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CMS collaboration explores how AI can be used to search for partner particles to the Higgs boson

Tue, 20/02/2024 - 14:21
CMS collaboration explores how AI can be used to search for partner particles to the Higgs boson Event display showing two collimated bursts of light. (Image: CMS collaboration)

As part of their quest to understand the building blocks of matter, physicists search for evidence of new particles that could confirm the existence of physics beyond the Standard Model (SM). Many of these beyond-SM theories postulate the need for additional partner particles to the Higgs boson. These partners would behave similarly to the SM Higgs boson, for example in terms of their “spin”, but would have a different mass.

To search for Higgs partner particles, scientists at the CMS collaboration look for the signatures of these particles in the data collected by the detector. One such signature is when the particles decay from a heavy Higgs partner (X) particle to two lighter partner particles (φ), which in turn each decay into collimated pairs of photons. Photon signatures are ideal to search for particles with unknown masses as they provide a clean, well-understood signature. However, if the φ is very light, the two photons will significantly overlap with each other and the tools usually applied for the photon identification fall apart.

This is where artificial intelligence (AI) comes in. It is well known that machine learning computer vision techniques can differentiate between many faces, and now such AI methodologies are becoming useful tools in particle physics.

The CMS experiment searched for the X and φ partners of the Higgs boson using the hypothetical process X→φφ, with both φ decaying to collimated photon pairs. To do this, they trained two AI algorithms to distinguish the overlapping pairs of photons from noise, as well as to precisely determine the mass of the particle from which they originated. A wide range of masses was explored. No evidence for such new particles was seen, enabling them to set upper limits on the production rate of this process. The result is the most sensitive search yet performed for such Higgs-like particles in this final state.

How can the scientists test the AI’s effectiveness? It is not as easy as verifying AI facial differentiation, where you can simply check by looking. Thankfully, the SM has well-understood processes, which CMS physicists used to validate and control the AI techniques. For example, the η meson, which also decays to two photons, provided an ideal test bench. Scientists at CMS were able to cleanly identify and reconstruct the η meson when searching for its decay into photons when they applied these AI techniques.

This analysis clearly shows that AI algorithms can be used to cleanly identify two-photon signatures from the noise and to search for new massive particles. These machine learning techniques are continuously improving and will continue to be used in unique analyses of LHC data, extending CMS searches to even more challenging cases.

Read more here



ndinmore Tue, 02/20/2024 - 13:21 Byline CMS collaboration Publication Date Wed, 02/21/2024 - 09:30

Hearing the sound of quark–gluon plasma

Thu, 08/02/2024 - 12:44
Hearing the sound of quark–gluon plasma

Neutron stars in the Universe, ultracold atomic gases in the laboratory, and the quark–gluon plasma created in collisions of atomic nuclei at the Large Hadron Collider (LHC): they may seem totally unrelated but, surprisingly enough, they have something in common. They are all a fluid-like state of matter made up of strongly interacting particles. Insights into the properties and behaviour of any of these almost perfect liquids may be key to understanding nature across scales that are orders of magnitude apart.

In a new paper, the CMS collaboration reports the most precise measurement to date of the speed at which sound travels in the quark–gluon plasma, offering new insights into this extremely hot state of matter.

Sound is a longitudinal wave that travels through a medium, producing compressions and rarefactions of matter in the same direction as its movement. The speed of sound depends on the medium’s properties, such as its density and viscosity. It can therefore be used as a probe of the medium.

At the LHC, the quark–gluon plasma is formed in collisions between heavy ions. In these collisions, for a very small fraction of a second, an enormous amount of energy is deposited in a volume whose maximum size is that of the nucleus of an atom. Quarks and gluons emerging from the collision move freely within this area, providing a fluid-like state of matter whose collective dynamics and macroscopic properties are well described by theory. The speed of sound in this environment can be obtained from the rate at which pressure changes in response to variations in energy density or, alternatively, from the rate at which temperature changes in response to variations in entropy, which is a measure of disorder in a system.

In heavy-ion collisions, the entropy can be inferred from the number of electrically charged particles emitted from the collisions. The temperature, on the other hand, can be deduced from the average transverse momentum (i.e. the momentum transverse to the collision axis) of those particles. Using data from lead–lead collisions at an energy of 5.02 trillion electronvolts per pair of nucleons (protons or neutrons), the CMS collaboration has measured for the first time how the temperature varies with the entropy in central heavy-ion collisions, in which the ions collide head on and overlap almost completely.

From this measurement, they obtained a value for the speed of sound in this medium that is nearly half the speed of light and has a record precision: in units of the speed of light, the squared speed of sound is 0.241, with a statistical uncertainty of 0.002 and a systematic uncertainty of 0.016. Using the mean transverse momentum, they also determined the effective temperature of the quark–gluon plasma to be 219 million electronvolts (MeV), with a systematic uncertainty of 8 MeV.

The results match the theoretical expectation and confirm that the quark–gluon plasma acts as a fluid made of particles that carry enormous amounts of energy.

abelchio Thu, 02/08/2024 - 11:44 Byline CMS collaboration Publication Date Fri, 02/16/2024 - 11:41

The January/February issue of the CERN Courier is out

Mon, 15/01/2024 - 16:33
The January/February issue of the CERN Courier is out

With just under two years of LHC operations remaining before the collider is shut down to make way for its high luminosity upgrade (HL-LHC), 2024 is a big year for teams across CERN and beyond. The focus now is on the validation of key technologies, tests of prototypes and the series production of equipment (p37).

Due to deliver high-brightness beams from 2029, the HL-LHC will bring rich physics opportunities for the four main experiments into the early 2040s.

The experience gained from the HL-LHC will also be key to the success of a future collider at CERN. On that note, a February session of the CERN Council is to assess impressive progress documented in the mid-term review of the Future Circular Collider feasibility study. Meanwhile, in December the US “P5” report expressed strong support for a Higgs factory in Europe or Japan (p7).

December also brought news from the CERN Council that CERN’s cooperation with Russia and Belarus will conclude at the expiry of their respective international collaboration agreements: 30 November and 27 June 2024. This issue looks at how the war has impacted particle physics in Ukraine through the experiences of researchers at institutes in Kharkiv, Kyiv, Odesa and Uzhhorod (p30). The lure of the CERN model (p43), 3D-printed detectors (p9), fair and transparent web search (p45), 40 years of the CERN Accelerator School (p25), and how to lead in collaborations (p50) are other must-reads.

Read the digital edition of this new issue on CDS. 

anschaef Mon, 01/15/2024 - 15:33 Publication Date Mon, 01/15/2024 - 15:25

CLOUD challenges current understanding of aerosol particle formation in polar and marine regions

Fri, 15/12/2023 - 12:03
CLOUD challenges current understanding of aerosol particle formation in polar and marine regions

Atmospheric aerosol particles exert a strong net cooling effect on the climate by making clouds brighter and more extensive, reflecting more sunlight back out to space. However, how aerosol particles form in the atmosphere remains poorly understood, especially in polar and marine regions. Globally, the main vapour driving particle formation is thought to be sulfuric acid, stabilised by ammonia. However, since ammonia is frequently lacking in polar and marine regions, models generally underpredict aerosol particles in these regions.

A new study from the CLOUD collaboration now challenges this view, by showing that iodine oxoacids are acting synergistically with sulfuric acid to greatly enhance the particle formation rates. The new findings, described in a paper published today in the journal Science, build on earlier CLOUD studies that showed that iodine oxoacids rapidly form particles even in the complete absence of sulfuric acid. The results imply that climate models are substantially underestimating the formation rates of aerosol particles in marine and polar regions.

“Our results show that climate models need to include iodine oxoacids along with sulfuric acid and other vapours,” says CLOUD spokesperson Jasper Kirkby. “This is particularly important in polar regions, which are highly sensitive to small changes in aerosol particles and clouds. Here, aerosol particles actually providea warming effect by absorbing infrared radiation otherwise lost to space and re-radiating it back down to thesurface.”

The CLOUD experiment is studying how aerosol particles form and grow from mixtures of vapours at atmospheric conditions in a large chamber. It differs from previous experiments both by maintaining ultra-low contaminants and by its precise control of all experimental parameters at conditions found in the real atmosphere. This includes the use of a CERN particle beam to simulate ions formed by galactic cosmic rays at any altitude in the troposphere.

The new CLOUD results show that iodine oxoacids greatly boost the formation rate of sulfuric acid particles. At iodine oxoacid concentrations that are typical of marine and polar regions – between 0.1 and 5 relative to those of sulfuric acid – the CLOUD measurements show that the formation rate of sulfuric acid particles is increased by 10- to 10 000-fold compared with previous estimates.

The CLOUD team found that this increase is due to two effects: first, iodous acid substitutes for ammonia to stabilise newly formed sulfuric acid particles against evaporation and, second, iodic acid facilitates the formation of charged sulfuric acid clusters. Using quantum chemistry, the collaboration has confirmed thesynergy between iodine oxoacids and sulfuric acid, and calculated particle formation rates that closely agree with the CLOUD measurements.

“Global marine iodine emissions have tripled in the past 70 years due to thinning sea ice and rising ozone concentrations, and this trend is likely to continue,” says Kirkby. “The resultant increase of marine aerosol particles and clouds, suggested by our findings, will have created a positive feedback that accelerates the loss of sea ice in polar regions, while simultaneously introducing a cooling effect at lower latitudes. The next generation of climate models will need to take iodine vapours into account.”

abelchio Fri, 12/15/2023 - 11:03 Publication Date Fri, 12/15/2023 - 10:46

Charm is better than beauty at going with the flow

Thu, 14/12/2023 - 12:22
Charm is better than beauty at going with the flow

When two lead ions collide at the Large Hadron Collider (LHC), they produce an extremely hot and dense state of matter in which quarks and gluons are not confined inside composite particles called hadrons. This fireball ­of particles – known as quark–gluon plasma and believed to have filled the Universe in the first few millionths of a second after the Big Bang– expands and cools down rapidly. The quarks and gluons then transform back into hadrons, which fly out of the collision zone towards particle detectors.

In collisions where the two lead ions do not collide head on, the overlap region between the ions has an elliptic shape that leaves an imprint on the flow of hadrons. Measurements of such elliptic flow provide a powerful way to study quark–gluon plasma. In a recent paper, the ALICE collaboration reported a new measurement of the elliptic flow of hadrons containing heavy quarks, which are particularly powerful probes of the plasma.

Unlike the gluons and light quarks that make up the bulk of the quark–gluon plasma created in heavy-ion collisions, heavy charm and beauty quarks are produced in the initial stages of the collisions, before the plasma forms. They therefore interact with the plasma throughout its entire evolution, from its expansion and cooling to its transformation into hadrons. Multiple interactions with the plasma’s constituents bring heavy quarks into thermal equilibrium with the medium within a time that is inversely proportional to the quark’s mass. Charm quarks are lighter than beauty quarks, so a shorter thermalisation time and a larger degree of thermalisation is expected for charm quarks than beauty quarks.

Once they thermalise with the plasma, charm quarks form D mesons and beauty quarks form B mesons, by combining with the medium’s light quarks (see figure above). Previous measurements have shown that the elliptic flow of such “prompt” D mesons, so named because they are produced right after the collisions, is almost as strong as that of the lightest hadrons, pions. Because the thermalisation time is expected to be longer for beauty quarks than charm quarks, the elliptic flow of B mesons is predicted to be weaker than that of prompt D mesons.

In its recent analysis of non-head-on lead–lead collisions that occurred during Run 2 of the LHC, the ALICE collaboration measured the elliptic flow of B mesons, by measuring the flow of “non-prompt” D mesons that are produced in the decays of B mesons (see figure above). Key to the analysis was the adoption of a machine-learning technique to separate the products of the decay of non-prompt D mesons from those of the prompt ones, as well as to supress the many background particle processes that mimic D meson production and decay.

The new measurement shows that the elliptic flow of the non-prompt D mesons is weaker than that of their prompt counterparts, in agreement with the expectation. The result sheds new light on the thermalisation of beauty quarks in the quark–gluon plasma, and paves the way for new ALICE measurements based on data from Run 3 of the LHC. With 40 times more collisions than the total recorded by ALICE in its previous periods of heavy-ion data taking, the new sample of lead–lead collisions taken in 2023 will allow the flow of charm and beauty particles to be studied in greater detail, shedding further light on their dynamics in the quark–gluon plasma.

abelchio Thu, 12/14/2023 - 11:22 Publication Date Fri, 12/15/2023 - 11:21

Making the leap from the impossible to the possible

Mon, 11/12/2023 - 15:53
Making the leap from the impossible to the possible

There are many open questions about the Standard Model of particle physics (SM), which is currently the best description we have of the world of particle physics. Experimental and theoretical physicists vie with other in a healthy competition to scrutinise the SM and identify parts of it that require further explanation, beyond the model’s well-known shortcomings, such as neutrino masses. Experiments carried out at the LHC and other facilities at CERN can detect specific signatures where data deviates slightly from theoretical predictions. It is crucial to continue to explore whether such potential deviations could either reveal new physics or be explained by the SM.

To distinguish the signal from the background in an experiment, theoretical physicists need to calculate all complex processes with extreme precision. This involves examining fine details, including observable quantities such as the number of events or kinematic details of a specific process that could unveil the footprint of an as-yet-unknown phenomenon. Such calculations improve, for instance, the accuracy of the mass measurements of the W boson and the top quark, as well as the strong coupling constant. The strong force and its coupling are the least well known of all in the SM, yet they govern almost every process at the LHC. In addition, precision calculations help to develop new techniques to describe scattering processes and how to simulate them efficiently.

These calculations were already challenging during the LEP era, but the LHC took them to a new level, leading to an explosion in computational complexity and thus the need for new methods to calculate scattering processes. 

Various aspects of precision calculations have become essential for data analysis at modern experiments: for example, they are needed for the computation of complex scattering amplitudes describing the final state immediately after a collision, such as the production of three particles after the collision of two protons. One prominent example is associated Higgs-boson production, specifically with two top quarks. Due to the many possible production mechanisms and final states, new physics can enter in many different ways. Theoretical physicists must therefore calculate each production mode to a high accuracy.

Computing scattering amplitudes is only one small piece of the wider field of precision calculations. Another is Monte Carlo event generators. These calculations aim to describe all stages of the scattering process, from the few particles produced in the collision to the hundreds of particles observed in the detector. At each stage, the underlying physics is interpreted probabilistically and simulated with Monte Carlo methods, which are essential for simulations that can be adopted by experiments as a robust control over systematic uncertainties in their analyses. One crucial example is the vector-boson fusion, where two quarks scatter and exchange a weak boson that creates a Higgs boson, among other particles. Computing this process with a Monte Carlo generator is a very complex but important task, as new physics can potentially hide in details of the final state.

“A few decades ago, this was not possible. Now, our ability to describe the data with up to 5% accuracy or better showcases the power of first-principle calculations and their ability to precisely reflect the complexity of a hadron collider environment, such as the LHC. I am really looking forward to what the era of the High-Luminosity LHC and future colliders will bring,” says Pier Monni, a theoretical physicist at CERN.

kbernhar Mon, 12/11/2023 - 14:53 Byline Kristiane Bernhard-Novotny Publication Date Mon, 12/11/2023 - 14:52

ALICE bags about twelve billion heavy-ion collisions

Tue, 28/11/2023 - 11:48
ALICE bags about twelve billion heavy-ion collisions

After a five-year pause, on the evening of 26 September, lead ions collided at the Large Hadron Collider (LHC) at an unprecedented high energy of 5.36 TeV per pair of nucleons (protons or neutrons) and a collision rate six times higher than before. The final lead-ion beam of this latest heavy-ion run was dumped early in the morning of 30 October, after a forced magnet ‘quench’, carried out to better understand the amount of deposited energy at which the LHC superconducting magnets lose their superconducting state. This improved understanding of the LHC machine will help to further increase the heavy-ion collision rate in the near future.

For this much-anticipated heavy-ion run, alongside improved beam parameters, the ALICE experiment – the LHC’s heavy-ion specialist – made use of its significantly upgraded detector with continuous readout electronics. This means that each and every collision can now be recorded and is thus available for physics analysis, whereas, in the past, only a fraction of collisions could be selected for recording. This continuous readout was achieved by revamping the experiment’s time projection chamber (TPC) detector and upgrading the readout electronics of all of the detectors. In addition, the new inner tracking system (ITS) detector, which is based on highly granular silicon pixel technology, provides sharp images of the collisions with its 10 m2 of active silicon area and nearly 13 billion pixels within the three-dimensional detector volume.

The resulting dramatic increase in the data rate was facilitated by the deployment of a new computing infrastructure for online data processing. This infrastructure includes a new data processing farm that sends the data produced by the experiment directly to CERN’s Data Centre, located about five kilometres from ALICE, through a dedicated high-speed optical-fibre connection that had to be established to cope with the increased data rate.

During the five-week run, ALICE recorded about 12 billion lead–lead collisions – 40 times more collisions than the total recorded by ALICE in the previous periods of heavy-ion data taking, from 2010 to 2018. The new data processing farm, consisting of 2800 graphics processing units (GPUs) and 50 000 central processing unit (CPU) cores, routinely digested collision data at a rate of up to 770 gigabytes per second. It then compressed the data to about 170 gigabytes per second before shipping it to the Data Centre for storage on disk and later, at a limited speed of 20 gigabytes per second, for storage on tape for long-term preservation.

The fresh data set – which amounts to 47.7 petabytes of disk space and is now being analysed – will advance physicists’ understanding of quark–gluon plasma, a state of matter in which quarks and gluons roam around freely for a very short time before forming the composite particles called hadrons that ALICE detects. The increased number of recorded collisions will allow the ALICE researchers to determine the temperature of the plasma using precise measurements of thermal radiation in the form of photons and pairs of electrons and positrons. It will also allow other properties of the nearly-perfect fluid to be measured with greater precision, especially using hadrons containing heavy charm and beauty quarks.

The number of lead–lead collisions collected by ALICE in 2023, expressed in terms of the cumulative number of collisions (right vertical axis) and a related quantity called integrated luminosity (left vertical axis). (Image: CERN) abelchio Tue, 11/28/2023 - 10:48 Byline ALICE collaboration Publication Date Fri, 12/01/2023 - 18:00

Exotic atomic nucleus sheds light on the world of quarks

Tue, 28/11/2023 - 11:12
Exotic atomic nucleus sheds light on the world of quarks

Experiments at CERN and the Accelerator Laboratory in Jyväskylä, Finland, have revealed that the radius of an exotic nucleus of aluminium, 26mAl,  is much larger than previously thought. The result, described in a paper just published in Physical Review Letters, sheds light on the effects of the weak force on quarks – the elementary particles that make up protons, neutrons and other composite particles.

Among the four known fundamental forces of nature – the electromagnetic force, the strong force, the weak force and gravity – the weak force can, with a certain probability, change the “flavour” of a quark. The Standard Model of particle physics, which describes all particles and their interactions with one another, does not predict the value of this probability, but, for a given quark flavour, does predict the sum of all possible probabilities to be exactly 1. Therefore, the probability sum offers a way to test the Standard Model and search for new physics: if the probability sum is found to be different from 1, it would imply new physics beyond the Standard Model.

Interestingly, the probability sum involving the up quark is presently in apparent tension with the expected unity, although the strength of the tension depends on the underlying theoretical calculations. This sum includes the respective probabilities of the down quark, the strange quark and the bottom quark transforming into the up quark.

The first of these probabilities manifests itself in the beta decay of an atomic nucleus, in which a neutron (made of one up quark and two down quarks) changes into a proton (composed of two up quarks and one down quark) or vice versa. However, due to the complex structure of the atomic nuclei that undergo beta decays, an exact determination of this probability is generally not feasible. Researchers thus turn to a subset of beta decays that are less sensitive to the effects of nuclear structure to determine the probability. Among the several quantities that are needed to characterise such “superallowed” beta decays is the (charge) radius of the decaying nucleus.

This is where the new result for the radius of the 26mAl nucleus, which undergoes a superallowed beta decay, comes in. The result was obtained by measuring the response of the 26mAl nucleus to laser light in experiments conducted at CERN’s ISOLDE facility and the Accelerator Laboratory’s IGISOL facility. The new radius, a weighted average of the ISOLDE and IGISOL datasets, is much larger than predicted, and the upshot is a weakening of the current apparent tension in the probability sum involving the up quark.

“Charge radii of other nuclei that undergo superallowed beta decays have been measured previously at ISOLDE and other facilities, and efforts are under way to determine the radius of 54Co at IGISOL,” explains ISOLDE physicist and lead author of the paper, Peter Plattner. “But 26mAl is a rather unique case as, although it is the most precisely studied of such nuclei, its radius has remained unknown until now, and, as it turns out, it is much larger than assumed in the calculation of the probability of the down quark transforming into the up quark.”

“Searches for new physics beyond the Standard Model, including those based on the probabilities of quarks changing flavour, are often a high-precision game,” says CERN theorist Andreas Juttner. “This result underlines the importance of scrutinising all relevant experimental and theoretical results in every possible way.”

Past and present particle physics experiments worldwide, including the LHCb experiment at the Large Hadron Collider, have contributed, and are continuing to contribute, significantly to our knowledge of the effects of the weak force on quarks through the determination of various probabilities of a quark flavour change. However, nuclear physics experiments on superallowed beta decays currently offer the best way to determine the probability of the down quark transforming into the up quark, and this may well remain the case for the foreseeable future.

ssanchis Tue, 11/28/2023 - 10:12 Publication Date Tue, 11/28/2023 - 10:30

Bikash Sinha (1945 – 2023)

Tue, 21/11/2023 - 10:39
Bikash Sinha (1945 – 2023)

Bikash Sinha, influential Indian scientist and pioneer in quark–gluon plasma and the early Universe, passed away on 11 August at the age of 78. As one of the ALICE experiment’s early visionaries and architects, his impact on heavy-ion physics is unmistakable.

Bikash Sinha was born on 16 June 1945 in Kandi, Murshidabad, in the state of West Bengal, India. After graduating from Presidency College, Kolkata, with a degree in physics in 1964, he went on to obtain the Tripos in Natural Sciences from King’s College, Cambridge, in 1967 and then a PhD in nuclear physics from the University of London in 1970. He returned to India at the invitation of nuclear physicist Raja Ramanna and joined the Bhabha Atomic Research Centre (BARC) in 1976. In the early 1980s, he started working in high-energy physics, particularly in relativistic heavy-ion collisions and the formation of quark–gluon plasma. He was appointed Director of the Variable Energy Cyclotron Centre (VECC) in 1987 and was concurrently Director of the Saha Institute of Nuclear Physics (SINP) from 1992 to June 2009. His numerous awards and honours include the Padma Shri Award in 2001 and the prestigious Padma Bhusan Award (the third-highest civilian award in India) in 2010 for his significant contribution to science and technology. He was also a member of the Scientific Advisory Council to the Prime Minister of India.

As director of two major institutes in Kolkata, his efforts put India on the map of nuclear and particle physics laboratories. He was a strong supporter of India’s engagement with the international scientific community via CERN’s programmes. His charismatic leadership and charming personality allowed him to successfully navigate the scientific bureaucracy surrounding a multi-agency funding model for the nascent ALICE collaboration. From modest beginnings at the CERN SPS in the early 1990s, armed with only a handful of collaborators, students and borrowed equipment, but with a grand vision and unbeatable spirit, he nourished and led the Indian team to become a major pillar of ALICE and a key player in heavy-ion physics.

Bikash was a synthesis of science, culture, philosophy and society. He initiated the creation of a medical cyclotron in Kolkata to diagnose and treat prostate cancer. Inspired by the great Indian poet and Nobel laureate Rabindranath Tagore, he organised a one-of-a-kind international conference, MMAP (Microcosmos, Macrocosmos, Accelerator and Philosophy), in May 2022, combining elementary particles, the Universe, accelerator physics and philosophy through poetry and songs.

He initiated a successful series of international conferences on the physics and astrophysics of quark–gluon plasma (ICPAQGP) that have been held in India since 1988, and he organised and chaired the Quark Matter Conference in India in 2008. His efforts helped India to become a prominent CERN non-Member State, culminating in its accession to Associate Member State in 2016.

While Bikash’s passing leaves an undeniable void, his legacy is a vibrant and thriving team, primed to continue his journey. We will always remember him for his charismatic personality, great kindness, openness and generosity. We honour his memory and, with our deepest condolences, extend our sympathy to his family.

His friends and colleagues in the ALICE collaboration

katebrad Tue, 11/21/2023 - 09:39 Publication Date Tue, 11/21/2023 - 09:16

The CMS collaboration at CERN presents its latest search for new exotic particles

Thu, 09/11/2023 - 16:48
The CMS collaboration at CERN presents its latest search for new exotic particles

The CMS experiment has presented its first search for new physics using data from Run 3 of the Large Hadron Collider. The new study looks at the possibility of “dark photon” production in the decay of Higgs bosons in the detector. Dark photons are exotic long-lived particles: “long-lived” because they have an average lifetime of more than a tenth of a billionth of a second – a very long lifetime in terms of particles produced in the LHC – and “exotic” because they are not part of the Standard Model of particle physics. The Standard Model is the leading theory of the fundamental building blocks of the Universe, but many physics questions remain unanswered, and so searches for phenomena beyond the Standard Model continue. CMS’s new result defines more constrained limits on the parameters of the decay of Higgs bosons to dark photons, further narrowing down the area in which physicists can search for them.

In theory, dark photons would travel a measurable distance in the CMS detector before they decay into “displaced muons”. If scientists were to retrace the tracks of these muons, they would find that they don’t reach all the way to the collision point, because the tracks come from a particle that has already moved some distance away, without any trace.

Run 3 of the LHC began in July 2022 and has a higher instantaneous luminosity than previous LHC runs, meaning there are more collisions happening at any one moment for researchers to analyse. The LHC produces tens of millions of collisions every second, but only a few thousand of them can be stored, as recording every collision would quickly consume all the available data storage. This is why CMS is equipped with a real-time data selection algorithm called the trigger, which decides whether or not a given collision is interesting. Therefore, it is not only a higher volume of data that could help to reveal evidence of the dark photon, but also the way in which the trigger system is fine-tuned to look for specific phenomena.

“We have really improved our ability to trigger on displaced muons,” says Juliette Alimena from the CMS experiment. “This allows us to collect much more events than before with muons that are displaced from the collision point by distances from a few hundred micrometres to several metres. Thanks to these improvements, if dark photons exist, CMS is now much more likely to find them.”

The CMS trigger system has been crucial to this search, and was especially refined between Runs 2 and 3 to search for exotic long-lived particles. As a result, the collaboration has been able to use the LHC more efficiently, obtaining a strong result using just a third of the amount of data as previous searches. To do this, the CMS team refined the trigger system by adding a new algorithm called a non-pointing muon algorithm. This improvement meant that even with just four to five months of data from Run 3 in 2022, more displaced-muon events were recorded than in the much larger 2016–18 Run 2 dataset. The new coverage of the triggers vastly increases the momentum ranges of the muons that are picked up, allowing the team to explore new regions where long-lived particles may be hiding.

The CMS team will continue using the most powerful techniques to analyse all data taken in the remaining years of Run 3 operations, with the aim of further exploring physics beyond the Standard Model.

Find out more:


ndinmore Thu, 11/09/2023 - 15:48 Publication Date Fri, 11/10/2023 - 10:00