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Claude Détraz was born on 20 March 1938 in Albi, in the south of France. He graduated from the École Normale Supérieure and began his career at CNRS in 1962 as a researcher studying atomic nuclei.
Détraz then joined the Institut de Physique Nucléaire d’Orsay, founded by Irène and Fréderic Joliot Curie, which has now been merged with its neighbouring laboratories in Orsay to form the Laboratoire de Physique des 2 Infinis Irène Joliot-Curie (IJCLab).
At CERN’s Proton Synchrotron (PS), in collaboration with Robert Klapisch’s team, he contributed to the discovery of the first evidence of deformation in exotic nuclei at a shell closure. Drawing on these results, he became convinced that the beams at GANIL could also become a unique tool in this field.
Détraz was a great scientist and a true visionary, who played a major role in nuclear and particle physics in France and Europe. As the Director of GANIL (the Grand Accélérateur National d’Ions Lourds in Caen) from 1982 to 1990, he launched several research projects on exotic nuclei. The legacy of these projects is still with us today and will continue into the future. He was one of the main founders of NuPECC (the Nuclear Physics Collaboration Committee) and was its first Chair from 1989 to 1992, cementing its position as the main coordinating committee for nuclear physics in Europe.
In 1991, Claude Détraz became a technical adviser in the office of the French Minister for Research, Hubert Curien, who was later the President of the CERN Council at the time of the LHC's approval in 1994. Through his involvement with decision-making bodies at all levels in France, Détraz made a major contribution to ensuring that the LHC project was approved. For example, he played a key role in Hubert Curien’s appointment as the President of the CERN Council, a position from which he was able to exert a major influence in the final phases of the decision.
As the Director of IN2P3 (Institut National de Physique Nucléaire et des Particules at CNRS) from 1992 to 1998, he helped to give the impetus, first with Robert Aymar and then with Catherine Cesarsky of the CEA, to France’s wholehearted participation in the LHC adventure. His involvement was essential in ensuring that France and its institutes played a leading role in the project.
In 1999, Luciano Maiani, CERN Director-General at that time, appointed him Director of Research, jointly with Roger Cashmore, until 2003. This was a period filled with important events for CERN, including the shutdown of LEP, the excavation of new caverns for the LHC and the start of a project to send neutrinos from CERN to the underground laboratory at Gran Sasso, to which Claude contributed substantially.
Throughout his career, Détraz promoted and supported interaction between scientific disciplines. As a nuclear physicist, he established strong links with particle physics. He was also one of the architects of the emergence of astroparticle physics, a discipline connecting the two infinities.
He received multiple honours both in France (commander of the Order of Merit, SFP Joliot-Curie prize, CNRS silver medal) and abroad (Gay Lussac-Humboldt prize from the Humboldt Foundation and an honoris causa doctorate from JINR Dubna).
I knew Claude Détraz throughout his time at GANIL, IN2P3 and CERN, and even afterwards. As well as being a brilliant scientist and occupying several high-level positions, he was a true “Enlightenment man” whom I appreciated for his commitment, efficiency, foresight and humanity. A man of great culture and finesse, he expressed himself in an elegant, convincing and moving way. His passing is a great loss that greatly saddens me. He was a shining light of our generation.
Michel Spiro, President of the International Union of Pure and Applied Physics, Chair of the CERN and Society Foundation Board, former Director of IN2P3 and former President of the CERN Council
The LHCb collaboration has observed a type of four-quark particle never seen before. The discovery, presented at a recent seminar at CERN and described in a paper posted today on the arXiv preprint server, is likely to be the first of a previously undiscovered class of particles.
The finding will help physicists better understand the complex ways in which quarks bind themselves together into composite particles such as the ubiquitous protons and neutrons that are found inside atomic nuclei.
Quarks typically combine together in groups of twos and threes to form particles called hadrons. For decades, however, theorists have predicted the existence of four-quark and five-quark hadrons, which are sometimes described as tetraquarks and pentaquarks, and in recent years experiments including the LHCb have confirmed the existence of several of these exotic hadrons. These particles made of unusual combinations of quarks are an ideal “laboratory” for studying one of the four known fundamental forces of nature, the strong interaction that binds protons, neutrons and the atomic nuclei that make up matter. Detailed knowledge of the strong interaction is also essential for determining whether new, unexpected processes are a sign of new physics or just standard physics.
“Particles made up of four quarks are already exotic, and the one we have just discovered is the first to be made up of four heavy quarks of the same type, specifically two charm quarks and two charm antiquarks,” says the outgoing spokesperson of the LHCb collaboration, Giovanni Passaleva. “Up until now, the LHCb and other experiments had only observed tetraquarks with two heavy quarks at most and none with more than two quarks of the same type.”
“These exotic heavy particles provide extreme and yet theoretically fairly simple cases with which to test models that can then be used to explain the nature of ordinary matter particles, like protons or neutrons. It is therefore very exciting to see them appear in collisions at the LHC for the first time,” explains the incoming LHCb spokesperson, Chris Parkes.
The LHCb team found the new tetraquark using the particle-hunting technique of looking for an excess of collision events, known as a “bump”, over a smooth background of events. Sifting through the full LHCb datasets from the first and second runs of the Large Hadron Collider, which took place from 2009 to 2013 and from 2015 to 2018 respectively, the researchers detected a bump in the mass distribution of a pair of J/ψ particles, which consist of a charm quark and a charm antiquark. The bump has a statistical significance of more than five standard deviations, the usual threshold for claiming the discovery of a new particle, and it corresponds to a mass at which particles composed of four charm quarks are predicted to exist.
As with previous tetraquark discoveries, it is not completely clear whether the new particle is a “true tetraquark”, that is, a system of four quarks tightly bound together, or a pair of two-quark particles weakly bound in a molecule-like structure. Either way, the new tetraquark will help theorists test models of quantum chromodynamics, the theory of the strong interaction.
Read more on the LHCb website.
There is strong evidence that dark matter exists and permeates the cosmos, yet all searches for the hypothetical particles that may make up this invisible form of matter have drawn a blank so far. In light of these null results, researchers have started to spread a wider net in their searches, exploring as many types of particle as possible, new regions in which the particles may lie hidden and new ways to probe them. The NA64 experiment collaboration has now widened the scope of its searches with a search for axions and axion-like particles – hypothetical particles that could mediate an interaction between dark matter and visible matter or comprise dark matter itself, depending on their exact properties.
The NA64 team targeted an unexplored area for axions and axion-like particles, a gap in the two-dimensional area of possible values of their mass and interaction strength with a pair of photons. This gap doesn’t include the regions where axions and axion-like particles could make up dark matter, but it includes an area where axions could explain the long-puzzling symmetry properties of the strong force, for which axions were originally proposed, as well as an area where axion-like particles could mediate an interaction between dark matter and visible matter.
To explore this gap, the NA64 team used an electron beam of 100 GeV energy from the Super Proton Synchrotron and directed it onto a fixed target. They then searched for axions and axion-like particles that would be produced in interactions between high-energy photons generated by the 100 GeV electrons in the target and virtual photons from the target’s atomic nuclei. The researchers looked for the particles both through their transformation, or “decay”, into a pair or photons in a detector placed right after the target or through the “missing energy” that the particles would carry away if they decayed downstream of the detector.
The NA64 team analysed data that was collected over the course of three years, between 2016 and 2018. Together, these data corresponded to some three hundred billion electrons hitting the target. The NA64 researchers found no sign of axions and axion-like particles in this dataset, but the null result allowed them to set limits on the allowed values of the interaction strength of axions and axion-like particles with two photons for particle masses below 55 MeV.
“We’re very excited to have added NA64 to the list of experiments that are hunting for axions as well as axion-like particles, which are a popular candidate for a mediator of a new force between visible and dark matter”, says NA64 collaboration spokesperson Sergei Gninenko. “Little by little, and together, these experiments are narrowing down the regions of where to look for, and perhaps find, these particles.”
Following almost two years of discussion and deliberation, the CERN Council today announced that it has updated the strategy that will guide the future of particle physics in Europe within the global particle-physics landscape. Presented during the open part of the Council’s meeting, held remotely due to the ongoing COVID-19 pandemic, the recommendations highlight the scientific impact of particle physics, as well as its technological, societal and human capital.
By probing ever-higher energy and thus smaller distance scales, particle physics has made discoveries that have transformed the scientific understanding of the world. Nevertheless, many of the mysteries about the universe, such as the nature of dark matter, and the preponderance of matter over antimatter, are still to be explored. The 2020 update of the European Strategy for Particle Physics proposes a vision for both the near- and the long-term future of the field, which maintains Europe's leading role in addressing the outstanding questions in particle physics and in the innovative technologies being developed within the field.
The highest scientific priorities identified in this update are the study of the Higgs boson - a unique particle that raises scientific profound questions about the fundamental laws of nature - and the exploration of the high-energy frontier. These are two crucial and complementary ways to address the open questions in particle physics.
“The Strategy is above all driven by science and thus presents the scientific priorities for the field,” says Ursula Bassler, President of the CERN Council. “The European Strategy Group (ESG) – a special body set up by the Council – successfully led a strategic reflection to which several hundred European physicists contributed.” The scientific vision outlined in the Strategy should serve as a guideline to CERN and facilitate a coherent science policy across Europe.
The successful completion of the High-Luminosity LHC in the coming decade, for which upgrade work is currently in progress at CERN, should remain the focal point of European particle physics. The strategy emphasises the importance of ramping up research and development (R&D) for advanced accelerator, detector and computing technologies, as a necessary prerequisite for all future projects. Delivering the near and long-term future research programme envisaged in this Strategy update requires both focused and transformational R&D, which also has many potential benefits to society.
The document also highlights the need to pursue an electron-positron collider acting as a “Higgs factory” as the highest-priority facility after the Large Hadron Collider (LHC). The Higgs boson was discovered at CERN in 2012 by scientists working on the LHC, and is expected to be a powerful tool to look for physics beyond the Standard Model. Such a machine would produce copious amounts of Higgs bosons in a very clean environment, would make dramatic progress in mapping the diverse interactions of the Higgs boson with other particles and would form an essential part of a rich research programme, allowing measurements of extremely high precision. Construction of this future collider at CERN could begin within a timescale of less than 10 years after the full exploitation of the High-Luminosity LHC, which is expected to complete operations in 2038.
The exploration of significantly higher energies than the LHC will allow new discoveries to be made and the answers to existing mysteries, such as the nature of dark matter, to potentially be found. In acknowledgement of the fact that the particle physics community is ready to prepare for the next step towards even higher energies and smaller scales, another significant recommendation of the Strategy is that Europe, in collaboration with the worldwide community, should undertake a technical and financial feasibility study for a next-generation hadron collider at the highest achievable energy, with a view to the longer term.
It is further recommended that Europe continue to support neutrino projects in Japan and the US. Cooperation with neighbouring fields is also important, such as astroparticle and nuclear physics, as well as continued collaboration with non-European countries.
“This is a very ambitious strategy, which outlines a bright future for Europe and for CERN with a prudent, step-wise approach. We will continue to invest in strong cooperative programmes between CERN and other research institutes in CERN’s Member States and beyond,” declares CERN Director-General Fabiola Gianotti. “These collaborations are key to sustained scientific and technological progress and bring many societal benefits.”
“The natural next step is to explore the feasibility of the high-priority recommendations, while continuing to pursue a diverse programme of high-impact projects,” explains ESG chair Halina Abramowicz. “Europe should keep the door open to participating in other headline projects that will serve the field as a whole, such as the proposed International Linear Collider project.”
Beyond the immediate scientific return, major research infrastructures such as CERN have broad societal impact, thanks to their technological, economic and human capital. Advances in accelerators, detectors and computing have a significant impact on areas like medical and biomedical technologies, aerospace applications, cultural heritage, artificial intelligence, energy, big data and robotics. Partnerships with large research infrastructures help drive innovation in industry. In terms of human capital, the training of early-career scientists, engineers, technicians and professionals provides a talent pool for industry and other fields of society.
The Strategy also highlights two other essential aspects: the environment and the importance of Open Science. “The environmental impact of particle physics activities should continue to be carefully studied and minimised. A detailed plan for the minimisation of environmental impact and for the saving and reuse of energy should be part of the approval process for any major project,” says the report. The technologies developed in particle physics to minimise the environmental impact of future facilities may also find more general applications in environmental protection.
The update of the European Strategy for Particle Physics announced today got under way in September 2018, when the CERN Council, comprising representatives from CERN’s Member and Associate Member States, established a European Strategy Group (ESG) to coordinate the process. The ESG worked in close consultation with the scientific community. Nearly two hundred submissions were discussed during an Open Symposium in Granada in May 2019 and distilled into the Physics Briefing Book, a scientific summary of the community’s input, prepared by the Physics Preparatory Group. The ESG converged on the final recommendations during a week-long drafting session held in Germany in January 2020. The group’s findings were presented to the CERN Council in March and were scheduled to be announced on 25 May, in Budapest. This was delayed due to the global Covid-19 situation but they have now been made publicly available.
For more information, consult the documents of the Update of the European Strategy for Particle Physics:
Geneva, 19 June 2020. Today, the CERN Council announced that it had unanimously updated the strategy intended to guide the future of particle physics in Europe within the global landscape (the document is available here). The updated recommendations highlight the scientific impact of particle physics and its technological, societal and human capital.
The 2020 update of the European Strategy for Particle Physics proposes a vision for both the near- and the long-term future of the field, which maintains Europe's leading role in particle physics and in the innovative technologies developed within the field.
The highest-priority physics recommendations are the study of the Higgs boson and the exploration of the high-energy frontier: two crucial and complementary ways to address the open questions in particle physics.
“The Strategy is above all driven by science and thus presents the scientific priorities for the field,” said Ursula Bassler, President of the CERN Council. “The European Strategy Group (ESG) – a special body set up by the Council – successfully led a strategic reflection to which several hundred European physicists contributed.” The scientific vision outlined in the Strategy should serve as a guideline to CERN and facilitate a coherent science policy across Europe.
The successful completion of the High-Luminosity LHC in the coming years, for which upgrade work is currently in progress at CERN, should remain the focal point of European particle physics.
The Strategy emphasises the importance of ramping up research and development (R&D) for advanced accelerator, detector and computing technologies as a necessary prerequisite for all future projects. Delivering the near and long-term future research programme envisaged in this Strategy update requires both focused and transformational R&D, which also has many potential benefits to society.
The document also highlights the need to pursue an “electron-positron Higgs factory” as the highest-priority facility after the Large Hadron Collider (LHC). Construction of this future collider at CERN could start within a timescale of less than 10 years after the full exploitation of the High-Luminosity LHC, which is expected to complete operations in 2038. The electron-positron collider would allow the properties of the Higgs boson to be measured with extremely high precision. The Higgs boson was discovered at CERN in 2012 by scientists working on the LHC, and is expected to be a powerful tool in the search for physics beyond the Standard Model.
Another significant recommendation of the Strategy is that Europe, in collaboration with the worldwide community, should undertake a feasibility study for a next-generation hadron collider at the highest achievable energy, in preparation for the longer-term scientific goal of exploring the high-energy frontier, with an electron-positron collider as a possible first stage.
It is further recommended that Europe continue to support neutrino projects in Japan and the US. Cooperation with neighbouring fields is also important, such as astroparticle and nuclear physics, as well as continued collaboration with non-European countries.
“This is a very ambitious strategy, which outlines a bright future for Europe and for CERN with a prudent, step-wise approach. We will continue to invest in strong cooperative programmes between CERN and other research institutes in CERN’s Member States and beyond,” declared CERN Director-General Fabiola Gianotti. “These collaborations are key to sustained scientific and technological progress and bring many societal benefits.”
“The natural next step is to explore the feasibility of the high-priority recommendations, while continuing to pursue a diverse programme of high-impact projects,” explains ESG Chair Halina Abramowicz. “Europe should keep the door open to participating in other headline projects that will serve the field as a whole, such as the proposed International Linear Collider project.”
Beyond the immediate scientific return, major research infrastructures such as CERN have vast societal impact, thanks to their technological, economic and human capital. Advances in accelerators, detectors and computing have a significant impact on areas like medical and biomedical technologies, aerospace applications, cultural heritage, artificial intelligence, energy, big data and robotics. Partnerships with large research infrastructures help drive innovation in industry.
In terms of human capital, the training of early-career scientists, engineers, technicians and professionals from diverse backgrounds is an essential part of high-energy physics programmes, which provide a talent pool for industry and other fields.
The Strategy also highlights two other essential aspects: the environment and the importance of Open Science. “The environmental impact of particle physics activities should continue to be carefully studied and minimised. A detailed plan for the minimisation of environmental impact and for the saving and reuse of energy should be part of the approval process for any major project,” says the report. The technologies developed in particle physics to minimise the environmental impact of future facilities may also find more general applications in environmental protection.
The update of the European Strategy for Particle Physics announced today got under way in September 2018 when the CERN Council, comprising representatives from CERN’s Member and Associate Member States, established a European Strategy Group (ESG) to coordinate the process. The ESG worked in close consultation with the scientific community. Nearly two hundred submissions were discussed during an Open Symposium in Granada in May 2019 and distilled into the Physics Briefing Book, a scientific summary of the community’s input, prepared by the Physics Preparatory Group. The ESG converged on the final recommendations during a week-long drafting session held in Germany in January 2020. The group’s findings were presented to the CERN Council in March and were scheduled to be announced on 25 May, in Budapest. This was delayed due to the global Covid-19 situation but they have now been made publicly available.
This media update is part of a series related to the 2020 Large Hadron Collider Physics conference, which took place from 25 to 30 May 2020. Originally planned to take place in Paris, the conference was held entirely online due to the COVID-19 pandemic.
At the LHCP conference this year, the ATLAS and CMS collaborations presented new results relating to a physics process called vector boson scattering. CMS also reported the first observation of the so-called “massive triboson production. Studying these processes to test the Standard Model is important as it could shed light on new physics. The results were presented online at the virtual LHCP conference, originally due to be held in Paris.
During proton collisions at the LHC, many particles, including the carriers of the electroweak force – photons and W and Z bosons – are produced. These bosons are often referred to simply as vector bosons, in the Standard Model, and one of the processes that leads to their pair production is called vector boson scattering.
Vector boson processes are an excellent probe to seek deviation from theoretical predictions. Two rare processes that are of particular interest as they probe the self-interactions of four vector bosons are diboson production via vector boson scattering and triboson production”. The observation and measurement of these processes are important as they test the electroweak symmetry breaking mechanism, whereby the unified electroweak force separates into electromagnetic and weak forces in the Standard Model, and are complementary to the measurements of Higgs boson production and decay.
In a vector boson scattering process, a vector boson is radiated from a quark in each proton and these vector bosons scatter off one another to produce a diboson final state. Triboson production refers instead to the production of three massive vector bosons.
At the LHCP conference, physicists from the ATLAS and CMS collaborations presented new searches for the production of a pair of Z bosons via electroweak production including the vector boson scattering mechanism. ATLAS observed this process at 5.5 sigma and CMS reported strong evidence. CMS also reported the first observation of a W boson produced in association with a photon through the vector boson scattering process, as well as more precise measurements of the same-sign WW production, and an observation of the vector boson scattering production of a W and a Z boson, complementing earlier ATLAS observations.
Another way to probe four-boson interaction is to study the very rare production of three massive bosons or tribosons. This April, the CMS experiment released a 5.7 sigma result of the triboson phenomenon, establishing it as a firm observation, following the first evidence of this process seen by the ATLAS experiment last year.
Most physics processes of fundamental particles involve two or more individual particles that interact with each other via an intermediary particle that is emitted or absorbed in the process.
“The more bosons produced, the rarer the event. This new observation of tribosons was very difficult because it is a much rarer process than the one that led to the Higgs boson discovery, and very interesting because it may reveal signs of new particles and anomalous interactions,” says Roberto Carlin, CMS spokesperson.
In the triboson and vector boson scattering processes, W and Z can interact with themselves to create more W and Z particles, producing two or three bosons. W and Z being highly unstable particles, they quickly decay into leptons (electrons, muons, taus and their corresponding neutrinos) or quarks. But such processes are extremely rare and the diboson and triboson events that physicists look for are mimicked by background processes, making them even more difficult for physicists to analyse.
“To separate signal from background, physicists have to be ingenious and employ advanced machine learning algorithms. This is a challenging task for such rare processes, and requires meticulous and thorough studies,” says Karl Jakobs, ATLAS spokesperson.
The measurements of vector boson scattering and triboson production presented at LHCP 2020 are consistent with the predictions made by the Standard Model, which remains our best understanding of fundamental particles and their interactions. The above observations also provide physicists with tools to probe quartic self-interaction between massive electroweak bosons. The current measurements place constraints on the strength at which these quartic interactions take place and increased precision from the use of new datasets could open up horizons for new physics at higher energy scales in the LHC and lead to possible discoveries of new particles.
By: Achintya Rao
4 JULY, 2020 · Voir en français
Our third story in the LHC Physics at Ten series takes us on a deeper dive into the Higgs boson
(Image: CERN)“For me, it’s an incredible thing that it happened in my lifetime!”
Peter Higgs was at a loss for words. The CMS and ATLAS collaborations had just announced the discovery of a new, Higgs-boson-like particle at the Large Hadron Collider.
4 July 2012: François Englert (left) listens as Peter Higgs speaks, after ATLAS and CMS announce their discovery (Image: Maximilien Brice/CERN)It had been 48 years since the publication of his paper that first predicted the existence of the particle that bears his name, not long after Robert Brout and François Englert proposed a new mechanism that would give mass to elementary bosons. More than 30 years had elapsed since the LHC was first conceived and around 20 years since the ATLAS and CMS collaborations were formed. After those long years filled with anticipation, it only took the Swedish Academy of Sciences a little over one year to award Englert and Higgs the 2013 Nobel Prize in Physics.
For Peter Higgs, the discovery of the Higgs boson was the end of a remarkable journey. For particle physics, it was the beginning of a new one.
(Image: CERN)
Higgs-like? Higgs-ish? Higgs-y?
“When you find something new, you have to understand exactly what it is that you have found,” remarks Giacinto Piacquadio, one of the conveners of the ATLAS collaboration’s Higgs group
This understanding is built up gradually over time. Back in July 2012, physicists were cautious about calling the new particle a Higgs boson, let alone the Higgs boson predicted by the Standard Model of particle physics. And with good reason: while the simplest theoretical formulations required there to be only one kind of Higgs boson, some extensions of the Standard Model proposed that there could be as many as five kinds of bosons that are involved in the mass-giving mechanism. So for the first few months after the discovery, it was referred to as Higgs-like, shorthand for “a particle that seems to behave like the Higgs boson predicted by the Standard Model but we need more data to be sure”.
The identification of two quantum-mechanical properties of the particle – quantum spin and parity – gave credence to the Standard-Model interpretation. Spin is the intrinsic spatial orientation of quantum particles, and parity refers to whether the properties of the particle remain the same when some of its spatial coordinates are flipped, like comparing the particle with a hypothetical mirror image. In the Standard Model, the Higgs boson has no spin (“0”) and “even” parity. At the time of the discovery, the fact that the Higgs boson transformed into photons meant that – unlike all other elementary bosons we know – its spin could not be 1: photons have a quantum spin of 1 themselves, so a particle transforming into two photons would have a spin of 0 (with the two spins of the photon cancelling out) or 2 (if the two spins add up).
Differences between the positive- and negative-parity theoretical scenarios (solid and dashed lines respectively) for a particle with spin 0. The data do not show evidence for the negative-parity scenario (Image: ATLAS/CERN)In science, you can never know something with 100% certainty, but you can rule out things that are not likely. Because spin-2 particles or parity-odd particles with spin 0 would leave subtly different signatures in the ATLAS and CMS detectors than the spin-0-parity-even particle they were looking for, the scientists were eventually able to rule out these more exotic possibilities by examining many more collision events and finding no evidence to support them. “We had to analyse two-and-a-half-times more data to drop the ‘-like’,” Piacquadio adds. By March 2013, scientists were confident calling the particle a Higgs boson.
The Goldilocks zoneThe Higgs boson was the last missing piece in the Standard Model. Crucially, its mass would determine how it could be observed. At 125 gigaelectronvolts (GeV), it turned out to be just right for studying the particle at the Large Hadron Collider.
We can never directly see a Higgs boson. Like most kinds of particle in nature, it is unstable and – immediately after being produced – transforms into lighter particles through a process known as particle decay. The ATLAS and CMS detectors can therefore see only the remnants of transformations, signatures that a Higgs boson might have been produced in the LHC’s collisions. Further, the downstream remnants of a Higgs transformation hold clues for how the particle was produced in the first place.
The Higgs boson’s mass was not predicted precisely by the Standard Model, but theorists knew that the processes that produced it and the kinds of particles it transformed into would depend on how heavy the boson actually was. They had prepared elaborate plots calculating the various probabilities for a Higgs boson of a given mass to transform into particular pairs of particles. According to these so-called “branching fractions”, a light Higgs boson of around 125 GeV would have the largest variety of transformation candidates that ATLAS and CMS could detect: pairs of W bosons, Z bosons, photons, bottom quarks, tau leptons and many others. The greater the variety of observable particles the Higgs can transform into, the greater the ability of scientists to study the interplay between these particles and the Higgs boson.
The rates at which a Higgs boson could undergo certain transformations (vertical axis) depending on its mass (horizontal axis) (Image: CERN)Although the Higgs field was conceived to explain the masses of the W and Z bosons, scientists realised that it could help account for the masses of the fermions, namely the particles of matter. If, due to its mass, they could only observe the interplay between the Higgs boson on one hand and the W and Z bosons on the other, the puzzle of the fermion masses would remain unsolved. Discovering the particle at a convenient mass was an unexpected kindness from nature. If it were slightly more massive, above 180 GeV or so, the options to study it at the time of its discovery would have been more limited.
The variety of available transformation products means that data from the individual channels can be combined together through sophisticated techniques to build up a greater understanding of the particle. “Doing so is not trivial,” says Giovanni Petrucciani, co-convener of the Higgs analysis group in CMS. “You have to treat the uncertainties similarly across all the individual analyses and interpret the results carefully, once you have applied complicated statistical machinery.” Combining data from the transformation of the Higgs boson to pairs of Z bosons and pairs of photons allowed ATLAS and CMS to discover the Higgs boson in 2012.
Photograph featured in the CERN courier article for issue 2019MarApr. Contains an image of ATLAS Higgs event, accompanied with a piece of event selection code of an CMS analysis reimplemented by theorists in open code CheckMATE. (Image: CERN)
Generation gaps
The LHC started operations at a collision energy of 7 teraelectronvolts (TeV) before ramping up to 8 TeV over the course of its first run (2010–2013). The data collected over this period not only led to the discovery of the Higgs boson but showed the relationship (“coupling”) between the Higgs boson and elementary bosons: it was observed transforming into pairs of Ws, Zs and photons. And, while transformations to gluons are impossible to observe, the scientists could probe this coupling through the Higgs production itself: the most abundant way for a Higgs to be created in proton–proton interactions is for two gluons – one from each proton – to fuse together, accounting for nearly 90% of Higgs bosons produced at the LHC.
A candidate for a Higgs boson transforming into two photons (Image: CMS/CERN)The next challenge was to observe the coupling to fermions, to cement the role of the Higgs field as the origin of mass of all elementary massive particles. These couplings had been probed indirectly: the Standard Model tells us that the gluon-fusion production mechanism and the Higgs transformation to photon pairs require the creation and annihilation of “virtual” top–antitop pairs. However, a direct observation of Higgs couplings to fermions was lacking.
Curiously, both kinds of fermions – quarks, which make up compound particles like protons, and leptons, like the familiar electron – come in three generations of particles, each heavier than the previous. And unlike bosons, whose coupling strengths to the Higgs are proportional to their masses, the Higgs-coupling strengths of fermions is proportional to the square of their masses.
The third generation of fermions – the heaviest – are therefore the most likely particles to manifest in processes involving the Higgs boson. “The connection between the Higgs and the top quark in particular is very exciting to look into,” remarks María Cepeda, Petrucciani’s fellow convener on CMS. Despite their relative abundance in such processes, these particles are challenging to identify. Since quarks cannot exist freely, two bottom quarks (a quark and an antiquark) emerging from a Higgs transformation rapidly combine with other quarks pulled out of the vacuum and form jets of particles. The experimentalists have to then tag jets of particles that carry the signature of a bottom quark, in order to isolate the signal. The top quark on the other hand is heavier than the Higgs and so a Higgs can never be observed transforming into two top quarks. Scientists have to therefore measure its coupling with the Higgs by looking for collision events in which a Higgs boson is produced in association with two top quarks. The second run of the LHC (2015–2018) was at an energy of 13 TeV and the large data volume collected allowed ATLAS and CMS to observe the interplay between the Higgs boson and the bottom quark, the top quark and the tau lepton.
A candidate for a Higgs boson transforming into a b-quark and a b-antiquark (Image: ATLAS/CERN)Couplings to the second generation of fermions are much weaker and neither ATLAS nor CMS have so far observed Higgs transformations into charm quarks, strange quarks or muons. The next run of the LHC (2021 onwards) is expected to provide enough data to begin to shed light on some of these interactions. “The LHC’s instantaneous luminosity – the rate at which it collides protons – has increased dramatically over its first two runs,” notes Piacquadio with excitement. “This means that the number of Higgs bosons produced by the LHC continues to rise, as do the odds that we observe them undergoing rarer transformations.”
But for the second generation of fermions, the LHC’s data volume over its whole operational life may not be enough to breach the 5σ statistical threshold to claim a Higgs transformation to all these particles. Although the High-Luminosity LHC, which will be the collider’s incarnation from 2026, is expected to allow ATLAS and CMS to see the Higgs transforming into pairs of muons, transformations to second-generation quarks will probably remain out of reach.
More data, more precisionThe Higgs boson holds the key to our understanding of nature beyond what is shown by the Standard Model.
ATLAS and CMS are, for example, looking for so-called “invisible decays” of the Higgs boson, in which it transforms into particles that the detectors cannot observe. These invisible particles might be manifestations of dark matter. And measurements of couplings that deviate from the theoretical predictions could provide an alternative explanation for the masses of the different generations of fermions, explaining why they exist in distinct generations to start with and possibly hinting at the existence of other Higgs bosons.
Yet, the Brout-Englert-Higgs mechanism remains among the least-understood phenomena in the Standard Model. Indeed, while scientists have dropped the “-like” suffix and have understood the Higgs boson remarkably since its discovery, they still do not know if what was observed is the Higgs boson predicted by the Standard Model. Couplings to the second-generation fermions remain elusive and the couplings that have been observed are known with an uncertainty of 10 to 20%, expected to reduce to the 2–4% range with the High-Luminosity LHC. Observation of as-yet-unseen phenomena and precision measurements of those that have been seen may require data volumes far greater than the LHC can provide over its lifetime.
The global particle-physics community is therefore keen on building a “Higgs factory”, a dedicated accelerator with a focus on producing Higgs bosons in unimaginably large quantities, to allow the continued exploration of this strange particle. A high-energy Higgs factory would also enable scientists to produce two Higgs bosons at a time, to address the question of the so-called “Higgs self-interaction”, the process through which the Higgs boson itself gains mass.
Since its discovery nearly eight years ago, ATLAS and CMS have published hundreds of papers on the Higgs boson and our understanding of the particle has grown incrementally but greatly. Today, we know with great precision what its mass is, what its most abundant transformation channels are and how it is produced in the first place. But a lot remains unknown, about both the Higgs boson and the quantum world in general.
The Higgs may be the most important discovery of the LHC so far, but there is much still to learn from this remarkable machine. Our next story in this series will take a look at searches for dark matter at the Large Hadron Collider.
Our third story in the LHC Physics at Ten series takes us on a deeper dive into the Higgs bosonWelcome to the largest accelerator complex in the world, which propels tiny pieces of matter to speeds of up to 99% of the speed of light. Four sophisticated machines form a chain around 15 kilometres long, in which protons, one of the tiny particles that make up matter, travel more than 1.7 million kilometres before being injected into the Large Hadron Collider (LHC). The LHC accelerates them further before colliding them about 40 million times per second inside four large detectors.
CERN’s accelerator complex has been shut down since December 2018 to allow major upgrades to take place. After several months of work, the accelerators will gradually be recommissioned. But before their doors are closed, CERN invites you to discover them during an event on Facebook and YouTube, which will be streamed live from the brand new Linac 4 linear accelerator. You will go on the same journey as the particles, accompanied by CERN scientists who will reveal the secrets of these high-performance machines.
On 8 June, CERN will celebrate the third birthday of the CERN Alumni Network. This network of former CERN personnel now has more than 6200 members who share information, help each other out and act as ambassadors for the Laboratory’s values of excellence and cooperation. This birthday celebration will bring together recruiters and CERN-grown talent.
Live from the accelerators on Facebook and YouTube
Join us on 8 June
4.00 p.m. CET: follow the journey of CERN’s alumni
5.00 p.m. CET: follow the journey of CERN’s particles
This media update is part of a series related to the 2020 Large Hadron Collider Physics conference, which took place from 25 to 30 May 2020. Originally planned to take place in Paris, the conference was held entirely online due to the COVID-19 pandemic.
The ATLAS and CMS collaborations presented their latest results on new signatures for detecting the Higgs boson at CERN’s Large Hadron Collider. These include searches for rare transformations of the Higgs boson into a Z boson – which is a carrier of one of the fundamental forces of nature – and a second particle. Observing and studying transformations that are predicted to be rare helps advance our understanding of particle physics and could also point the way to new physics if observations differ from the predictions. The results also included searches for signs of Higgs transformations into “invisible” particles, which could shine light on potential dark-matter particles. The analyses involved nearly 140 inverse femtobarns of data, or around 10 million billion proton–proton collisions, recorded between 2015 and 2018.
The ATLAS and CMS detectors can never see a Higgs boson directly: an ephemeral particle, it transforms (or “decays”) into lighter particles almost immediately after being produced in proton–proton collisions, and the lighter particles leave telltale signatures in the detectors. However, similar signatures may be produced by other Standard-Model processes. Scientists must therefore first identify the individual pieces that match this signature and then build up enough statistical evidence to confirm that the collisions had indeed produced Higgs bosons.
When it was discovered in 2012, the Higgs boson was observed mainly in transformations into pairs of Z bosons and pairs of photons. These so-called “decay channels” have relatively clean signatures making them more easily detectable, and they have been observed at the LHC. Other transformations are predicted to occur only very rarely, or to have a less clear signature, and are therefore challenging to spot.
At LHCP, ATLAS presented the latest results of their searches for one such rare process, in which a Higgs boson transforms into a Z boson and a photon (γ). The Z thus produced, itself being unstable, transforms into pairs of leptons, either electrons or muons, leaving a signature of two leptons and a photon in the detector. Given the low probability of observing a Higgs transformation to Zγ with the data volume analysed, ATLAS was able to rule out the possibility that more than 0.55% of Higgs bosons produced in the LHC would transform into Zγ. “With this analysis,” says Karl Jakobs, spokesperson of the ATLAS collaboration, “we can show that our experimental sensitivity for this signature has now reached close to the Standard Model’s prediction.” The extracted best value for the H→Zγ signal strength, defined as the ratio of the observed to the predicted Standard-Model signal yield, is found to be 2.0+1.0−0.9.
CMS presented the results of the first search for Higgs transformations also involving a Z boson but accompanied by a ρ (rho) or φ (phi) meson. The Z boson once again transforms into pairs of leptons, while the second particle transforms into pairs of pions (ππ) in the case of the ρ and into pairs of kaons (KK) in the case of the φ. “These transformations are extremely rare,” says Roberto Carlin, spokesperson of the CMS collaboration, “and are not expected to be observed at the LHC unless physics from beyond the Standard Model is involved.” The data analysed allowed CMS to rule out that more than approximately 1.9% of Higgs bosons could transform into Zρ and more than 0.6% could transform into Zφ. While these limits are much greater than the predictions from the Standard Model, they demonstrate the ability of the detectors to make inroads in the search for physics beyond the Standard Model.
The so-called “dark sector” includes hypothetical particles that could make up dark matter, the mysterious element that accounts for more than five times the mass of ordinary matter in the universe. Scientists believe that the Higgs boson could hold clues as to the nature of dark-matter particles, as some extensions of the Standard Model propose that a Higgs boson could transform into dark-matter particles. These particles would not interact with the ATLAS and CMS detectors, meaning they remain “invisible” to them. This would allow them to escape direct detection and manifest as “missing energy” in the collision event. At LHCP, ATLAS presented their latest upper limit – of 13% – on the probability that a Higgs boson could transform into invisible particles known as weakly interacting massive particles, or WIMPs, while CMS presented results from a new search into Higgs transformations to four leptons via at least one intermediate “dark photon”, also presenting limits on the probability of such a transformation occurring at the LHC.
The Higgs boson continues to prove invaluable in helping scientists test the Standard Model of particle physics and seek physics that may lie beyond. These are only some of the many results concerning the Higgs boson that were presented at LHCP. You can read more about them on the ATLAS and CMS websites.
Technical noteWhen data volumes are not high enough to claim a definite observation of a particular process, physicists can predict the limits that they expect to place on the process. In the case of Higgs transformations, these limits are based on the product of two terms: the rate at which a Higgs boson is produced in proton–proton collisions (production cross-section) and the rate at which it will undergo a particular transformation to lighter particles (branching fraction).
ATLAS expected to place an upper limit of 1.7 times the Standard Model expectation for the process involving Higgs transformations to a Z boson and a photon (H→Zγ) if such a transformation were not present; the collaboration was able to place an upper limit of 3.6 times this value, approaching the sensitivity to the Standard Model’s predictions. The CMS searches were for a much rarer process, predicted by the Standard Model to occur only once in every million Higgs transformations, and the collaboration was able to set upper limits of about 1000 times the Standard Model expectations for the H→Zρ and H→Zφ processes.
Links to the papers and notesDive into the interior of neutron stars and you’ll find, guess what, neutrons. But it’s not as simple as that. The deeper the dive, the fuzzier and denser the interior gets. There’s no shortage of theories as to what might make up the centre of these cosmic objects. One hypothesis is that it’s filled with free quarks, not confined inside neutrons. Another is that it’s made of hyperons, particles that contain at least one quark of the “strange” type. Another still is that it consists of an exotic state of matter called a kaon condensate.
In a paper just published in the journal Nature Physics, a quintet of researchers including Aleksi Kurkela from CERN’s Theory department provides evidence that massive neutron stars can contain cores filled with free quarks. Such quark matter resembles the dense state of free quarks and gluons that is thought to have existed shortly after the Big Bang and can be recreated at particle colliders on Earth, such as the Large Hadron Collider.
To reach this evidence, the researchers combined information from astronomical observations of neutron stars with theoretical calculations. While astronomical observations provide some information about the stars’ interior, they don’t reveal their exact make-up.
The theoretical calculations involved describing the state of matter inside a neutron star from the crust all the way down to the centre. To do this, the researchers used so-called equations of state, which relate the pressure of a state of matter to the energy density – the amount of energy packed into a system or region of space per unit volume.
The team then plugged two pieces of information from astronomical data into these calculations: the observation that neutron stars can have masses equivalent to two Suns; and the possible values of a property called tidal deformability for a neutron star with a mass of about 1.4 times that of the Sun. The tidal deformability describes the stiffness of a star in response to stresses caused by the gravitational pull of a companion star, and was previously derived from observations of gravitational waves (ripples in the fabric of spacetime) emitted by the merger of two neutron stars.
From this combination of theory and data, the researchers find that the cores of neutron stars with a mass 1.4 times that of the Sun should be filled with neutrons. By contrast, more massive stars can contain large quark-matter cores. For example, a 2-solar-mass neutron star with a radius of about 12 km could have a quark-matter core with a radius of about 6.5 km – about half of the star’s radius.
“Our analysis does not completely rule out the existence of massive stars with neutron cores but it demonstrates that quark-matter cores are not an exotic alternative,” says Kurkela. “We can’t wait to incorporate new neutron-star data into our analysis and see how they will affect this conclusion.”
Ever since astronauts attached the 7.5 tonne AMS detector to the International Space Station in May 2011, the space-based magnetic spectrometer, which was assembled at CERN, has collected data on more than 150 billion cosmic rays – charged particles that travel through space with energies up to trillions of electron volts. It’s an impressive amount of data, which has provided a wealth of information about these cosmic particles, but remarkably, as the spokesperson of the AMS team Sam Ting has previously noted, none of the AMS results were predicted. In a paper just published in Physical Review Letters, the AMS team reports measurements of heavy primary cosmic rays that, again, are unexpected.
Primary cosmic rays are produced in supernovae explosions in our galaxy, the Milky Way, and beyond. The most common are nuclei of hydrogen, that is, protons, but they can also take other forms, such as heavier nuclei and electrons or their antimatter counterparts. AMS and other experiments have previously measured the number, or more precisely the so-called flux, of several of these types of cosmic rays and how the flux varies with particle energy and rigidity – a measure of a charged particle’s momentum in a magnetic field. But until now there have been no measurements of how the fluxes of the heavy nuclei of neon, magnesium and silicon change with rigidity. Such measurements would help shed new light on the exact nature of primary cosmic rays and how they journey through space.
In its latest paper, the AMS team describes flux measurements of these three cosmic nuclei in the rigidity range from 2.15 GV to 3.0 TV. These measurements are based on 1.8 million neon nuclei, 2.2 million magnesium nuclei and 1.6 million silicon nuclei, collected by AMS during its first 7 years of operation (19 May 2011 to 26 May 2018). The neon, magnesium and silicon fluxes display unexpectedly identical rigidity dependence above 86.5 GV, including an also unexpected deviation above 200 GV from the single-power-law dependence predicted by the conventional theory of cosmic-ray origin and propagation. What’s more, the observed rigidity dependence is surprisingly different from that of the lighter primary helium, carbon and oxygen cosmic rays, which has been previously measured by AMS.
The cosmic-ray plot continues to thicken. The AMS researchers have seen deviations from expected cosmic-ray behaviour before, including a rigidity dependence of the primary helium, carbon and oxygen cosmic rays that is distinctly different from that of the secondary lithium, beryllium and boron cosmic rays; secondary cosmic rays are produced by interactions between the primary cosmic rays and the interstellar medium.
“Historically, cosmic rays are classified into two distinct classes – primaries and secondaries. Our new data on heavy primary cosmic rays show that primary cosmic rays have at least two distinct classes.” says Ting. “This is totally unexpected based on our previous knowledge of cosmic rays.”
The new and surprising data is likely to keep theorists busy rethinking and reworking current cosmic-ray models. “Our previous observations have already generated new developments in cosmic-ray models. The new observations will provide additional challenges for the new models,” says Ting. And if the data that the detector is currently taking and sending back to CERN for analysis – after a successful series of spacewalks that has extended its lifetime – throws up more surprises, theorists are likely to become even busier.
Watch the video below and relive the drama of the complex spacewalks that have extended the remaining lifetime of the AMS detector to match that of the International Space Station itself.
Video: CERNRead more about the spacewalks and the previous AMS results in this CERN Courier article.
This media update is part of a series related to the 2020 Large Hadron Collider Physics conference, taking place 25–30 May 2020. Originally planned to be held in Paris, the conference is now taking place entirely online due to the COVID-19 pandemic.
The ALICE, CMS and LHCb collaborations at CERN present new measurements that show how charmed particles – particles containing charm quarks – can serve as “messengers” of two forms of matter made up of quarks and gluons: hadrons, which make up most of the visible matter in the present-day universe; and the quark–gluon plasma, which is thought to have existed in the early universe and can be recreated in heavy-ion collisions at the Large Hadron Collider (LHC). By studying charmed particles, physicists can learn more about hadrons, in which quarks are bound by gluons, as well as the quark–gluon plasma, in which quarks and gluons are not confined within hadrons.
The main results are:
Looking forward, the LHC collaborations aim to make more precise measurements of these messengers of the quark world using data from the next LHC run, which will benefit from largely upgraded experiment set-ups.
Read more below for a comprehensive description of these results.
Charm quark results related to hadronsThe LHCb and CMS collaborations describe results from their studies of a hadron known as χc1(3872). The particle was discovered in 2003 by the Belle experiment in Japan but it has remained unclear whether it is a two-quark hadron, a more exotic hadron such as a tetraquark – a system of four quarks tightly bound together – or a pair of two-quark particles weakly bound in a molecule-like structure.
Pinning down the nature of χc1(3872) could extend physicists’ understanding of how quarks bind into hadrons. The new studies by the CMS and LHCb collaborations shed new light on – but do not yet fully reveal – the nature of this particle.
Using sophisticated analysis techniques and two different datasets, the LHCb team obtained the most precise measurements yet of the particle’s mass and determined for the first time and with a significance of more than five standard deviations the particle’s “width”, a parameter that determines the particle’s lifetime.
Until now researchers had only been able to obtain upper limits on the allowed values of this parameter. The LHCb researchers detected χc1(3872) particles in their datasets using the classic “bump”-hunting technique of searching for an excess (the bump) of collision events over a smooth background. Each dataset led to a measurement of the mass and width, and the results from both datasets agree with each other.
“Our results are not only the most precise yet, they also show that the mass of χc1(3872) is remarkably close to the sum of the masses of the D0 and D*0 charmed mesons,” says LHCb spokesperson Giovanni Passaleva. “This is consistent with χc1(3872) being a pair of two-quark particles loosely bound together, but it does not fully rule out the tetraquark hypothesis or other possibilities.”
Meanwhile, analysing a large dataset recorded over the course of three years, the CMS collaboration observed for the first time the transformation, or “decay”, of the B0s particle into the χc1(3872) and a ϕ meson. This two-quark particle, B0s, is a relative of the B+ meson, in the decay of which the Belle experiment first detected χc1(3872). Like the LHCb team, the CMS team detected χc1(3872) using the bump technique.
“Our result is particularly interesting because we found that the rate at which the B0s decays to the hadron χc1(3872) and the ϕ meson is similar to that of the B0 into χc1(3872) and an anti-K0 meson, whereas it is about twice as low as that for the previously observed B+ decay into χc1(3872) and the K+ meson,” says CMS spokesperson Roberto Carlin. “In these decays, different quarks, other than the bottom quark, play a role,” Carlin explains. “The fact that the decay rates do not follow an obvious pattern may shed light on the nature of χc1(3872).”
Charm quark results related to the quark–gluon plasmaAt the other end of the quark-binding spectrum, the ALICE collaboration measured the so-called elliptic flow of hadrons containing a charm quark, either bound to a light quark (forming a D meson) or to an anticharm (making a J/ψ meson) in heavy-ion collisions. Hadrons containing heavy quarks, charm or bottom, are excellent messengers of the quark–gluon plasma formed in these collisions. They are produced in the initial stages of the collisions, before the emergence of the plasma, and thus interact with the plasma constituents throughout its entire evolution, from its rapid expansion to its cooling and its eventual transformation into hadrons.
When heavy nuclei do not collide head on, the plasma is elongated and its expansion leads to a dominant elliptical modulation of the hadrons’ momentum distribution, or flow. The ALICE team found that, at low momentum, the elliptic flow of D mesons is not as large as that of pions, which contain only light quarks, whereas the elliptic flow of J/ψ mesons is lower than both but distinctly observed.
“This pattern indicates that the heavy charm quarks are dragged by the quark–gluon plasma’s expansion,” says ALICE spokesperson Luciano Musa, “but likely to a lesser extent than light quarks, and that both D and J/ψ mesons at low momentum are in part formed by the binding, or recombination, of flowing quarks.”
An illustration of heavy-ion collisions recorded by ALICE. The colored lines represent the reconstructed trajectories fo charged particles produced from the collision. (Image: CERN)Another measurement performed by the ALICE team – of the flow of electrons originating from decays of B hadrons, containing a bottom quark – indicates that bottom quarks are also sensitive to the elongated shape of the quark–gluon plasma. Upsilon particles, which are made up of a bottom quark and its antiquark, as opposed to a charm and anticharm like the J/ψ, do not exhibit significant flow, likely because of their much larger mass and the small number of bottom quarks available for recombination.
Read more on the CMS and LHCb websites:
Original papers:
This media update is part of a series related to the 2020 Large Hadron Collider Physics conference, taking place from 25 to 30 May 2020. Originally planned to take place in Paris, the conference is being held entirely online due to the COVID-19 pandemic.
The ALICE collaboration has presented new results on the production rates of antideuterons based on data collected at the highest collision energy delivered so far at the Large Hadron Collider. The antideuteron is composed of an antiproton and an antineutron. The new measurements are important because the presence of antideuterons in space is a promising indirect signature of dark matter candidates. The results mark a step forward in the search for dark matter.
Recent astrophysical and cosmological results point towards dark matter being the dominant form of matter in the universe, accounting for approximately 85% of all matter. The nature of dark matter remains a great mystery, and cracking its secrets would open a new door for physics.
Detecting antideuterons in space could be an indirect signature of dark matter, since they could be produced during the annihilation or decay of neutralinos or sneutrinos, which are hypothetical dark matter particles.
Various experiments are on the hunt for antideuterons in the Universe, including the AMS detector on the International Space Station. However, before inferring the existence of dark matter from the detection of these nuclei, scientists must account for both their rates of production by other sources (namely, collisions between cosmic rays and nuclei in the interstellar medium) and the rates of their annihilation caused by encountering matter on their journey. In order to assert that the detected antideuteron is related to the presence of dark matter, the production and annihilation rates must be well understood.
By colliding protons in the LHC, ALICE scientists mimicked antideuteron production through cosmic ray collisions, and could thus measure the production rate associated with this phenomenon. These measurements provide a fundamental basis for modelling antideuteron production processes in space. By comparing the amount of antideuterons detected with that of their matter counterparts (deuterons, which do not annihilate in the detector), they were able to determine, for the first time, the annihilation probability of low-energy antideuterons.
These measurements will contribute to future antideuteron studies in the Earth’s vicinity, and help physicists determine whether they are signatures of the presence of dark matter particles, or if on the contrary they are manifestations of known phenomena.
In the future, these types of studies at ALICE could be extended to heavier antinuclei. “The LHC and the ALICE experiment represent a unique facility to study antimatter nuclei,” says ALICE Spokesperson Luciano Musa. “This research will continue to provide a crucial reference for the interpretation of future astrophysical dark matter searches.”
Further reading:
An international team of experimentalists and theorists working at CERN’s nuclear-physics facility ISOLDE have succeeded in performing the first ever laser-spectroscopy measurements of a short-lived radioactive molecule, radium monofluoride. For physicists studying molecules, laser spectroscopy, in which laser light is shone on molecules to reveal their energy structure, is a staple tool in the toolbox. Until now, however, researchers hadn’t been able to use the technique to study short-lived radioactive molecules, which contain one or more unstable nuclei. Compared to atoms, such molecules offer a superior means to explore fundamental symmetries of nature and to search for new physics phenomena. The results, published today in the journal Nature, represent a pivotal step towards using these molecules for fundamental physics research and beyond.
“Our measurements demonstrate that radium monofluoride molecules can be chilled down to temperatures that would allow researchers to investigate them in extraordinary detail,” says principal investigator Ronald Garcia Ruiz. “Our results pave the way to high-precision studies of short-lived radioactive molecules, which offer a new and unique laboratory for research in fundamental physics and other fields.”
Radium monofluoride molecules are particularly interesting because they contain radium, some isotopes of which have nuclei shaped like a pear, with more mass at one end than the other. These exotic pear shapes amplify processes that break fundamental symmetries of nature and could reveal new physics phenomena beyond the Standard Model.
For example, processes that break time-reversal symmetry – that is, that vary if you swap forwards in time for backwards – would give particles an electric dipole moment. This can be thought of as a shift of the cloud of virtual particles that surround every elementary particle away from the centre of mass. The Standard Model predicts a non-zero but very small electric dipole moment, but theories beyond the Standard Model often predict larger values. Nuclear pear shapes would amplify a putative electric dipole moment and would thus offer a sensitive means to probe new phenomena beyond the Standard Model – one that would be complementary to searches for new physics at high-energy particle colliders such as the Large Hadron Collider.
The current experiment builds on theoretical investigations of the energy structure of radium monofluoride. Based on these investigations it was predicted that the molecule is amenable to laser cooling, whereby lasers are used to cool down atoms or molecules for high-precision studies. “This laser-spectroscopy study of radium monofluoride at ISOLDE provides strong evidence that the molecules can indeed be laser cooled,” says ISOLDE spokesperson Gerda Neyens.
Garcia Ruiz and colleagues used the following method to obtain their results. After producing radioactive radium isotopes by firing protons from the CERN’s Proton Synchrotron Booster on a uranium carbide target, radium monofluoride ions were formed by surrounding the target with carbon tetrafluoride gas. The radium monofluoride ions were then sent through ISOLDE’s Collinear Resonance Ionisation Spectroscopy (CRIS) setup, where the ions were turned into neutral molecules that were subsequently subjected to a laser beam that boosted them to excited energy states at specific laser frequencies. A subset of these excited molecules was then ionised with a second laser beam and deflected onto a particle detector for analysis.
By analysing the measured spectra of ionised excited molecules, the team was able to identify the low-lying energy levels of the molecules and some of the properties that demonstrate that the molecules can be laser cooled for future precision studies.
“Our technique allowed the study of radium monofluoride molecules that have lifetimes as short as a few days and are produced at rates lower than one million molecules per second,” says Garcia Ruiz.
In addition to their potential in exploring fundamental symmetries, molecules made of short-lived isotopes can be highly abundant in space, for example in supernovae remnants or in the gas ejected from mergers of neutron stars.
“We anticipate that the approach can also be employed to perform laser spectroscopy on other molecules, including those composed of isotopes with lifetimes of a few tens of milliseconds,” Garcia Ruiz adds. This will allow future studies of bespoke molecules, designed to amplify symmetry-violating properties.
Video presenting ISOLDE, CERN’s nuclear-physics facility (Video: CERN)As a layman I would now say… I think we have it.
“It” was the Higgs boson, the almost-mythical entity that had put particle physics in the global spotlight, and the man proclaiming to be a mere layman was none other than CERN’s Director-General, Rolf Heuer. Heuer spoke in the Laboratory’s main auditorium on 4 July 2012, moments after the CMS and ATLAS collaborations at the Large Hadron Collider announced the discovery of a new elementary particle, which we now know is a Higgs boson. Applause reverberated in Geneva from as far away as Melbourne, Australia, where delegates of the International Conference on High Energy Physics were connected via video-conference.
So what exactly is so special about this particle?
Read the latest feature in the “LHC Physics at 10” series to find out.
Registration is now open for the Neutrino 2020 online conference. Half-day sessions will take place from Mondays to Thursdays between 22 June and 2 July, and will include both plenary talks and poster sessions. A block schedule is posted on the conference webpage and a detailed agenda will be available soon.
Please note that registration for the online conference is required even if you had previously registered for the in-person event. There is no registration fee to attend. Please register before June 8.
Over the past few weeks, we have received 570 poster abstracts for the online conference. Instructions on poster preparation are also now available for presenters on the conference webpage.
We appreciate that everyone is experiencing the present COVID-19 situation in different ways. We look forward to bringing the community together, despite these challenging times, as a chance to talk about neutrino physics and hopefully make some new connections.
Steve Brice, Marvin Marshak, Sam Zeller
for the Neutrino 2020 Local Organizing Committee
email: nu2020@fnal.gov @fnal.gov>
Sam Zeller
MicroBooNE Co-Spokesperson
Deputy Division Head @fnal.gov>
Neutrino Division
Fermi National Accelerator Laboratory
(630) 840-6879 office @fnal.gov>
Physicists from the ATLAS collaboration at CERN’s Large Hadron Collider searched for particles of dark matter by looking for transformation of the Higgs boson into particles that cannot be directly detected by the ATLAS experiment (“invisible particles”). Presence of such particles in the collision debris would create an energy imbalance with visible particles, which can be measured. The scientists sifted through the full dataset from Run 2 of the LHC (2015–2018), around ten million billion (1016) proton–proton collisions, seeking events in which a Higgs boson was produced via a specific, well-identifiable process (known as vector-boson fusion) and then transformed into undetected particles.
The data show no excess of such characteristic events over the expected background. ATLAS concluded at a 95% confidence level that no more than 13% of Higgs bosons produced in the LHC could transform into invisible particles. These findings place the strongest limits so far on Higgs transformations to such invisible particles.
Dark matter, which makes up around 85% of the mass of the universe, has only been observed indirectly, through gravitational effects. No particles of this substance have been observed in a laboratory. Further, even if they are produced in collisions at the LHC, physicists expect that dark-matter particles would escape interaction with the gigantic detectors located at the collision points (be “invisible” to the detector), resulting in “missing energy” in the collision debris.
However, dark matter has mass, and considering the Higgs boson’s relation to mass, physicists have suggested that dark-matter particles could interact with the Higgs boson: a Higgs boson could transform (or “decay”) into dark-matter particles shortly after being produced in the LHC’s collisions. Collision events in which a Higgs is produced through vector-boson fusion contain additional conical jets of particles directed towards the forward regions of ATLAS, close to the LHC beam pipe. The missing energy resulting from the individual particles would, on the other hand, be aligned towards the vertical plane perpendicular to the beam pipe. Combining these two characteristics gives scientists a unique signature in the quest for dark matter.
Although no excess was observed, the search provided important constraints on low-mass dark matter, which complement direct searches for dark matter performed at other facilities. It was also an important demonstration of the novel techniques that scientists are applying in research at the LHC. The Higgs boson, discovered in 2012, has quickly evolved into an invaluable means of searching for signs of physics beyond the Standard Model of particle physics.
A team of researchers from the ASACUSA collaboration have taken experimental equipment from CERN to the Paul Scherrer Institut (PSI) near Zurich to create a theoretically predicted but never before verified exotic atom and made first measurements of how it absorbs and resonates with light. The results, published today in the journal Nature, mark the first time such spectroscopic measurements have been made on an exotic atom containing a meson, a particle consisting of two fundamental particles called quarks.
Replace an electron in an atom with a heavy, negatively charged particle, and you get a so-called exotic atom. Such atoms usually have very short lifetimes, and they provide excellent tools for studying the properties of the replacement particle and to search for physics phenomena not predicted by the Standard Model.
“Spectroscopic measurements of exotic atoms containing mesons could be used to determine with high precision the mass and other properties of the constituent mesons, as well as to place limits on possible new forces involving mesons,” says ASACUSA co-spokesperson Masaki Hori. “For the meson used in this study, one of the lightest mesons, we might eventually be able to determine its mass with a precision of less than about one part in a hundred million. That would be 100 times more precise than has been achieved so far, and would allow a precise comparison with the Standard Model prediction to be made.”
The new atom verified by the experiment consists of a nucleus from an isotope of helium (helium-4), an electron and a negatively charged pion in a high-lying energy state. Its lifetime is more than a thousand times longer than any other atom containing a pion. To make such atoms, the team took negatively charged pions provided by PSI’s 590 MeV ring cyclotron facility – the world’s most intense source of such pions – and focused them using a magnet into a target containing superfluid helium (superfluids are fluids that flow without any resistance). Both the target and the magnet were made at CERN and brought to PSI for this study.
Next, to confirm that the atoms had indeed been created and to study how they absorb and resonate with light, the researchers fired laser light of various frequencies at the target and searched for instances in which the pions made a quantum jump between different energy levels of their host atoms.
After some trial and error playing with different laser frequencies, the researchers were able to identify a specific jump. This jump was predicted to result in the absorption of the pion by the helium nucleus and the subsequent breaking of the latter into a proton, a neutron and a composite particle made up of a proton and a neutron. The researchers detected these fragments using an array of particle detectors that was also made at CERN and brought to PSI, thereby confirming that the pions had indeed made the jump.
Next on the researchers’ agenda is to improve the precision with which the jump was identified and to search for other jumps, with the view to using them to measure the mass of the pions and test the Standard Model.
Experimental apparatus used to synthesise pionic helium atoms at the Paul Scherrer Institute (Image: Masaki Hori / ASACUSA collaboration)
By: Achintya Rao
4 MAY, 2020
Our second story in the LHC Physics at Ten series visits the LHC’s most important discovery so far
(Image: CERN)As a layman I would now say… I think we have it.
“It” was the Higgs boson, the almost-mythical entity that had put particle physics in the global spotlight, and the man proclaiming to be a mere layman was none other than CERN’s Director-General, Rolf Heuer. Heuer spoke in the Laboratory’s main auditorium on 4 July 2012, moments after the CMS and ATLAS collaborations at the Large Hadron Collider announced the discovery of a new elementary particle, which we now know is a Higgs boson. Applause reverberated in Geneva from as far away as Melbourne, Australia, where delegates of the International Conference on High Energy Physics were connected via video-conference.
4 July 2012: A packed auditorium at CERN listens keenly to the announcement from CMS and ATLAS (Image: Maximilien Brice/CERN)So what exactly is so special about this particle?
“Easy! It is the first and only elementary scalar particle we have observed,” grins Rebeca Gonzalez Suarez, who, as a doctoral student, was involved in the CMS search for the Higgs boson. Easy for a physicist, perhaps…
(Image: CERN)
Elegance and symmetries
At the subatomic scale, the universe is a complex choreography of elementary particles interacting with one another through fundamental forces, which can be explained using a term that physicists of all persuasions turn to: elegance.
“In the 1960s, theoretical physicists were working on an elegant way of describing the fundamental laws of nature in terms of quantum field theory,” says Pier Monni, of CERN’s Theory department. In quantum field theory, both matter particles (fermions such as electrons, or the quarks inside protons) and the force carriers (bosons such as the photon, or the gluons that bind quarks) are manifestations of underlying, fundamental quantum fields. Today we call this elegant description the Standard Model of particle physics.
The Standard Model of particle physics represented in a single equation (Image: CERN)The Standard Model is based on the notion of symmetries in nature, that the physical properties they describe remain unchanged under some transformation, such as a rotation in space. Using this notion, physicists can provide a unified set of equations for both electromagnetism (electricity, magnetism, light) and the weak nuclear force (radioactivity). The force which is thus unified is dubbed the electroweak force.
But these very symmetries presented a glaring problem: “The symmetries explained the electroweak force but in order to keep the symmetries valid, they forbid its force-carrying particles from having mass,” explains Fabio Cerutti, who co-led Higgs groups at ATLAS on two separate occasions. “The photon, which carries electromagnetism, we knew was massless; the W and Z bosons, carriers of the weak force, could not be.” Although the W and Z had not been directly observed at the time, physicists knew that if they were to have no mass, processes such as beta decay would have occurred at infinite rates – a physical impossibility – while other processes would have probabilities greater than one at high energies.
In 1964, two papers – one by Robert Brout and François Englert, the other by Peter Higgs – purported to have a solution: a new mechanism that would break the electroweak symmetry. The Brout-Englert-Higgs mechanism introduced a new quantum field that today we call the Higgs field, whose quantum manifestation is the Higgs boson. Only particles that interact with the Higgs field acquire mass. “It is exactly this mechanism,” Cerutti adds, “that creates all the complexity of the Standard Model.”
Originally conceived to explain the masses of the W and Z bosons only, scientists soon found they could extend the Brout-Englert-Higgs mechanism to account for the mass of all massive elementary particles. “To accommodate the mass of the W and Z bosons, we don’t need the same Higgs field to give mass to any other particles such as electrons or quarks,” remarks Kerstin Tackmann, a co-convener of the Higgs group on ATLAS. “But it is a convenient way to do so!”
The mathematical puzzle had been solved decades ago but whether the maths described physical reality remained to be tested.
Artistic view of the Brout-Englert-Higgs Field (Image: CERN)
Something in nothing
The Higgs field is peculiar in two particular ways.
Imagine an empty region of space, a perfect vacuum, without any matter present in it. Quantum field theory tells us that this hypothetical region is not really empty: particle–antiparticle pairs associated with different quantum fields pop into existence briefly before annihilating, transforming into energy. However, the “expectation value” of these fields in a vacuum is zero, implying that on average we can expect there to be no particles within the perfect vacuum. The Higgs field on the other hand has a really high vacuum expectation value. “This non-zero vacuum expectation value,” Tackmann elaborates, “means that the Higgs field is everywhere.” Its omnipresence is what allows the Higgs field to affect all known massive elementary particles in the entire universe.
When the universe had just come into being and was extremely hot, its energy density was higher than the energy associated with the vacuum expectation value of the Higgs field. As a result, the symmetries of the Standard Model could hold, allowing particles such as the W and Z to be massless. As the universe started to cool down, the energy density dropped, until – fractions of a second after the Big Bang – it fell below that of the Higgs field. This resulted in the symmetries being broken and certain particles gained mass.
The other property of the Higgs field is what makes it impossible to observe directly. Quantum fields, both observed and hypothesised, come in different varieties. Vector fields are like the wind: they have both magnitude and direction. Consequently, vector bosons have an intrinsic angular momentum that physicists call quantum spin. Scalar fields have only magnitude and no direction, like temperature, and scalar bosons have no quantum spin. Before 2012 we had only ever observed vector fields at the quantum level, such as the electromagnetic field.
“You can observe a field by observing a particle interacting with it, like electrons bending in a magnetic field,” Monni explains. “Or you can observe it by producing the quantum particle associated with the field, such as a photon.” But the Higgs field, with its constant non-zero value, cannot be switched on or off like the electromagnetic field. Scientists had only one option to prove it exists: create – and observe – the Higgs boson.
r-z view (vertical plane containing the beam) with labels (Image: CERN)
Bump-hunting at the Large Hadron Collider
Particle collisions at sufficiently high energies are necessary to produce a Higgs boson, but for a long time physicists were hunting in the dark: they did not know what this energy range was.
They had searched for signs of the Higgs boson in particle-collision debris at the Large Electron–Positron collider (LEP), which was the Large Hadron Collider’s direct predecessor, and at Fermilab’s Tevatron in the US. The Large Hadron Collider had the capacity to explore the entire predicted energy range where the Higgs boson could appear, and the two general-purpose particle detectors at the LHC – ATLAS and CMS – were meant to provide a definitive answer on its existence. For some, like Monni, the LHC’s calling was irresistible, leading him to switch careers from aerospace engineering to theoretical physics.
Gonzalez Suarez’s colleagues and friends were in the CMS and ATLAS control rooms when the LHC embarked on its high-energy journey on 30 March 2010. She herself was in her office at CERN’s main site in Geneva. “I was writing my doctoral thesis on one screen and looking at the live stream of the collisions on a second. I wanted to know if the code I had written to identify particles produced in the collisions worked!”
When two protons collide within the LHC, it is their constituent quarks and gluons that interact with one another. These high-energy interactions can, through well-predicted quantum effects, produce a Higgs boson, which would immediately transform – or “decay” – into lighter particles that ATLAS and CMS could observe. The scientists therefore needed to build up enough evidence to suggest that particles that could have appeared from a Higgs production and transformation were indeed the result of such a process.
ATLAS observed the Higgs boson in transformations to two photons by collecting data over time. If the animation does not load, please visit the CDS record (Image: ATLAS/CERN)“When the LHC programme started, popular belief was that we would only see a Higgs boson after several years of of data collection,” recounts Vivek Sharma, who co-led the CMS search when the LHC began operations. Sharma and his colleagues presented a plan to CMS in September 2010 of how to tackle the problem with half that data. It required a thorough understanding of not only one’s own detector hardware, its reach and its limitations, but also a team with a variety of technical expertise. “By the time ATLAS and CMS gave a joint talk to CERN’s Scientific Policy Committee in March 2011,” Sharma continues, “there was a force building up that the Higgs boson could be hunted with even smaller datasets.”
A routine end-of-year seminar by ATLAS and CMS in December 2011 overloaded CERN’s webcast servers, as thousands tuned in to hear the latest updates from the collaborations. Early signs of the Higgs boson were there: both detectors had seen bumps in their data that were starting to look distinct from any statistical fluctuations or noise. But the results lacked the necessary statistical certainty to claim discovery. The world had to wait nearly seven months before Joe Incandela of CMS and Fabiola Gianotti of ATLAS could do so in July 2012. The collaborations had performed better than expected to discover the Higgs boson with just two years of data from the LHC.
In CERN’s auditorium, Peter Higgs wiped away tears of joy, and François Englert paid tribute to his late colleague and collaborator, Robert Brout, who did not live to see proof of the mechanism that bears his name.
Gonzalez Suarez celebrated with mixed emotions. Her post-doctoral research took her away from Higgs research before the discovery, and eventually from CMS, to the ATLAS collaboration. “The discovery of the Higgs boson was a historic event, but we are still only at the beginning in our understanding of this new particle.”
The road from data to discovery was challenging. But what have we learnt about the Higgs boson since then? Find out more in part two of the Higgs saga (coming soon).
Recent years have seen the study of the Higgs boson progress from the discovery age to the measurement age. Among the latest studies of the properties of this unique particle by the ATLAS and CMS collaborations are measurements that shed further light on its interaction with top quarks – which, as the heaviest elementary particle, have the strongest interactions with the Higgs boson. In addition to allowing a determination of the strength of the top-Higgs interaction, the analyses open a new window on charge-parity (CP) violation.
Discovered unexpectedly more than 50 years ago, CP violation reveals a fundamental asymmetry in nature that causes rare differences in the rates of processes involving matter particles and their antimatter counterparts, and is therefore thought to be an essential ingredient to explaining the observed abundance of matter over antimatter in the universe. While the Standard Model of particle physics can explain CP violation, the amount of CP violation observed so far in experiments – recently in the behaviour of charm quarks by the LHCb collaboration – is too small to account for the cosmological matter–antimatter imbalance. Searching for new sources of CP violation is thus of great interest to physicists.
In their recent studies, the CMS and ATLAS teams independently performed a direct test of the properties of the top–Higgs interaction. The studies are based on the full dataset of Run 2 of the LHC, which allowed for more precise measurements and analyses of the collision events where the Higgs boson is produced in association with one or two top quarks before decaying into two photons. The detection of this extremely rare association, which was first observed by the two collaborations in 2018, required the full capacities of the detectors and analysis techniques.
As predicted by the Standard Model, no signs of CP violation were found in the top–Higgs interaction by either experiment. The top–Higgs production rate, a measure of the strength of the interaction between the particles, was also found by both experiments to be in line with previous results and consistent with the Standard Model predictions.
Following these first investigations of CP violation in the top–Higgs interaction, ATLAS and CMS physicists plan to study other Higgs-boson decay channels as part of the decades-long search for the origin of the universe’s missing antimatter.
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