summer Study and Workshop Synthesis”. As the audience held its breath, JoAnne Hewett of SLAC National Accelerator Laboratory and chair of the US High Energy Physics Advisory Panel (HEPAP) announced that the new P5 chair would be Berkeley professor Hitoshi Murayama.

We wish to acknowledge the work of the University of Washington local organising committee, led by Professors Gordon Watts and Shih-Chieh Hsu, for running an outstanding workshop under difficult circumstances. We thank the four units of APS whose work is closely related to HEP, namely astrophysics, nuclear physics, physics of gravity, and especially the physics of particle beams, for their many contributions to this Snowmass process. Finally, we express our thanks and admiration to the Snowmass community, who produced great physics studies despite the many challenges facing them in this period.

 

Joel Butler, US Division of Particles and Fields, Chairperson, 2022, Fermilab, Batavia, IL, US.
Sekhar Chivukula, University of California, San Diego, San Diego, CA, US. (DPF Chairperson-elect)
Andre de Gouvea, Northwestern University, Evanston, IL, US. (DPF Vice-Chairperson)
Tao Han, University of Pittsburgh, Pittsburgh, PA, US. (DPF Chairperson, 2021)
Young- Kee Kim, University of Chicago, Chicago, IL, US. (DPF chairperson, 2020)
Priscilla Cushman, University of Minnesota, Minneapolis, MN, US. (DPF recent chairperson, 2019)

 

thortala Tue, 08/09/2022 - 12:35 Byline Joel Butler Publication Date Tue, 08/09/2022 - 12:32

Higgs10: Ten things we’ve learned about the Higgs boson in the past ten years

Wed, 27/07/2022 - 12:18
(Image: CERN) Higgs10: Ten things we’ve learned about the Higgs boson in the past ten years

By: Monica Dunford& Andre David

19 July, 2022 · Voir en français

In the seventh part of the Higgs10 series, we give an overview of ten years of Higgs boson research.

 

Since its discovery in 2012, the Higgs boson has become one of the most powerful tools to probe our understanding of nature and, with that, examine some of the biggest open questions in physics today. But what have we physicists learned about the particle in the past ten years? 

A scalar particle exists in nature

During the early hours of 4 July 2012, the foyer outside the main CERN lecture hall looked more like the lead-up to a rock concert than the main building of the world’s leading particle physics lab. Dozens of groggy-eyed students slowly rolled up their sleeping bags, stretching out after a long night on the hard floor. A line hundreds long snaked through the foyer, around the restaurant and out the door. The excitement in the line was pulsating – even though the odds of making it into the auditorium were small, just to be there was a thrill. We had found it. A scalar particle existed in nature and 4 July 2012 was its debut.

It’s heavy and short-lived

The first measurements of the new scalar particle, H(125), relied on two experimental channels: 4-lepton decays and 2-photon decays. Although these are not the most abundant decay channels, they are the best in determining the scalar particle’s mass. The measured mass of about 125 GeV is maximally interesting: it is much heavier than was expected for popular models of supersymmetry, it puts the universe in a precarious position between being stable and metastable, and it has a rich phenomenology. In contrast to its heavy mass, the particle’s lifetime is short; it is gone in 10-22 of a second.

It has no electric charge and no spin

The discovery of the H(125) via its decay to two photons immediately established that the new particle had no electric charge and strongly disfavoured it to have spin of 1. The exact spin of the new particle can be probed by examining the angular distributions of the final-state products in decays to two protons, two W bosons and two Z bosons. The spin 0 hypothesis has held up against a myriad of other possible assignments.

Measurements of the interaction strength between the H(125) and some of the Standard Model particles. The red line represents the Standard Model expectation. Recent progress has increased the reach to second generations fermions, like the muon, and first results concerning charm quarks. (Image: ATLAS)

It interacts with other bosons

How the new boson interacts with other particles can be probed in both how it decays and how it is produced. With its discovery via decays to two photons and two Z bosons, it was readily concluded that the H(125) particle couples to bosons (in the case of photons, indirectly). This was further reaffirmed with measurements of decays to two W bosons. Furthermore, the production of the H(125) through couplings to bosons is measured when two vector bosons (force carriers such as W and Z bosons) fuse to produce the scalar or when the scalar radiates from a heavy boson (so-called V+H production).

It interacts with fermions

The Standard Model (SM) predicts that the strength of the coupling between the H(125) and other particles is proportional to their masses. Studying fermions tests these couplings over three fermion generations spanning three orders of magnitude of masses. For the heaviest fermions, all couplings have been measured – to top quarks (via measurements of ttH production), to beauty quarks and to tau leptons. Now, the experimental challenge lies in reaching the second generation, whose coupling with the Higgs boson is weaker. First evidence of decays to muons are emerging and both the ATLAS and CMS experiments are homing in on decays to charm quarks. 

It could be a portal for dark matter

If dark matter consists of elementary particle(s), the SM simply does not predict any of them. If the H(125) and dark matter particles interact in nature, one possible signature is that of “invisible” Higgs boson decays. Such searches limit these decays to be lower than 15% and, consequently, set limits on interactions between this Higgs boson and possible dark matter particles and on the models that predict them. The SM predicts only a diminutive branching fraction of 0.1% – to four neutrinos. 

Limits on Higgs boson pair production, a process that is sensitive to the Higgs boson self-interaction and the shape of the Higgs potential. Results are presented as a function of time along with projections for the full HL-LHC dataset that should provide enough sensitivity to challenge the SM prediction (red horizontal line). (Image: CMS)

It may touch the structure of the universe

The inclusion of the Brout-Englert-Higgs mechanism in the SM leads to precise predictions of how the universe evolved during one of its earliest stages, the electroweak epoch. A scalar field can influence several aspects of cosmology and even play a role in the observed matter–antimatter asymmetry in the universe. Depending on the shape of the vacuum potential, the universe could be metastable and decay, and one way to probe this shape is to measure the different ways in which the H(125) interacts with itself. One of the signatures that can be used to access this self-interaction is the production of Higgs boson pairs. While existing analyses of LHC data have already started to exclude some non-SM alternatives, more data and future accelerators – like Higgs factories – will allow us to explore this critical area.

It seems to be a lone child

The SM is minimalistic as far as scalars are concerned: it predicts one single elementary scalar particle, with distinct types of interactions. In straightforward extensions to the minimal SM, more than one Higgs boson is predicted, resulting in different sets of interactions. Therefore, a vigorous programme of searches for other Higgs bosons – lighter and heavier, neutral and charged (and doubly charged) – has been undertaken. With other possibilities being strongly reduced, H(125) is presently the only scalar we know of in nature.

It’s a new player in the team pushing past the SM

This Higgs boson is the newest player joining the team of particles that we use to understand the nature of the universe. Matter–antimatter asymmetry, dark matter, unification of all forces; these are some of the questions where a coherent and precise exploration of the properties of particles like the Z and W bosons, the beauty and top quarks and now the H(125), probe energy regimes far beyond those directly accessible at colliders. One possibility is to extend the SM with generic interactions that represent the effect of particles and interactions beyond the direct reach of present colliders. Making use of all the information from H(125) and its team members in a consistent fashion may point us in the direction of the next standard model.

It’s just the beginning

While we have established several properties and interactions of the H(125), much remains to be learned about this Higgs boson. Far from just being the last prediction from the SM, the discovery of the H(125) and its singular scalar quality provides an important instrument to further our understanding of nature at its deepest. Is there really only one Higgs boson in nature? Do its properties differ from the SM predictions? Can it show us what is beyond the electroweak scale? Might it interact with dark matter particles? Will we be able to use it to measure the shape of the vacuum potential of the universe?

Ten years ago, before the discovery of this formidable tool, these questions were beyond our reach. The H(125) has opened new doors, inviting us to walk through.

Since its discovery in 2012, the Higgs boson has become one of the most powerful tools to probe our understanding of nature and, with that, examine some of the biggest open questions in physics today.

Higgs10: Ten things we’ve learned about the Higgs boson in the past ten years

Tue, 19/07/2022 - 16:36
Higgs10: Ten things we’ve learned about the Higgs boson in the past ten years

Since its discovery in 2012, the Higgs boson has become one of the most powerful tools to probe our understanding of nature and, with that, examine some of the biggest open questions in physics today. But what have we physicists learned about the particle in the past ten years? 

A scalar particle exists in nature

During the early hours of 4 July 2012, the foyer outside the main CERN lecture hall looked more like the lead-up to a rock concert than the main building of the world’s leading particle physics lab. Dozens of groggy-eyed students slowly rolled up their sleeping bags, stretching out after a long night on the hard floor. A line hundreds long snaked through the foyer, around the restaurant and out the door. The excitement in the line was pulsating – even though the odds of making it into the auditorium were small, just to be there was a thrill. We had found it. A scalar particle existed in nature and 4 July 2012 was its debut.

It’s heavy and short-lived

The first measurements of the new scalar particle, H(125), relied on two experimental channels: 4-lepton decays and 2-photon decays. Although these are not the most abundant decay channels, they are the best in determining the scalar particle’s mass. The measured mass of about 125 GeV is maximally interesting: it is much heavier than was expected for popular models of supersymmetry, it puts the universe in a precarious position between being stable and metastable, and it has a rich phenomenology. In contrast to its heavy mass, the particle’s lifetime is short; it is gone in 1/10^22 of a second.

It has no electric charge and no spin

The discovery of the H(125) via its decay to two photons immediately established that the new particle had no electric charge and strongly disfavoured it to have spin of 1. The exact spin of the new particle can be probed by examining the angular distributions of the final-state products in decays to two protons, two W bosons and two Z bosons. The spin 0 hypothesis has held up against a myriad of other possible assignments.

Measurements of the interaction strength between the H(125) and some of the Standard Model particles. The red line represents the Standard Model expectation. Recent progress has increased the reach to second generations fermions, like the muon, and first results concerning charm quarks. (Image: ATLAS)

It interacts with other bosons

How the new boson interacts with other particles can be probed in both how it decays and how it is produced. With its discovery via decays to two photons and two Z bosons, it was readily concluded that the H(125) particle couples to bosons (in the case of photons, indirectly). This was further reaffirmed with measurements of decays to two W bosons. Furthermore, the production of the H(125) through couplings to bosons is measured when two vector bosons (force carriers such as W and Z bosons) fuse to produce the scalar or when the scalar radiates from a heavy boson (so-called V+H production).

It interacts with fermions

The Standard Model (SM) predicts that the strength of the coupling between the H(125) and other particles is proportional to their masses. Studying fermions tests these couplings over three fermion generations spanning three orders of magnitude of masses. For the heaviest fermions, all couplings have been measured – to top quarks (via measurements of ttH production), to beauty quarks and to tau leptons. Now, the experimental challenge lies in reaching the second generation, whose coupling with the Higgs boson is weaker. First evidence of decays to muons are emerging and both the ATLAS and CMS experiments are homing in on decays to charm quarks. 

It could be a portal for dark matter

If dark matter consists of elementary particle(s), the SM simply does not predict any of them. If the H(125) and dark matter particles interact in nature, one possible signature is that of “invisible” Higgs boson decays. Such searches limit these decays to be lower than 15% and, consequently, set limits on interactions between this Higgs boson and possible dark matter particles and on the models that predict them. The SM predicts only a diminutive branching fraction of 0.1% – to four neutrinos. 

Limits on Higgs boson pair production, a process that is sensitive to the Higgs boson self-interaction and the shape of the Higgs potential. Results are presented as a function of time along with projections for the full HL-LHC dataset that should provide enough sensitivity to challenge the SM prediction (red horizontal line). (Image: CMS)

It may touch the structure of the universe

The inclusion of the Brout-Englert-Higgs mechanism in the SM leads to precise predictions of how the universe evolved during one of its earliest stages, the electroweak epoch. A scalar field can influence several aspects of cosmology and even play a role in the observed matter–antimatter asymmetry in the universe. Depending on the shape of the vacuum potential, the universe could be metastable and decay, and one way to probe this shape is to measure the different ways in which the H(125) interacts with itself. One of the signatures that can be used to access this self-interaction is the production of Higgs boson pairs. While existing analyses of LHC data have already started to exclude some non-SM alternatives, more data and future accelerators – like Higgs factories – will allow us to explore this critical area.

It seems to be a lone child

The SM is minimalistic as far as scalars are concerned: it predicts one single elementary scalar particle, with distinct types of interactions. In straightforward extensions to the minimal SM, more than one Higgs boson is predicted, resulting in different sets of interactions. Therefore, a vigorous programme of searches for other Higgs bosons – lighter and heavier, neutral and charged (and doubly charged) – has been undertaken. With other possibilities being strongly reduced, H(125) is presently the only scalar we know of in nature.

It’s a new player in the team pushing past the SM

This Higgs boson is the newest player joining the team of particles that we use to understand the nature of the universe. Matter–antimatter asymmetry, dark matter, unification of all forces; these are some of the questions where a coherent and precise exploration of the properties of particles like the Z and W bosons, the beauty and top quarks and now the H(125), probe energy regimes far beyond those directly accessible at colliders. One possibility is to extend the SM with generic interactions that represent the effect of particles and interactions beyond the direct reach of present colliders. Making use of all the information from H(125) and its team members in a consistent fashion may point us in the direction of the next standard model.

It’s just the beginning

While we have established several properties and interactions of the H(125), much remains to be learned about this Higgs boson. Far from just being the last prediction from the SM, the discovery of the H(125) and its singular scalar quality provides an important instrument to further our understanding of nature at its deepest. Is there really only one Higgs boson in nature? Do its properties differ from the SM predictions? Can it show us what is beyond the electroweak scale? Might it interact with dark matter particles? Will we be able to use it to measure the shape of the vacuum potential of the universe?

Ten years ago, before the discovery of this formidable tool, these questions were beyond our reach. The H(125) has opened new doors, inviting us to walk through.

thortala Tue, 07/19/2022 - 15:36 Byline Monica Dunford André David Publication Date Tue, 07/19/2022 - 15:34

ATLAS measures joint polarisation of carriers of the weak force

Wed, 13/07/2022 - 14:32
ATLAS measures joint polarisation of carriers of the weak force

In the Standard Model of particle physics, the Brout–Englert–Higgs mechanism provides mass to elementary particles. While physicists are carrying out direct studies of the Higgs boson to test this mechanism, probes of other particles that have mass can also provide insight. For instance, the W and Z bosons – the carriers of the weak force – get their mass from the Higgs mechanism. This impacts their polarisation, that is, the degree by which their quantum spin is aligned to a given direction. The W and Z bosons have a spin of 1 and can be longitudinally polarised as a direct consequence of their being massive – in other words, their spin can be oriented perpendicular to their direction of motion.

The simultaneous production of two W or Z bosons (or “diboson” production) allows physicists to study fundamental interactions between bosons. These rare processes have yet to be fully tested against Standard Model predictions, and studying the polarisation of the produced bosons is a way to potentially unveil new physics effects. While the polarisation of W and Z bosons separately has been studied since the era of the Large Electron–Positron (LEP) collider, the predecessor to the Large Hadron Collider (LHC), two such bosons produced simultaneously with a longitudinal polarisation have never been observed. With the wealth of data collected during Run 2 of the LHC and innovative analysis methods, ATLAS researchers are now able to study the joint-polarisation states of diboson production events.

In a new study presented at the ICHEP 2022 conference, ATLAS physicists have been able to observe events with both a W and a Z boson simultaneously polarised longitudinally for the very first time. To achieve this result, the researchers identified events containing both a W boson and a Z boson. They focused on events where the bosons transform, or “decay”, into particles called leptons, as these leave the clearest signature in the ATLAS detector. The polarisation of the parent bosons in such WZ events manifests itself in angular observables that have very distinct distributions for different polarisation states.

However, not all of the four possible WZ joint-polarisation states – longitudinal–longitudinal, longitudinal–transverse, transverse–longitudinal and transverse–transverse – are equally probable. The most interesting events, with both bosons exhibiting a longitudinal polarisation, are well hidden – they represent only about 7% of all WZ events, amounting to just 1200 of the 17 100 WZ events studied by ATLAS.

To overcome the main experimental challenges, researchers developed dedicated machine-learning algorithms to extract the fractions of the four types of joint-polarisation events with a relative uncertainty of about 20%, at most. They found that the Standard Model predictions for these fractions always lie within the 95.5% confidence level region of the measurements, meaning that there is no significant tension with the theory. Researchers also found that the product of the two single-boson longitudinal polarisation fractions is about 50% below the actual longitudinal–longitudinal joint-polarisation fraction. This is a direct measure of the role played by correlations between the two bosons and demonstrates that the two single-boson polarisations are not independent.

This result is a fascinating look into some of the most fundamental structures of the Standard Model itself. And the feasibility of joint-polarisation measurements provides new opportunities to look for new physics phenomena, targeting more specific (and rarer) processes. Building on the novel techniques developed here, physicists can now envisage the even more challenging joint-polarisation measurement of the scattering of two longitudinally polarised bosons.

_____

Read more on the ATLAS website.

abelchio Wed, 07/13/2022 - 13:32 Byline ATLAS collaboration Publication Date Thu, 07/14/2022 - 10:33

Higgs10: When spring 2012 turned to summer

Thu, 07/07/2022 - 10:41
(Image: CERN) Higgs10: When spring 2012 turned to summer

By: Fabiola Gianotti& Joe Incandela

04 July, 2022 · Voir en français

In the sixth part of the Higgs10 series, the ATLAS and CMS experiments announce the discovery on 4 July 2012

At the Higgs boson search update seminar on 13 December 2011, things were already looking promising. The data had allowed us to constrain the Higgs boson mass to the range from around 115 to 130 GeV, and both ATLAS and CMS had tantalising hints of a new particle around a mass of 125 GeV. Those hints were not yet sufficiently strong to claim a discovery, the local significance was between 2.6 and 3.6 sigma, but they were enough to ensure that the eyes of the world would be on CERN as the data taking resumed in spring 2012 at a larger centre-of-mass energy: 8 TeV, compared with 7 TeV in 2011.

The year’s big high-energy physics conference, ICHEP, was to be held in Melbourne, Australia, starting on 6 July 2012. We both had our tickets booked, and an update on the Higgs boson search was a key part of the agenda. Plans were made for a two-way link to relay the Higgs boson sessions live from Melbourne to CERN’s Main Auditorium. Meanwhile, both experiments got on with data taking and analysis.

It was around mid-June that things started to get really interesting. By then, ATLAS had been seeing an excess of events in the two-photon channel, at the same mass as the excess reported at the end of 2011 based on an independent data sample, but nothing in the rarer four-lepton channels. It was clear to us both that we needed to see a signal in the gamma-gamma and lepton channels before going to the Director-General, Rolf Heuer. In the middle of June, CMS unblinded its analysis to find a four-sigma signal in the two-photon channel, and three-sigma in the leptons. Meanwhile, ATLAS’s Higgs boson sample received its first four-lepton candidates. We went to see Rolf.

The following weeks were incredibly intense. It was imperative that the collaborations maintained complete confidentiality, and it was impressive how well that was respected, not only outside but also within CERN. ATLAS did not know what exactly CMS had, and vice versa. The two of us would keep each other informed almost daily, but we did not disclose the other experiment’s results to our respective collaborations. Together with Rolf Heuer, we were the only ones who had the full picture of what was going on. This was essential to maintain confidentiality, but also to avoid ATLAS and CMS influencing each other, and to ensure that emotions did not affect the ongoing work. The pressure was enormous, people were working around the clock making millions of checks and cross-checks, and they had to remain calm and focused. The rest of CERN, and indeed anyone following particle physics, must have felt the energy emanating from the community, because the sense of anticipation was tangible.

The CERN Council met during the week of 18 June and decided that, whatever ATLAS and CMS had to say about the status of the Higgs boson searches, it should be said at CERN. We rapidly changed our travel plans, and CERN announced a Higgs boson update seminar for 4 July – the latest date compatible with both of us being in Melbourne in time for the plenary sessions of ICHEP. The primary direction of the two-way link with Melbourne was reversed: those arriving early for the conference would now follow the seminar at CERN remotely. The Council’s decision was taken as a sign that we had an announcement to make but, at that point, we were not telling anyone what we’d be announcing. Nevertheless, eminent theorists such as Carl Hagen and Gerry Guralnik decided to attend, and we invited all the other theorists who had been involved with developing the theory back in the 60s. As a result, François Englert and Peter Higgs also joined us on the day. Two years earlier, the four had shared the APS’s Sakurai Prize along with Robert Brout and Tom Kibble, both now sadly departed, for their work on spontaneous symmetry breaking in gauge theories.

Fabiola Gianotti, Rolf Heuer and Joe Incandela in a packed CERN auditorium on the day of the announcement of the discovery of the Higgs boson (Image: AFP/Denis Balibouse)

It went right down to the wire. The results were still being checked and double-checked until just days before the seminar, and we were putting the final touches to our presentations until minutes before the seminar began. Walking into the auditorium, past the people rolling up their sleeping bags because they’d camped out overnight to ensure their places, we felt tremendous pressure along with great pride for what our community had managed to achieve over the decades. Then, as soon as it began, it seemed that a huge weight was lifted. The room was a sea of faces, ranging from those of people whose working lives had been devoted to building the LHC, ATLAS and CMS, to those whose careers were just beginning. Everyone was with us and, because the results were so compelling, neither of us needed our banks of back-up slides in case we were called upon to justify the details.

It was an amazing day, seen live around the world by half a million people, and reported by the media to over a billion. The media focused heavily upon us as the spokespersons but of course we were just the messengers. This success was the culmination of a multigenerational effort spanning decades. The capability of the particle physics community to deliver beyond expectation was truly inspiring. From the original theory through phenomenology to the design and construction of the accelerator, the detectors and the computing infrastructure, the tiny signal we were able to tease out from a large background was a credit to everyone who played a part. It was the triumph of a community that was able to achieve what many would have deemed impossible, bringing together expertise from every branch of the field.

Plots shown by Joe Incandela for CMS and Fabiola Gianotti for ATLAS show a clear 5 sigma discovery signal. (Image: CERN)

Peter Higgs was treated like a rock star on the day. His reaction gives the measure of our field: when pressed for comment by the media, he replied that this was a day for the experiments, and there would be plenty of time to talk to the theorists later. A little over a year later, François Englert and Peter Higgs shared the Nobel Prize in physics for “the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, which recently was confirmed through the discovery of the predicted fundamental particle by the ATLAS and CMS experiments at CERN's Large Hadron Collider.”

Fabiola Gianotti and Joe Incandela, Spokespersons of the ATLAS and CMS experiments at the time of the Higgs boson’s discovery.

 

It was just a few short weeks in mid-2012, but they were so intense that it felt like years. As 4 July drew near, the ATLAS and CMS experiments could sense that they were homing in on something big.

Higgs10: Big Bang Day

Thu, 07/07/2022 - 10:40
A chain of LHC dipole magnets inside the tunnel at point 1 (ATLAS) towards the end of Long Shutdown 2 (LS2).When the Large Hadron Collider (LHC) begins Run 3 next year, operators aim to increase the energy of the proton beams to an unprecedented 6.8 TeV. (Image: CERN) Higgs10: Big Bang Day

By: Lyn Evans

20 June, 2022 · Voir en français

In the fifth part of the Higgs10 series, the LHC circulates its first beams on 10 September 2008.

It was at 9.30 a.m. on 10 September 2008 that the LHC’s first beam was injected, in the full glare of the global media spotlight. Just under one hour later, a beam had been successfully steered all the way around the ring, to scenes of great emotion at the Laboratory. A long wait was over, LHC page 1 became the focus of everyone’s attention around the Lab, and a new era of research seemed about to get under way, but the sense of euphoria was to be short-lived.

In the days that followed, things went well, but then disaster struck: during a ramp to full energy, one of the 10 000 superconducting joints between the magnets failed, causing extensive damage that took more than a year to recover from.

It was unheard of to start a machine like the LHC in the public eye, but I’m assured we had little choice. In the months and weeks before the start-up, particle physics had never seen so much media attention. A small number of individuals on social media had managed to stir up the myth that the LHC would create a world-eating black hole, and the newspapers were full of it. They were going to come to CERN whether we asked them or not, so we invited them in on the basis that it would be better to have them inside the Lab than outside, telling the world that CERN was starting up the “black hole machine” behind locked doors. Over 300 media outlets came, BBC Radio 4 did an unprecedented full day of outside broadcast from CERN, and an estimated billion people watched as I gave the countdown to that first beam. I thought I was just talking to physicists in the main auditorium!

The scene in the CERN Control Centre on 10 September 2008, when beams went round the LHC for the first time. (Image: CERN)

Those joyful events of 10 September firmly established CERN’s place in the public eye, while the failure of a magnet interconnection just over a week later ensured the Laboratory would stay there. There was, and there remains, fascination with the human endeavour that particle physics represents, and the media were kind to us on the whole. But for me, the most important part of the story was somewhat lost.

The LHC is unique. Like any energy-frontier accelerator, it is its own prototype, and building it was a learning experience from the start. Despite the serious nature of the setback in September 2008, it was really just another step, albeit a big one, on a long learning curve. As with previous setbacks, the LHC team was hard at work the next day to ensure that we could recover as fast as possible. We soon understood the problem, and we had all the spares we needed. It took a year to put right, but we knew straight away what we had to do.

Image of a LHC beam screen recorded on 10 September 2008, showing two spots corresponding to the successful circulation of protons once around the machine. (Image: CERN)

It’s a great tribute to the global particle physics community that setbacks are confronted with a confident, positive approach. In 2004, after we’d installed a full sector of the cryogenic distribution line (QRL), it failed and had to be removed from the tunnel. To me, this was a much bigger issue than the 2008 event, since it required the whole LHC installation schedule to be rearranged while the contractor made good the problem with considerable help from CERN. Our Director-General at the time, Robert Aymar, was an engineer, and he understood the magnitude of the problem perfectly. He was the unsung hero in liberating the resources needed to get it fixed. It’s also thanks to him that we have Linac4, a key part of the HL-LHC project, whose construction began during his mandate. Later, in 2007, one of the so-called inner triplets, which perform the final focus of the beams, failed a high-pressure test in the LHC tunnel. It was remarkable how quickly CERN staff came up with an innovative and elegant solution, and implemented it with the help of colleagues from Fermilab, KEK and the Lawrence Berkeley National Laboratory.

Following repairs and consolidation, on 29 November 2009 there were beams circulating again in the LHC, and full commissioning could get under way. The experiments had had an extra year to prepare, and although I’m sure they’d have preferred beam in 2008, they were in perfect shape to start data taking. Every cloud has a silver lining. This time, start-up went very quickly. The injector chain worked beautifully, as always, with even higher performance than we’d anticipated: a great tribute to our predecessors who built those machines from the 1950s onwards. We’d also learned a lot from LEP, and instrumentation was very much improved. The LHC physics programme, at an initial energy of 3.5 TeV per beam, began in earnest in March 2010.

I’m an accelerator physicist, but I want to finish by talking about the experiments. It’s not only the LHC that took technology way beyond anything that had ever been done before. Like the accelerator team, the experimental collaborations had also learned much from their predecessors. The previous generation of hadron collider experiments had luminosities two orders of magnitude lower to deal with, they had around a million readout channels compared with the LHC experiments’ up to 100 million, and their data rates and volumes were also much smaller. It’s thanks to the efforts of a global, multidisciplinary collaboration that the LHC project delivered so well on its promise right from the moment data taking began, re-measuring everything we’d learned before about the Standard Model of particle physics in the first few months of operation, and then going on to new discoveries. But that’s a story for another day.

On 10 September 2008, the LHC circulated its first beams. It may not have been all plain sailing from then on, but the adventure had begun.

CMS measures rare particle decay with high precision

Mon, 04/07/2022 - 15:48
CMS measures rare particle decay with high precision

At CERN’s Large Hadron Collider (LHC), studies of rare processes allow scientists to infer the presence of heavy particles, including undiscovered particles, that cannot be directly produced. Such particles are widely anticipated to exist beyond the Standard Model, and could help explain some of the enigmas of the universe, such as the existence of dark matter, the masses of neutrinos (elusive particles originally thought to be massless) and the universe’s matter–antimatter asymmetry.

One such process is the rare decay of neutral B mesons to a muon and antimuon pair: the heavier cousin of the electron paired with its corresponding antiparticle. There are two types of neutral B mesons: the B0 meson consists of a beauty antiquark and a down quark, while for the Bs meson the down quark is replaced by a strange quark. If there are no new particles affecting these rare decays, researchers have predicted that only one in 250 million Bs mesons will decay into a muon–antimuon pair; for the B0 meson, the process is even more rare, at only one in 10 billion.

Scientists have been searching for experimental confirmation of these decays since the 1980s. Only recently, in 2014, was the first observation of the Bs to muons decay reported in a combined analysis of data taken by the LHCb and CMS collaborations, later confirmed by the ATLAS, CMS and LHCb experiments individually. However, the B0 decay still eludes any attempt to observe it.

Using data from Run 2 of the LHC, the CMS experiment has released a new study of the decay rate and the lifetime of the Bs meson decay, as well as a search for the B0 decay. The new study, presented at the International Conference on High Energy Physics (ICHEP), benefits from not only a large amount of data analysed, but also advanced machine-learning algorithms that single out the rare decay events from the overwhelming background of events produced by millions of particle collisions per second.

The results revealed a very clear signal of the Bs meson decaying to a muon–antimuon pair. The precision of the decay rate measurement exceeds that achieved in previous measurements in other experiments.

Both the observed Bs decay rate, found to be 3.8 ± 0.4 parts in a billion, and its lifetime measurement of 1.8 ± 0.2 picoseconds (one picosecond is one trillionth of a second), are very close to the values predicted by the Standard Model.

As for the B0 decay, although no evidence of it was found from these results, physicists can state with 95% statistical confidence that its decay rate is less than 1 part in 5 billion.

In recent years, a number of anomalies have been observed in other rare B meson decays, with discrepancies between the theoretical predictions and the data – indicating the potential existence of new particles. The new CMS result is much closer to theoretical predictions than these other rare decays and so could help scientists to understand the nature of the anomalies.

Rare B meson decays continue to be of great interest to scientists. With the Bs meson to muons decay firmly established and measured with high precision, scientists are now setting their sights on the ultimate prize: the B0 decay. With large data sets anticipated from LHC Run 3, they hope to catch the first glimpse of this extremely rare process and learn more about the puzzling anomalies.

Read more on the CMS website.

ndinmore Mon, 07/04/2022 - 14:48 Byline CMS collaboration Publication Date Mon, 07/11/2022 - 14:44

LHCb discovers three new exotic particles

Mon, 04/07/2022 - 12:55
LHCb discovers three new exotic particles

The international LHCb collaboration at the Large Hadron Collider (LHC) has observed three never-before-seen particles: a new kind of “pentaquark” and the first-ever pair of “tetraquarks”, which includes a new type of tetraquark. The findings, presented today at a CERN seminar, add three new exotic members to the growing list of new hadrons found at the LHC. They will help physicists better understand how quarks bind together into these composite particles.

Quarks are elementary particles and come in six flavours: up, down, charm, strange, top and bottom. They usually combine together in groups of twos and threes to form hadrons such as the protons and neutrons that make up atomic nuclei. More rarely, however, they can also combine into four-quark and five-quark particles, or “tetraquarks” and “pentaquarks”. These exotic hadrons were predicted by theorists at the same time as conventional hadrons, about six decades ago, but only relatively recently, in the past 20 years, have they been observed by LHCb and other experiments.

Most of the exotic hadrons discovered in the past two decades are tetraquarks or pentaquarks containing a charm quark and a charm antiquark, with the remaining two or three quarks being an up, down or strange quark or their antiquarks. But in the past two years, LHCb has discovered different kinds of exotic hadrons. Two years ago, the collaboration discovered a tetraquark made up of two charm quarks and two charm antiquarks, and two “open-charm” tetraquarks consisting of a charm antiquark, an up quark, a down quark and a strange antiquark. And last year it found the first-ever instance of a “double open-charm” tetraquark with two charm quarks and an up and a down antiquark. Open charm means that the particle contains a charm quark without an equivalent antiquark.

The discoveries announced today by the LHCb collaboration include new kinds of exotic hadrons. The first kind, observed in an analysis of “decays” of negatively charged B mesons, is a pentaquark made up of a charm quark and a charm antiquark and an up, a down and a strange quark. It is the first pentaquark found to contain a strange quark. The finding has a whopping statistical significance of 15 standard deviations, far beyond the 5 standard deviations that are required to claim the observation of a particle in particle physics.

The two new tetraquarks, illustrated here as single units of tightly bound quarks. One of the particles is composed of a charm quark, a strange antiquark and an up quark and a down antiquark (left), and the other is made up of a charm quark, a strange antiquark and an up antiquark and down quark (right) (Image: CERN)

The second kind is a doubly electrically charged tetraquark. It is an open-charm tetraquark composed of a charm quark, a strange antiquark, and an up quark and a down antiquark, and it was spotted together with its neutral counterpart in a joint analysis of decays of positively charged and neutral B mesons. The new tetraquarks, observed with a statistical significance of 6.5 (doubly charged particle) and 8 (neutral particle) standard deviations, represent the first time a pair of tetraquarks has been observed.

“The more analyses we perform, the more kinds of exotic hadrons we find,” says LHCb physics coordinator Niels Tuning. “We’re witnessing a period of discovery similar to the 1950s, when a ‘particle zoo’ of hadrons started being discovered and ultimately led to the quark model of conventional hadrons in the 1960s. We’re creating ‘particle zoo 2.0’.”

“Finding new kinds of tetraquarks and pentaquarks and measuring their properties will help theorists develop a unified model of exotic hadrons, the exact nature of which is largely unknown,” says LHCb spokesperson Chris Parkes. “It will also help to better understand conventional hadrons.”

While some theoretical models describe exotic hadrons as single units of tightly bound quarks, other models envisage them as pairs of standard hadrons loosely bound in a molecule-like structure. Only time and more studies of exotic hadrons will tell if these particles are one, the other or both.

Further information: 

Read more on the LHCb website.

Illustrations: https://cds.cern.ch/record/2814136

ochriste Mon, 07/04/2022 - 11:55 Publication Date Tue, 07/05/2022 - 11:00

ATLAS and CMS release results of most comprehensive studies yet of Higgs boson’s properties

Mon, 04/07/2022 - 11:21
ATLAS and CMS release results of most comprehensive studies yet of Higgs boson’s properties

Today, exactly ten years after announcing the discovery of the Higgs boson, the international ATLAS and CMS collaborations at the Large Hadron Collider (LHC) report the results of their most comprehensive studies yet of the properties of this unique particle. The independent studies, described in two papers published today in Nature, show that the particle’s properties are remarkably consistent with those of the Higgs boson predicted by the Standard Model of particle physics. The studies also show that the particle is increasingly becoming a powerful means to search for new, unknown phenomena that – if found – could help shed light on some of the biggest mysteries of physics, such as the nature of the mysterious dark matter present in the universe.

The Higgs boson is the particle manifestation of an all-pervading quantum field, known as the Higgs field, that is fundamental to describe the universe as we know it. Without this field, elementary particles such as the quark constituents of the protons and neutrons of atomic nuclei, as well as the electrons that surround the nuclei, would not have mass, and nor would the heavy particles (W bosons) that carry the charged weak force, which initiates the nuclear reaction that powers the Sun.

To explore the full potential of the LHC data for the study of the Higgs boson, including its interactions with other particles, ATLAS and CMS combine numerous complementary processes in which the Higgs boson is produced and “decays” into other particles.

This is what the collaborations have done in their new, independent studies, using their full LHC Run 2 data sets, which each include over 10 000 trillion proton–proton collisions and about 8 million Higgs bosons – 30 times more than at the time of the particle’s discovery. The new studies each combine an unprecedented number and variety of Higgs boson production and decay processes to obtain the most precise and detailed set of measurements to date of their rates, as well as of the strengths of the Higgs boson’s interactions with other particles.

All of the measurements are remarkably consistent with the Standard Model predictions within a range of uncertainties depending, among other criteria, on the abundance of a given process. For the Higgs boson’s interaction strength with the carriers of the weak force, an uncertainty of 6% is achieved. By way of comparison, similar analyses with the full Run 1 data sets resulted in a 15% uncertainty for that interaction strength.

“After just ten years of Higgs boson exploration at the LHC, the ATLAS and CMS experiments have provided a detailed map of its interactions with force carriers and matter particles,” says ATLAS spokesperson Andreas Hoecker. “The Higgs sector is directly connected with very profound questions related to the evolution of the early universe and its stability, as well as to the striking mass pattern of matter particles. The Higgs boson discovery has sparked an exciting, deep and broad experimental effort that will extend throughout the full LHC programme.”

“Sketching such a portrait of the Higgs boson this early on was unthinkable before the LHC started operating,” says CMS spokesperson Luca Malgeri. “The reasons for this achievement are manifold and include the exceptional performances of the LHC and of the ATLAS and CMS detectors, and the ingenious data analysis techniques employed.”

The new combination analyses also provide, among other new results, stringent bounds on the Higgs boson’s interaction with itself and also on new, unknown phenomena beyond the Standard Model, such as on Higgs boson decays into invisible particles that may make up dark matter.

ATLAS and CMS will continue revealing the nature of the Higgs boson using data from the LHC’s Run 3, which starts tomorrow at a new high-energy frontier, and from the collider’s major upgrade, the High-Luminosity LHC (HL-LHC), from 2029. With about 18 million Higgs bosons projected to be produced in each experiment in Run 3 and some 180 million in the HL-LHC’s runs, the collaborations expect to not only reduce significantly the measurement uncertainties of the Higgs boson’s interactions determined so far but also to observe some of the Higgs boson’s interactions with the lighter matter particles and to obtain the first significant evidence of the boson’s interaction with itself.

Find out more in the ATLAS and CMS Nature papers.

mailys Mon, 07/04/2022 - 10:21 Publication Date Mon, 07/04/2022 - 11:00

LHC Run 3: physics at record energy starts tomorrow

Fri, 01/07/2022 - 17:18
LHC Run 3: physics at record energy starts tomorrow

A new period of data taking begins on Tuesday, 5 July for the experiments at the world’s most powerful particle accelerator, the Large Hadron Collider (LHC), after more than three years of upgrade and maintenance work. Beams have already been circulating in CERN’s accelerator complex since April, with the LHC machine and its injectors being recommissioned to operate with new higher-intensity beams and increased energy. Now, the LHC operators are ready to announce “stable beams”, the condition allowing the experiments to switch on all their subsystems and begin taking the data that will be used for physics analysis. The LHC will run around the clock for close to four years at a record energy of 13.6 trillion electronvolts (TeV), providing greater precision and discovery potential than ever before.

“We will be focusing the proton beams at the interaction points to less than 10 micron beam size, to increase the collision rate. Compared to Run 1, in which the Higgs was discovered with 12 inverse femtobarns, now in Run 3 we will be delivering 280 inverse femtobarns1. This is a significant increase, paving the way for new discoveries,” says Director for Accelerators and Technology Mike Lamont.

The four big LHC experiments have performed major upgrades to their data readout and selection systems, with new detector systems and computing infrastructure. The changes will allow them to collect significantly larger data samples, with data of higher quality than in previous runs. The ATLAS and CMS detectors expect to record more collisions during Run 3 than in the two previous runs combined. The LHCb experiment underwent a complete revamp and looks to increase its data taking rate by a factor of ten, while ALICE is aiming at a staggering fifty-fold increase in the number of recorded collisions.

With the increased data samples and higher collision energy, Run 3 will further expand the already very diverse LHC physics programme. Scientists at the experiments will probe the nature of the Higgs boson with unprecedented precision and in new channels. They may observe previously inaccessible processes, and will be able to improve the measurement precision of numerous known processes addressing fundamental questions, such as the origin of the matter–antimatter asymmetry in the universe. Scientists will study the properties of matter under extreme temperature and density, and will also be searching for candidates for dark matter and for other new phenomena, either through direct searches or – indirectly – through precise measurements of properties of known particles.

“We’re looking forward to measurements of the Higgs boson decay to second-generation particles such as muons. This would be an entirely new result in the Higgs boson saga, confirming for the first time that second-generation particles also get mass through the Higgs mechanism,” says CERN theorist Michelangelo Mangano.

“We will measure the strengths of the Higgs boson interactions with matter and force particles to unprecedented precision, and we will further our searches for Higgs boson decays to dark matter particles as well as searches for additional Higgs bosons,” says Andreas Hoecker, spokesperson of the ATLAS collaboration. “It is not at all clear whether the Higgs mechanism realised in nature is the minimal one featuring only a single Higgs particle.”

A closely watched topic will be the studies of a class of rare processes in which an unexpected difference (lepton flavour asymmetry) between electrons and their cousin particles, muons, was studied by the LHCb experiment in the data from previous LHC runs. “Data acquired during Run 3 with our brand new detector will allow us to improve the precision by a factor of two and to confirm or exclude possible deviations from lepton flavour universality,” says Chris Parkes, spokesperson of the LHCb collaboration. Theories explaining the anomalies observed by LHCb typically also predict new effects in different processes. These will be the target of specific studies performed by ATLAS and CMS. “This complementary approach is essential; if we’re able to confirm new effects in this way it will be a major discovery in particle physics,” says Luca Malgeri, spokesperson of the CMS collaboration.

The heavy-ion collision programme will allow the investigation of quark–gluon plasma (QGP) – a state of matter that existed in the first 10 microseconds after the Big Bang – with unprecedented accuracy. “We expect to be moving from a phase where we observed many interesting properties of the quark–gluon plasma to a phase in which we precisely quantify those properties and connect them to the dynamics of its constituents,” says Luciano Musa, spokesperson of the ALICE collaboration. In addition to the main lead–lead runs, a short period with oxygen collisions will be included for the first time, with the goal of exploring the emergence of QGP-like effects in small colliding systems.

The smallest experiments at the LHC – TOTEM, LHCf, MoEDAL, with its entirely new subdetector MAPP, and the recently installed FASER and SND@LHC – are also poised to explore phenomena within and beyond the Standard Model, from magnetic monopoles to neutrinos and cosmic rays.

A new physics season is starting, with a broad and promising scientific programme in store. The launch of LHC Run 3 will be streamed live on CERN’s social media channels and high-quality Eurovision satellite link starting at 4.00 p.m. (CEST) on 5 July. Live commentary from the CERN Control Centre, available in five languages (English, French, German, Italian and Spanish), will walk the viewers through the operation stages that take proton beams from injection into the LHC to collisions for physics at the four interaction points where the experiments are located. A live Q&A session with experts from the accelerators and experiments will conclude the live stream.

Further information 

To follow the live stream on EBU satellite, you will need to create an account. The event will be accessible here.

Pictures of the day will be added here.

Run 3 background information can be found here.

1 An inverse femtobarn is a measure of the number of collisions or the amount of data collected. One inverse femtobarn corresponds to approximately 100 trillion (100 x 1012) proton–proton collisions.

mailys Fri, 07/01/2022 - 16:18 Publication Date Mon, 07/04/2022 - 08:00

The Higgs boson, ten years after its discovery

Fri, 01/07/2022 - 14:29
The Higgs boson, ten years after its discovery

Geneva, 4 July 2022. Ten years ago, on July 4 2012, the ATLAS and CMS collaborations at the Large Hadron Collider (LHC) announced the discovery of a new particle with features consistent with those of the Higgs boson predicted by the Standard Model of particle physics. The discovery was a landmark in the history of science and captured the world’s attention. One year later it won François Englert and Peter Higgs the Nobel Prize in Physics for their prediction made decades earlier, together with the late Robert Brout, of a new fundamental field, known as the Higgs field, that pervades the universe, manifests itself as the Higgs boson and gives mass to the elementary particles.

“The discovery of the Higgs boson was a monumental milestone in particle physics. It marked both the end of a decades-long journey of exploration and the beginning of a new era of studies of this very special particle,” says Fabiola Gianotti, CERN’s Director-General and the project leader (‘spokesperson’) of the ATLAS experiment at the time of the discovery. “I remember with emotion the day of the announcement, a day of immense joy for the worldwide particle physics community and for all the people who worked tirelessly over decades to make this discovery possible.”

In just ten years physicists have made tremendous steps forward in our understanding of the universe, not only confirming early on that the particle discovered in 2012 is indeed the Higgs boson but also allowing researchers to start building a picture of how the pervasive presence of a Higgs field throughout the universe was established a tenth of a billionth of a second after the Big Bang.

The new journey so far

The new particle discovered by the international ATLAS and CMS collaborations in 2012 appeared very much like the Higgs boson predicted by the Standard Model. But was it actually that long-sought-after particle? As soon as the discovery had been made, ATLAS and CMS set out to investigate in detail whether the properties of the particle they had discovered truly matched those predicted by the Standard Model. By using data from the disintegration, or ‘decay’, of the new particle into two photons, the carriers of the electromagnetic force, the experiments have demonstrated that the new particle has no intrinsic angular momentum, or quantum spin – exactly like the Higgs boson predicted by the Standard Model. By contrast, all other known elementary particles have spin: the matter particles, such as the ‘up’ and ‘down’ quarks that form protons and neutrons, and the force-carrying particles, such as the W and Z bosons.

By observing the Higgs bosons being produced from and decaying into pairs of W or Z bosons, ATLAS and CMS confirmed that these gain their mass through their interactions with the Higgs field, as predicted by the Standard Model. The strength of these interactions explains the short range of the weak force, which is responsible for a form of radioactivity and initiates the nuclear fusion reaction that powers the Sun.

The experiments have also demonstrated that the top quark, bottom quark and tau lepton – which are the heaviest fermions – obtain their mass from their interactions with the Higgs field, again as predicted by the Standard Model. They did so by observing, in the case of the top quark, the Higgs boson being produced together with pairs of top quarks, and in the cases of the bottom quark and tau lepton, the boson’s decay into pairs of bottom quarks and tau leptons respectively. These observations confirmed the existence of an interaction, or force, called the Yukawa interaction, which is part of the Standard Model but is unlike all other forces in the Standard Model: it is mediated by the Higgs boson, and its strength is not quantized, that is, it doesn’t come in multiples of a certain unit.

ATLAS and CMS measured the Higgs boson’s mass to be 125 billion electronvolts (GeV), with an impressive precision of almost one per mil. The mass of the Higgs boson is a fundamental constant of nature that is not predicted by the Standard Model. Moreover, together with the mass of the heaviest known elementary particle, the top quark, and other parameters, the Higgs boson’s mass may determine the stability of the universe’s vacuum.

These are just a few of the concrete results of ten years of exploration of the Higgs boson at the world’s largest and most powerful collider – the only place in the world where this unique particle can be produced and studied in detail.

“The large data samples provided by the LHC, the exceptional performance of the ATLAS and CMS detectors, and new analysis techniques have allowed both collaborations to extend the sensitivity of their Higgs-boson measurements beyond what was thought possible when the experiments were designed,” says ATLAS spokesperson Andreas Hoecker.

In addition, since the LHC started colliding protons at record energies in 2010, and thanks to the unprecedented sensitivity and precision of the four main experiments, the LHC collaborations have discovered more than 60 composite particles predicted by the Standard Model, some of which are exotic ‘tetraquarks’ and ‘pentaquarks’. The experiments have also revealed a series of intriguing hints of deviations from the Standard Model that compel further investigation, and have studied the quark–gluon plasma that filled the universe in its early moments in unprecedented detail. They have also observed many rare particle processes, made ever more precise measurements of Standard Model phenomena, and broken new ground in searches for new particles beyond those predicted by the Standard Model, including particles that may make up the dark matter that accounts for most of the mass of the universe.

The results of these searches add important pieces to our understanding of fundamental physics. “Discoveries in particle physics don’t have to mean new particles,” says CERN’s Director for Research and Computing, Joachim Mnich. “The LHC results obtained over a decade of operation of the machine have allowed us to spread a much wider net in our searches, setting strong bounds on possible extensions of the Standard Model, and to come up with new search and data-analysis techniques.”

Remarkably, all of the LHC results obtained so far are based on just 5% of the total amount of data that the collider will deliver in its lifetime. “With this ‘small’ sample, the LHC has allowed big steps forward in our understanding of elementary particles and their interactions,” says CERN theorist Michelangelo Mangano. “And while all the results obtained so far are consistent with the Standard Model, there is still plenty of room for new phenomena beyond what is predicted by this theory.”

“The Higgs boson itself may point to new phenomena, including some that could be responsible for the dark matter in the universe,” says CMS spokesperson Luca Malgeri. “ATLAS and CMS are performing many searches to probe all forms of unexpected processes involving the Higgs boson.”

The journey that still lies ahead

What’s left to be learned about the Higgs field and the Higgs boson ten years on? A lot. Does the Higgs field also give mass to the lighter fermions or could another mechanism be at play? Is the Higgs boson an elementary or composite particle? Can it interact with dark matter and reveal the nature of this mysterious form of matter? What generates the Higgs boson’s mass and self-interaction? Does it have twins or relatives?

Finding the answers to these and other intriguing questions will not only further our understanding of the universe at the smallest scales but may also help unlock some of the biggest mysteries of the universe as a whole, such as how it came to be the way it is and what its ultimate fate might be. The Higgs boson’s self-interaction, in particular, might hold the keys to a better understanding of the imbalance between matter and antimatter and the stability of the vacuum in the universe.

While answers to some of these questions might be provided by data from the imminent third run of the LHC or from the collider’s major upgrade, the high-luminosity LHC, from 2029 onwards, answers to other enigmas are thought to be beyond the reach of the LHC, requiring a future ‘Higgs factory’. For this reason, CERN and its international partners are investigating the technical and financial feasibility of a much larger and more powerful machine, the Future Circular Collider, in response to a recommendation made in the latest update of the European Strategy for Particle Physics.

“High-energy colliders remain the most powerful microscope at our disposal to explore nature at the smallest scales and to discover the fundamental laws that govern the universe,” says Gian Giudice, head of CERN’s Theory department. “Moreover, these machines also bring tremendous societal benefits.”

Historically, the accelerator, detector and computing technologies associated with high-energy colliders have had a major positive impact on society, with inventions such as the World Wide Web, the detector developments that led to the PET (Positron Emission Tomography) scanner, and the design of accelerators for hadron therapy in the treatment of cancers. Furthermore, the design, construction and operation of particle physics colliders and experiments have resulted in the training of new generations of scientists and professionals in other fields, and in a unique model of international collaboration.

Further information

Video news release : https://videos.cern.ch/record/2296228

Pictures of the 4 July 2022 event will be added here.

Higgs boson background information can be found here.

mailys Fri, 07/01/2022 - 13:29 Publication Date Mon, 07/04/2022 - 08:00

Searching for matter–antimatter asymmetry with the Higgs boson

Wed, 22/06/2022 - 17:47
Searching for matter–antimatter asymmetry with the Higgs boson

Symmetries make the world go round, but so do asymmetries. A case in point is an asymmetry known as charge–parity (CP) asymmetry, which is required to explain why matter vastly outnumbers antimatter in the present-day universe even though both forms of matter should have been created in equal amounts in the Big Bang.

The Standard Model of particle physics – the theory that best describes the building blocks of matter and their interactions – includes sources of CP asymmetry, and some of these sources have been confirmed in experiments. However, these Standard Model sources collectively generate an amount of CP asymmetry that is far too small to account for the matter–antimatter imbalance in the universe, prompting physicists to look for new sources of CP asymmetry.

In two recent independent investigations, the international ATLAS and CMS collaborations at the Large Hadron Collider (LHC) turned to the Higgs boson that they discovered ten years ago to see if this unique particle hides a new, unknown source of CP asymmetry.

The ATLAS and CMS teams had previously searched for – and found no signs of – CP asymmetry in the interactions of the Higgs boson with other bosons as well as with the heaviest known fundamental particle, the top quark. In their latest studies, ATLAS and CMS searched for this asymmetry in the interaction between the Higgs boson and the tau lepton, a heavier version of the electron.

To search for this asymmetry, ATLAS and CMS first looked for Higgs bosons transforming, or “decaying”, into pairs of tau leptons in proton–proton collision data recorded by the experiments during the second run of the LHC (2015–2018). They then analysed this decay’s motion, or “kinematics”, which depends on an angle, called the mixing angle, that quantifies the amount of CP asymmetry in the interaction between the Higgs boson and the tau lepton.

In the Standard Model, the mixing angle is zero and thus the interaction is CP symmetric, meaning that it remains the same under a transformation that swaps a particle with the mirror image of its antiparticle. In theories that extend the Standard Model, however, the angle may deviate from zero and the interaction may be partially or fully CP asymmetric depending on the angle; an angle of -90 or +90 degrees corresponds to a fully CP-asymmetric interaction, whereas any angle in between, except 0 degrees, corresponds to a partially CP-asymmetric interaction.

After analysing their samples of Higgs boson decays into tau leptons, the ATLAS team obtained a mixing angle of 9 ± 16 degrees and the CMS team −1 ± 19 degrees, both of which exclude a fully CP-asymmetric Higgs boson­–tau lepton interaction with a statistical significance of about three standard deviations.

The results are consistent with the Standard Model within the present measurement precision. More data will allow researchers to either confirm this conclusion or spot CP asymmetry in the Higgs boson–tau lepton interaction, which would have a profound impact on our understanding of the history of the universe.

With the third run of the LHC set to start soon, the ATLAS and CMS collaborations won’t need to wait too long before they can feed more data into their analysis kits to find out whether or not the Higgs boson hides a new source of CP asymmetry.

abelchio Wed, 06/22/2022 - 16:47 Byline Ana Lopes Publication Date Thu, 06/23/2022 - 12:00

CMS on the lookout for new physics

Fri, 17/06/2022 - 15:18
CMS on the lookout for new physics

With Run 3 of the LHC just around the corner, the LHC experiments are still publishing new results based on the previous runs’ data. Despite no new discoveries being announced, small deviations from expectations are appearing in a small number of analyses. At the current level these deviations can still be attributed to random fluctuations in data, but they indicate regions that need to be investigated closely once the new stream of collisions arrives. Below are a few examples published recently by the CMS collaboration.

In 2017 CMS recorded a spectacular collision event containing four particle jets in the final state. The invariant mass of all four jets was 8 TeV and the jets could be divided into two pairs with a 1.9 TeV invariant mass each. Such a configuration could be produced if a new particle with an 8 TeV mass was created in the collision of proton beams, and subsequently decayed into a pair of – again, new – particles, with masses of 1.9 TeV. In a new analysis recently published by CMS, a search for such twin pairs of jets with matching invariant masses is performed for data collected up to the end of LHC Run 2. Surprisingly, a second event with similarly striking properties was found, with a 4-jet mass of 8.6 TeV and 2-jet masses of 2.15 TeV. These two events can be clearly seen in the plot below, where the 4-jet events are plotted as a function of the 2-jet and 4-jet mass.

Number of events observed (colour scale), plotted as a function of four-jet mass and the average mass of the two dijets. The two points in the top right correspond to the two interesting events. (Image: CMS)

While nearly all observed events with two pairs of jets are produced by strong interactions between the colliding photons, events with such high invariant masses are extremely unlikely. The probability of seeing two events at these masses without any new phenomena being present is of the order of 1 in 20 000, corresponding to a local significance of 3.9σ. While this may appear to be a very strong signal at first, given that the area of masses that are being analysed is large it is important to also look at global significance, which indicates the probability of observing an excess anywhere in the analysed region. For the two events the global significance is only 1.6σ.

Two other searches for new heavy particles are reporting small excesses in data. In a search for high mass resonances decaying into a pair of W bosons (that then decay into leptons) the highest deviation corresponds to a signal hypothesis with a mass of 650 GeV, with local significance at 3.8σ and global significance of 2.6σ. In a search for heavy particles decaying into a pair of bosons (WW, WZ or other combinations, also including Higgs bosons) that subsequently decay into pairs of jets, the data diverge from expectations in two places. The signal hypothesis is a W’ boson with a mass of 2.1 or 2.9 TeV, decaying into a WZ pair and the highest local significance is 3.6σ, with a global significance of 2.3σ.

Another new result comes from searches looking for extra Higgs boson particles decaying into tau pairs. For a new particle with a 100 GeV mass there is a small excess seen in the data with 3.1σ local and 2.7σ global significance. Interestingly, this coincides with a similar excess seen by CMS in a previous search for low-mass resonances in the two-photon final state. Another excess is visible in the high-mass range, with the largest deviation from the expectation observed for a mass of 1.2 TeV with a local (global) significance of 2.8σ (2.4σ).

The tau pair final state was also used to look for hypothetical new particles called leptoquarks. This is of particular interest since leptoquarks could potentially explain the flavour anomalies that have been observed by the LHCb experiment, so if the anomalies are indeed a manifestation of some new phenomena, this would be a way to independently look at these phenomena from a different angle. No excess has been found by CMS so far, but the analysis is only just beginning to be sensitive to the range of leptoquark parameters that could fit the flavour anomalies, so more data is needed to fully explore the leptoquark hypothesis.

The new LHC data-taking period is set to start in July, at higher energy and with significantly upgraded detectors, promising a fresh stream of data for searches for new phenomena.

Read more in the CERN Courier and CMS publications here, here and here

ptraczyk Fri, 06/17/2022 - 14:18 Byline Piotr Traczyk Publication Date Fri, 06/17/2022 - 14:13