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Fig. 1
Schematic representation of the right-handed Cartesian coordinate system adopted to describe the detector. Credit: The European Physical Journal C (2022).
Fig. 2
Left: schematic representation of the detector longitudinal sampling structure. Right: transverse view of the last active layer. Different colors represent different materials: copper (orange), stainless steel and lead (gray), air (white) and active sensors made of silicon (black)
There are more instructive images in the science paper.
A team of researchers from CERN, Massachusetts Institute of Technology, and Staffordshire University have implemented a new algorithm for reconstructing particles at the Large Hadron Collider.
The Large Hadron Collider (LHC) is the most powerful particle accelerator ever built which sits in a tunnel 100 meters underground at CERN, the European Organization for Nuclear Research, near Geneva in Switzerland. It is the site of long-running experiments which enable physicists worldwide to learn more about the nature of the universe.
The project is part of the Compact Muon Solenoid (CMS) experiment [below] —one of seven installed experiments which uses detectors to analyze the particles produced by collisions in the accelerator.
The subject of a new academic paper published in European Physical Journal C [below], the project has been carried out ahead of the high luminosity upgrade of the Large Hadron Collider.
The High Luminosity Large Hadron Collider (HL-LHC) project aims to crank up the performance of the LHC in order to increase the potential for discoveries after 2029. The HL-LHC will increase the number of proton-proton interactions in an event from 40 to 200.
Professor Raheel Nawaz, Pro Vice-Chancellor for Digital Transformation, at Staffordshire University, has supervised the research. He explained that “limiting the increase of computing resource consumption at large pileups is a necessary step for the success of the HL-LHC physics program and we are advocating the use of modern machine learning techniques to perform particle reconstruction as a possible solution to this problem.”
He added that “this project has been both a joy and a privilege to work on and is likely to dictate the future direction of research on particle reconstruction by using a more advanced AI-based solution.”
Dr. Jan Kieseler from the Experimental Physics Department at CERN added that “this is the first single-shot reconstruction of about 1,000 particles from and in an unprecedentedly challenging environment with 200 simultaneous interactions each proton-proton collision. Showing that this novel approach, combining dedicated graph neural network layers (GravNet) and training methods (Object Condensation), can be extended to such challenging tasks while staying within resource constraints represents an important milestone towards future particle reconstruction.”
Shah Rukh Qasim, leading this project as part of his Ph.D. at CERN and Manchester Metropolitan University, says that “the amount of progress we have made on this project in the last three years is truly remarkable. It was hard to imagine we would reach this milestone when we started.”
Professor Martin Jones, Vice-Chancellor and Chief Executive at Staffordshire University, added that “CERN is one of the world’s most respected centers for scientific research and I congratulate the researchers on this project which is effectively paving the way for even greater discoveries in years to come.”
“Artificial Intelligence is continuously evolving to benefit many different industries and to know that academics at Staffordshire University and elsewhere are contributing to the research behind such advancements is both exciting and significant.”
CERN FASER is designed to study the interactions of high-energy neutrinos and search for new as-yet-undiscovered light and weakly interacting particles. Such particles are dominantly produced along the beam collision axis and may be long-lived particles, travelling hundreds of metres before decaying. The existence of such new particles is predicted by many models beyond the Standard Model that attempt to solve some of the biggest puzzles in physics, such as the nature of dark matter and the origin of neutrino masses.
The SND@LHC experiment consists of an emulsion/tungsten target for neutrinos (yellow) interleaved with electronic tracking devices (grey), followed downstream by a detector (brown) to identify muons and measure the energy of the neutrinos. (Image: Antonio Crupano/SND@LHC)
For decades, researchers at Fermilab have been world leaders — precisely measuring known physics and scouring the data, looking for hints that our current theories are incomplete and that we need to revisit and revise the Standard Model. These hints might even require that it be replaced by a newer, more accurate model. This heritage continued at the recent Rencontres de Moriond conference, held annually in the Italian town of La Thuile, where new results could indicate that discoveries are right around the corner.
One physics measurement, led by physicists at the Fermilab LHC Physics Center, caused quite a buzz in the particle physics community. It studied the simultaneous creation of four high-energy “jets,” which are created when quarks and gluons are knocked out of a collision between two protons. The recent result is hard to reconcile with accepted theory and suggests that undiscovered phenomena could exist.
CMS event in which four jets were produced. The Standard Model predicts that events like these should be very rare, implying that possibly new physical phenomena is at work. Photo: CMS collaboration.
Senior scientist Rob Harris has long been recognized as a world leader in the study of high-energy jets produced at a hadron collider. He and Rutgers University postdoctoral researcher Marc Osherson have been digging through CMS data, looking for these “four jet” events. Osherson is part of a group led by Professor Eva Halkiadakis, a former LPC Fellow.
Because the Standard Model has been so thoroughly explored at lower-energy facilities like the Fermilab Tevatron [below] and the CERN LEP accelerator, many researchers believe that the best path forward is to create the highest-energy collisions current technology can provide, and this is only possible at the CERN Large Hadron Collider.
Furthermore, collisions mediated by the strong nuclear force are the most common, and it is through the collision and creation of quarks and gluons that researchers will be able to study the highest-energy particle production. It is for this reason that researchers are keen to investigate the highest-energy jets possible.
Earlier measurements studied events in which two protons collided and produced two high-energy jets. The idea is that the energy from the proton collisions would create some sort of new state of matter or involve new physics that would turn into quarks and/or gluons. However, the data from these earlier studies agreed quite well with the, suggesting no need for changes to currently accepted theory.
So, researchers considered another scenario, one in which four jets were simultaneously produced. These jets were grouped in pairs, and each pair came from an intermediate particle. Because physicists do not have a well-accepted theory to guide them, they used the placeholder “X” to denote the intermediate particle.
Essentially, they were looking for the process where two protons (p) collided and created two X particles. The X particles each then decayed into two jets (j) (i.e., pp → XX → (jj)(jj)) in what is called “non-resonant” production. Researchers also considered another hypothesis, where there was yet another intermediate particle that they called a Y, and the Y then decayed into the two X particles (i.e., pp→Y→XX→(jj)(jj)), which is called “resonant” production. Non-resonant production is similar to how top quarks were discovered at the Tevatron, and resonant production is similar to how the Higgs boson was discovered at the LHC.
The fascinating thing is that researchers found two events that were consistent with a particle with a mass of 8 TeV decaying into two objects, each with a mass of 2 TeV. Events with these properties are very unlikely in the Standard Model and point to the possibility that new physics exists.
The researchers used specific theoretical models to evaluate the significance of this observation. They found that for resonant production, the observation had a local significance of 3.9σ and a global significance of 1.6σ for an object Y with a mass of 8.6 TeV and a mass of 2.15 TeV for X. A local significance is the significance of specifically what you observed, while a global significance considers all possibilities. A significance above 3.0 is considered to be evidence of the existence of something.
For non-resonant prediction, the Y particle isn’t produced. Instead, this analysis had a global significance of 3.6σ and a local significance of 2.5σ for an X particle with a mass of 0.95 TeV.
These significances are tantalizingly close to the significance of 3σ necessary for claiming evidence of a discovery, but still far away from the significance of 5σ required to claim that something has been observed. So, it’s too early to get very excited.
However, with the imminent restart of the LHC, scientists are delighted at the prospect of collecting additional data to see if this observation is the first signs of new physics or just a statistical fluke. Even more exciting is that the LHC will resume operations at a collision energy of 13.6 TeV, which is higher than the previous operating energy of 13 TeV. This small increase in collision energy will double the production rate of events with the properties similar to those already observed.
Measurements involving quarks are not the only place where tantalizing measurements are observed. Experiments studying leptons have also found exciting hints of new physics. In the spring, the g-2 experiment unveiled a deviation from SM expectation with a significance of 4.2σ, which indicates the possibility of undiscovered particles that interact with electrons and muons.
And the LHCb experiment followed up in the studies of b quark decays and found that the decays into electrons and muons weren’t the same. This disagrees with predictions from the Standard Model, which means this is another possible hint of undiscovered physics.
The next several years are going to be very exciting.
Fermilab’s Tevatron was a landmark particle accelerator; until the startup in 2008 of the The European Southern Observatory [La Observatorio Europeo Austral][Observatoire européen austral][Europäische Südsternwarte](EU)(CL)[CERN]Large Hadron Collider(CH) near Geneva, Switzerland, it was the most powerful particle accelerator in the world, accelerating antiprotons to energies of 500 GeV, and producing proton-proton collisions with energies of up to 1.6 TeV, the first accelerator to reach one “tera-electron-volt” energy. At 3.9 miles (6.3 km), it was the world’s fourth-largest particle accelerator in circumference. One of its most important achievements was the 1995 discovery of the top quark, announced by research teams using the Tevatron’s CDF and DØ detectors. It was shut down in 2011.
In addition to high-energy collider physics, Fermilab hosts a series of fixed-target and neutrino experiments, such as The MicroBooNE (Micro Booster Neutrino Experiment),
Completed neutrino experiments include MINOS (Main Injector Neutrino Oscillation Search), MINOS+, MiniBooNE and SciBooNE (SciBar Booster Neutrino Experiment).
The MiniBooNE detector was a 40-foot (12 m) diameter sphere containing 800 tons of mineral oil lined with 1,520 phototube detectors. An estimated 1 million neutrino events were recorded each year.
SciBooNE sat in the same neutrino beam as MiniBooNE but had fine-grained tracking capabilities. The NOνA experiment uses, and the MINOS experiment used, Fermilab’s NuMI (Neutrinos at the Main Injector) beam, which is an intense beam of neutrinos that travels 455 miles (732 km) through the Earth to the Soudan Mine in Minnesota and the Ash River, Minnesota, site of the NOνA far detector.
The ICARUS neutrino experiment was moved from CERN to Fermilab.
In the public realm, Fermilab is home to a native prairie ecosystem restoration project and hosts many cultural events: public science lectures and symposia, classical and contemporary music concerts, folk dancing and arts galleries. The site is open from dawn to dusk to visitors who present valid photo identification.
Asteroid 11998 Fermilab is named in honor of the laboratory.
Weston, Illinois, was a community next to Batavia voted out of existence by its village board in 1966 to provide a site for Fermilab.
The laboratory was founded in 1969 as the National Accelerator Laboratory; it was renamed in honor of Enrico Fermi in 1974. The laboratory’s first director was Robert Rathbun Wilson, under whom the laboratory opened ahead of time and under budget. Many of the sculptures on the site are of his creation. He is the namesake of the site’s high-rise laboratory building, whose unique shape has become the symbol for Fermilab and which is the center of activity on the campus.
After Wilson stepped down in 1978 to protest the lack of funding for the lab, Leon M. Lederman took on the job. It was under his guidance that the original accelerator was replaced with the Tevatron, an accelerator capable of colliding protons and antiprotons at a combined energy of 1.96 TeV. Lederman stepped down in 1989. The science education center at the site was named in his honor.
The later directors include:
John Peoples, 1989 to 1996
Michael S. Witherell, July 1999 to June 2005
Piermaria Oddone, July 2005 to July 2013
Nigel Lockyer, September 2013 to the present
Fermilab continues to participate in the work at the Large Hadron Collider (LHC); it serves as a Tier 1 site in the Worldwide LHC Computing Grid and hosts 1000 U.S. scientists who work on the CMS project.
Physicist Cristián Peña grew up in Talca, a small town a few hours south of Santiago, Chile. “The Andes run all the way through the country,” he says. “No matter where you look, you always have the mountains.”
At the age of 13, he first aspired to climb them.
Over the years, as his mountaineering skills grew, so did his inventory of tools. Ice axes, crampons and ropes expanded his horizons.
Peña’s current focus is the CMS detector, one of two large, general-purpose detectors at the Large Hadron Collider. Peña and colleagues want to use CMS to search for a class of theoretical particles with long lifetimes.
While working through the problem, they realized that an ideal long-lived particle detector is already installed inside CMS: the CMS muon system. The question was whether they could hack it to do something new.
Courtesy of CMS Collaboration.
Long-lived particles
When scientists designed the CMS detector in the 1990s, they had the most popular and mathematically resilient models of particle physics in mind. As far as they knew, the most interesting particles would live just a fraction of a fraction of a second before transforming into well understood secondary particles, such as photons and electrons. CMS would catch signals from those secondary particles and use them as a trail back to the original.
The prompt-decay assumption worked in the search for Higgs bosons. But scientists are now realizing that this “live fast, die young” model might not apply to every interesting thing that comes out of a collision at the LHC. Peña says he sees this as a sign that it’s time for the experiment to evolve.
“If you’re a little kid and you walk a mile in the forest, it’s all completely new,” he says. “Now we have more experience and want to push new frontiers.”
For CMS scientists, that means finding better ways to look for particles with long lifetimes.
Long-lived particles are not a radical new concept. Neutrons, for example, live for about 14 minutes outside the confines of an atomic nucleus. And protons are so long-lived that scientists aren’t sure whether they decay at all. If undiscovered particles are moving into the detector before becoming visible, they could be hiding in plain sight.
“Previously, we hadn’t really thought to look for long-lived particles,” says Christina Wang, a graduate student at The California Institute of Technology (US) working on the CMS experiment. “Now, we have to find new ways to use the CMS detector to see them.”
A new idea
Peña was thinking about long-lived particles while attending a conference in Aspen, Colorado, in March 2019.
“There were a bunch of whiteboards, and we were throwing around ideas,” he says. “In that type of situation, you go with the vibe. There’s a lot of creativity and you start thinking outside the box.”
Peña and his colleagues visualized what an ideal long-lived particle detector might look like. They would need a detector that was far from the collision point. And they would need shielding to filter out the secondary particles that are the stars of the show in traditional searches.
“When you look at the CMS muon system,” Peña says, “that’s exactly what it is.”
Muons, often called the heavier cousins of electrons, are produced during the high-energy collisions inside the LHC. A muon can travel long distances, which is why CMS and its sister experiment, ATLAS, have massive detectors in their outer layers solely dedicated to capturing and recording muon tracks.
Peña ran a quick simulation to see if the CMS muon system would be sensitive to the firework-like signatures of long-lived particles. “It was quick and dirty,” he says, “but it looked feasible.”
After the conference, Peña returned to his regular activities. A few months later, Caltech rising sophomore Nathan Suri joined Professor Maria Spiropulu’s lab as a summer student, working with Wang. Peña, who was also collaborating with Spiropulu’s research group, assigned Suri the muon detector idea as his summer project.
“I was always encouraged to give ideas to young, talented people and let them run with it,” Peña says.
Suri was excited to take on the challenge. “I was in love with the originality of the project,” he says. “I was eager to sink my teeth into it.”
Testing the concept
Suri started by scanning event displays of simulated long-lived particle decays to look for any shared visual patterns. He then explored the original technical design report for the CMS muon detector system to see just how sensitive it could be to these patterns.
“Looking at the unique detector design and highly sensitive elements, I was able to realize what a powerful tool it was,” he says.
By the end of the summer, Suri’s work had shown that not only was it feasible to use the muon system to detect long-lived particles, but that CMS scientists could use pre-existing LHC data to get a jump start on the search.
“At this point, the floodgates opened,” Suri says.
In fall 2019, Wang took the lead on the project. Suri had shown that the idea was possible; Wang wanted to know if it was realistic.
So far, they had been working with processed data from the muon system, which was not adapted to the kind of search they wanted to do. “All the reconstruction techniques used in the muon system are optimized to detect muons,” Wang says.
Wang, Peña and Caltech Professor Si Xie set-up a Zoom meeting with muon system experts to ask for advice.
“They were really surprised that we wanted to use the muon system to infer long-lived particles,” Wang says. “They were like, ‘It’s not designed to do that.’ They thought it was a weird idea.”
The experts suggested the team should try looking at the raw data instead.
Doing so would require extracting unprocessed information from tapes and then developing new software and simulations that could reinterpret thousands of raw detector hits. The task would be arduous, if not impossible.
After the muon system experts left the call, Wang remembers, “we were still in the Zoom room and like, ‘Do we want to continue this?’”
She says it was not a serious question. Of course they did.
A trigger of their own
In fall 2020, Martin Kwok started a postdoctoral position at Fermilab. “We’re encouraged to talk to as many groups as we can and think about what we want to work on most,” he says.
He met with Fermilab researcher Artur Apresyan, who told him about the collaboration with Caltech to convert the CMS muon system into a long-lived particle detector. “It was immediately attractive,” Kwok says. “It’s not very often that we get to explore new uses for our detector.”
Wang and her colleagues had forged ahead with the idea, extracting, processing, and analyzing raw data recorded by the CMS muon system between 2016 and 2018.
It had worked, but the dataset they had available to study was not ideal.
The LHC generates around a billion collisions every second—much more than scientists can record and process. So scientists use filters called triggers to quickly evaluate and sort fresh collision data.
For every billion collisions, only about 1000 are deemed “interesting” by the triggers and saved for further analysis. Wang and her colleagues had determined the filters closest to what they were looking for were the ones programmed to look for signs of dark matter.
Apresyan pitched to Kwok that he could design a new trigger, one actually meant to look for signs of long-lived particles. They could install it in the CMS muon system before the LHC restarts operation in spring 2022.
With a dedicated trigger, they could increase the number of events deemed “interesting” for long-lived particle searches by up to a factor of 30. “It’s not often that we see a 30-times increase in our ability to capture potential signal events,” Kwok says.
Kwok was up for the challenge. And it was a challenge.
“The price of doing something different—of doing something innovative—is that you have to invent your own tools,” Kwok says.
The CMS collaboration consists of thousands of scientists all using collective research tools that they developed and honed over the last two decades. “It’s a bit like building with Legos,” Kwok says. “All the pieces are there, and depending on how you use and combine them, you can make almost anything.”
But developing this specialized trigger was less like picking the right Legos and more like creating a new Lego piece out of melted plastic.
Kwok dug into the experiment’s archives in search of his raw materials. He found an old piece of software that had been developed by CMS but rarely used. “This left-over tool that faded out of popularity turned out to be very handy,” he says.
Kwok and his collaborators also had to investigate if integrating a new trigger into the muon system was even possible. “There’s only so much bandwidth in the electronics to send information upstream,” Kwok says.
“I’m thankful that our collaboration ancestors designed the CMS muon system with a few unused bits. Otherwise, we would have had to reinvent the whole triggering scheme.”
What started as a feasibility study has now evolved into an international effort, with many more institutions contributing to data analysis and trigger R&D. The US institutions contributing to this research are funded by the Department of Energy (US) and the National Science Foundation (US).
“Because we don’t have dedicated long-lived particle triggers yet, we have a low efficiency,” Wang says. “But we showed that it’s possible—and not only possible, but we are overhauling the CMS trigger system to further improve the sensitivity.”
The LHC is scheduled to continue into the 2030s, with several major accelerator and detector upgrades along the way. Wang says that to keep probing nature at its most fundamental level, scientists must remain at the frontier of detector technology and question every assumption.
“Then new areas to explore will naturally follow,” she says. “Long-lived particles are just one of these new areas. We’re just getting started.”
The ATLAS and CMS experiments at the Large Hadron Collider (LHC) have performed luminosity measurements with spectacular precision. A recent physics briefing from CMS complements earlier ATLAS results and shows that by combining multiple methods, both experiments have reached a precision better than 2%. For physics analyses – such as searches for new particles, rare processes or measurements of the properties of known particles – it is not only important for accelerators to increase luminosity, but also for physicists to understand it with the best possible precision.
Luminosity is one of the fundamental parameters to measure an accelerator’s performance. In the LHC, the circulating beams of protons are not continuous beams but are grouped into packets, or “bunches”, of about 100 billion protons. These bunches collide with oncoming bunches 40 million times per second at the interaction points within particle detectors. But when two such bunches pass through each other, only a few protons from each bunch end up interacting with the protons circulating in the opposite direction. Luminosity is a measure of the number of these interactions. Two main aspects of luminosity are instantaneous luminosity, describing the number of collisions happening in a unit of time (for example every second), and integrated luminosity, measuring the total number of collisions produced over a period of time.
Integrated luminosity is usually expressed in units of “inverse femtobarns” (fb-1). A femtobarn is a unit of cross-section, a measure of the probability for a process to occur in a particle interaction. This is best illustrated with an example: the total cross-section for Higgs boson production in proton–proton collisions at 13 TeV at the LHC is of the order of 6000 fb. This means that every time the LHC delivers 1 fb-1 of integrated luminosity, about 6000 fb x 1 fb-1 = 6000 Higgs bosons are produced.
Knowing the integrated luminosity allows physicists to compare observations with theoretical predictions and simulations. For example, physicists can look for dark matter particles that escape collisions undetected by looking at energies and momenta of all particles produced in a collision. If there is an imbalance, it could be caused by an undetected, potentially dark matter, particle carrying energy away. This is a powerful method of searching for a large class of new phenomena, but it has to take into account many effects, such as neutrinos produced in the collisions. Neutrinos also escape undetected and leave an energy imbalance, so in principle, they are indistinguishable from the new phenomena. To see if something unexpected has been produced, physicists have to look at the numbers.
So if 11000 events show an energy imbalance, and the simulations predict 10000 events containing neutrinos, this could be significant. But if physicists only know luminosity with a precision of 10%, they could have easily had 11000 neutrino events, but there were just 10% more collisions than assumed. Clearly, a precise determination of luminosity is critical.
There are also types of analyses that depend much less on absolute knowledge of the number of collisions. For example, in measurements of ratios of different particle decays, such as the recent LHCb measurement.
Here, uncertainties in luminosity get cancelled out in the ratio calculations. Other searches for new particles look for peaks in mass distribution and so rely more on the shape of the observed distribution and less on the absolute number of events. But these also need to know the luminosity for any kind of interpretation of the results.
Ultimately, the greater the precision of the luminosity measurement, the more physicists can understand their observations and delve into hidden corners beyond our current knowledge.
All research-quality data recorded by CMS during the first two years of LHC operation are now publicly available.
An artistic representation of the CMS detector made of pulsating lines. (Image: Achintya Rao/CERN.)
The CMS collaboration at CERN has released into the open 18 new datasets, comprising proton–proton collision data recorded by CMS at the Large Hadron Collider (LHC) during the second half of 2011. LHC data are unique and are of interest to the scientific community as well as to those in education. Preserving the data and the knowledge to analyse them is critical. CMS has therefore committed to releasing its research data openly, with up to 100% being made available 10 years after recording them; the embargo gives the scientists working on CMS adequate time to analyse the data themselves.
The total data volume of this latest release is 96 terabytes. Not only does this batch complement the data from the first half of 2011, released back in 2016, it also provides additional tools, workflows and examples as well as improved documentation for analysing the data using cloud technologies. The data and related materials are available on the CERN Open Data portal, an open repository built using CERN’s home-grown and open source software, Invenio.
Previous releases from CMS included the full recorded data volume from 2010 and half the volumes from 2011 and 2012 (the first “run” of the LHC). Special “derived datasets”, some for education and others for data science, have allowed people around the world to “rediscover” the Higgs boson in CMS open data. Novel papers have also been published using CMS data, by scientists unaffiliated with the collaboration.
In the past, those interested in analysing CMS open data needed to install the CMS software onto virtual machines to re-create the appropriate analysis environment. This made it challenging to scale up a full analysis for research use, a task that requires considerable computing resources. With this batch, CMS has updated the documentation for using software containers with all the software pre-installed and added workflows running on them, allowing the data to be easily analysed in the cloud, either at universities or using commercial providers. Some of the new workflows are also integrated with REANA, the CERN platform for reusable analyses.
CMS and the CERN Open Data team have been working closely with current and potential users of the open data – in schools, in higher education and in research – to improve the services offered. The search functionality of the portal has been updated with feedback from teachers who participated in dedicated workshops at CERN in previous years, the documentation has been enhanced based on conversations with research users and a new online forum has been established to provide support. In September, CMS is organising a virtual workshop for theoretical physicists interested in using the open data.
“We are thrilled to be able to release these new data and tools from CMS into the public domain,” says Kati Lassila-Perini, who has co-led the CMS project for open data and data preservation since its inception. “We look forward to seeing how the steps we have taken to improve the usability of our public data are received by the community of users, be it in education or in research.”
The ATLAS [below] and CMS [below] experiments at CERN have announced new results which show that the Higgs boson decays into two muons.
Candidate event displays of a Higgs boson decaying into two muons as recorded by CMS (left) and ATLAS (right). (Image: CERN)
Geneva. At the 40th ICHEP conference, the ATLAS and CMS experiments announced new results which show that the Higgs boson decays into two muons. The muon is a heavier copy of the electron, one of the elementary particles that constitute the matter content of the Universe. While electrons are classified as a first-generation particle, muons belong to the second generation. The physics process of the Higgs boson decaying into muons is a rare phenomenon as only about one Higgs boson in 5000 decays into muons. These new results have pivotal importance for fundamental physics because they indicate for the first time that the Higgs boson interacts with second-generation elementary particles.
Physicists at CERN have been studying the Higgs boson since its discovery in 2012 in order to probe the properties of this very special particle. The Higgs boson, produced from proton collisions at the Large Hadron Collider, disintegrates – referred to as decay – almost instantaneously into other particles. One of the main methods of studying the Higgs boson’s properties is by analysing how it decays into the various fundamental particles and the rate of disintegration.
CMS achieved evidence of this decay with 3σ, which means that the chance of seeing the Higgs boson decaying into a muon pair from statistical fluctuation is less than one in 700. ATLAS’s 2σ result means the chances are one in 40 [strange, lower statistical signifance but greater probability, never saw that before] . The combination of both results would increase the significance well above 3σ and provides strong evidence for the Higgs boson decay to two muons.
“CMS is proud to have achieved this sensitivity to the decay of Higgs bosons to muons, and to show the first experimental evidence for this process. The Higgs boson seems to interact also with second-generation particles in agreement with the prediction of the Standard Model, a result that will be further refined with the data we expect to collect in the next run,” said Roberto Carlin, spokesperson for the CMS experiment.
The Higgs boson is the quantum manifestation of the Higgs field, which gives mass to elementary particles it interacts with, via the Brout-Englert-Higgs mechanism. By measuring the rate at which the Higgs boson decays into different particles, physicists can infer the strength of their interaction with the Higgs field: the higher the rate of decay into a given particle, the stronger its interaction with the field. So far, the ATLAS and CMS experiments have observed the Higgs boson decays into different types of bosons such as W and Z, and heavier fermions such as tau leptons. The interaction with the heaviest quarks, the top and bottom, was measured in 2018. Muons are much lighter in comparison and their interaction with the Higgs field is weaker. Interactions between the Higgs boson and muons had, therefore, not previously been seen at the LHC.
Standard Model of Particle Physics, Quantum Diaries
“This evidence of Higgs boson decays to second-generation matter particles complements a highly successful Run 2 Higgs physics programme. The measurements of the Higgs boson’s properties have reached a new stage in precision and rare decay modes can be addressed. These achievements rely on the large LHC dataset, the outstanding efficiency and performance of the ATLAS detector and the use of novel analysis techniques,” said Karl Jakobs, ATLAS spokesperson.
What makes these studies even more challenging is that, at the LHC, for every predicted Higgs boson decaying to two muons, there are thousands of muon pairs produced through other processes that mimic the expected experimental signature. The characteristic signature of the Higgs boson’s decay to muons is a small excess of events that cluster near a muon-pair mass of 125 GeV, which is the mass of the Higgs boson. Isolating the Higgs boson to muon-pair interactions is no easy feat. To do so, both experiments measure the energy, momentum and angles of muon candidates from the Higgs boson’s decay. In addition, the sensitivity of the analyses was improved through methods such as sophisticated background modelling strategies and other advanced techniques such as machine-learning algorithms. CMS combined four separate analyses, each optimised to categorise physics events with possible signals of a specific Higgs boson production mode. ATLAS divided their events into 20 categories that targeted specific Higgs boson production modes.
The results, which are so far consistent with the Standard Model predictions, used the full data set collected from the second run of the LHC. With more data to be recorded from the particle accelerator’s next run and with the High-Luminosity LHC, the ATLAS and CMS collaborations expect to reach the sensitivity (5 sigma) needed to establish the discovery of the Higgs boson decay to two muons and constrain possible theories of physics beyond the Standard Model that would affect this decay mode of the Higgs boson.
An extremely fast new detector inside the CMS detector will allow physicists to get a sharper image of particle collisions.
Some of the best commercially available high-speed cameras can capture thousands of frames every second. They produce startling videos of water balloons popping and hummingbirds flying in ultra-slow motion.
But what if you want to capture an image of a process so fast that it looks blurry if the shutter is open for even a billionth of a second? This is the type of challenge scientists on experiments like CMS and ATLAS face as they study particle collisions at CERN’s Large Hadron Collider.
When the LHC is operating to its full potential, bunches of about 100 billion protons cross each other’s paths every 25 nanoseconds. During each crossing, which lasts about 2 nanoseconds, about 50 protons collide and produce new particles. Figuring out which particle came from which collision can be a daunting task.
“Usually in ATLAS and CMS, we measure the charge, energy and momentum of a particle, and also try to infer where it was produced,” says Karri DiPetrillo, a postdoctoral fellow working on the CMS experiment at the US Department of Energy’s Fermilab. “We’ve had timing measurements before—on the order of nanoseconds, which is sufficient to assign particles to the correct bunch crossing, but not enough to resolve the individual collisions within the same bunch.”
Thanks to a new type of detector DiPetrillo and her collaborators are building for the CMS experiment, this is about to change.
CERN/CMS Detector
Physicists on the CMS experiment are devising a new detector capable of creating a more accurate timestamp for passing particles. The detector will separate the 2-nanosecond bursts of particles into several consecutive snapshots—a feat a bit like taking 30 billion pictures a second.
This will help physicists with a mounting challenge at the LHC: collision pileup.
Picking apart which particle tracks came from which collision is a challenge. A planned upgrade to the intensity of the LHC will increase the number of collisions per bunch crossing by a factor of four—that is from 50 to 200 proton collisions—making that challenge even greater.
Currently, physicists look at where the collisions occurred along the beamline as a way to identify which particular tracks came from which collision. The new timing detector will add another dimension to that.
“These time stamps will enable us to determine when in time different collisions occurred, effectively separating individual bunch crossings into multiple ‘frames,’” says DiPetrillo.
DiPetrillo and fellow US scientists working on the project are supported by DOE’s Office of Science, which is also contributing support for the detector development.
According to DiPetrillo, being able to separate the collisions based on when they occur will have huge downstream impacts on every aspect of the research. “Disentangling different collisions cleans up our understanding of an event so well that we’ll effectively gain three more years of data at the High-Luminosity LHC. This increase in statistics will give us more precise measurements, and more chances to find new particles we’ve never seen before,” she says.
The precise time stamps will also help physicists search for heavy, slow moving particles they might have missed in the past.
“Most particles produced at the LHC travel at close to the speed of light,” DiPetrillo says. “But a very heavy particle would travel slower. If we see a particle arriving much later than expected, our timing detector could flag that for us.”
The new timing detector inside CMS will consist of a 5-meter-long cylindrical barrel made from 160,000 individual scintillating crystals, each approximately the width and length of a matchstick. This crystal barrel will be capped on its open ends with disks containing delicately layered radiation-hard silicon sensors. The barrel, about 2 meters in diameter, will surround the inner detectors that compose CMS’s tracking system closest to the collision point. DiPetrillo and her colleagues are currently working out how the various sensors and electronics at each end of the barrel will coordinate to give a time stamp within 30 to 50 picoseconds.
“Normally when a particle passes through a detector, the energy it deposits is converted into an electrical pulse that rises steeply and the falls slowly over the course of a few nanoseconds,” says Joel Butler, the Fermilab scientist coordinating this project. “To register one of these passing particles in under 50 picoseconds, we need a signal that reaches its peaks even faster.”
Scientists can use the steep rising slopes of these signals to separate the collisions not only in space, but also in time. In the barrel of the detector, a particle passing through the crystals will release a burst of light that will be recorded by specialized electronics. Based on when the intense flash of light arrives at each sensor, physicists will be able to calculate the particle’s exact location and when it passed. Particles will also produce a quick pulse in the endcaps, which are made from a new type of silicon sensor that amplifies the signal. Each silicon sensor is about the size of a domino and can determine the location of a passing particle to within 1.3 millimeters.
The physicists working on the timing detector plan to have all the components ready and installed inside CMS for the start-up of the High Luminosity LHC in 2027
“High-precision timing is a new concept in high-energy physics,” says DiPetrillo. “I think it will be the direction we pursue for future detectors and colliders because of its huge physics potential. For me, it’s an incredibly exciting and novel project to be on right now.”
In its ongoing quest to understand the nature of the universe’s fundamental constituents, the CMS collaboration has reached another milestone.
CERN/CMS Detector
In October 2019, the U.S. contingent of the CMS collaboration presented their plans to upgrade the CMS particle detector for the high-luminosity phase of the Large Hadron Collider at CERN.
CERN LHC Tunnel
The upgrades would enable CMS to handle the challenging environment brought on by the upcoming increase in the LHC’s particle collision rate, fully exploiting the discovery potential of the upgraded machine.
In response, on Dec. 19, 2019, the Department of Energy Office of Science gave the plan its stamp of CD-1 approval, signaling that it favorably evaluated the project’s conceptual design, schedule range and cost, among other factors.
“This is a major achievement because it paves the way for the next major steps in our project, in which funds are allocated to start the production phase,” said scientist Anadi Canepa, head of the Fermilab CMS Department. “The U.S. project team was extremely satisfied. Preparing for CD-1 was a monumental effort.”
The CMS detector upgrade team met in October 2019 for a DOE review. Photo: Reidar Hahn, Fermilab
The LHC’s increase in beam intensity is planned for 2027, when it will become the High-Luminosity LHC. Racing around its 17-mile circumference, the upgraded collider’s proton beams will smash together to reveal even more about the nature of the subatomic realm thanks to a 10-fold increase in collision rate compared to the LHC’s design value.
The cranked up intensity means that the High-Luminosity LHC will deliver an unprecedented amount of data, and the giant detectors that sit in the path of the beam have to be able to withstand the higher data delivery rate and radiation dose. In preparation, USCMS will upgrade the CMS detector to keep up with the increase in data output, not to mention to harsher collision environment.
The collaboration plans to upgrade the detector with state-of-the-art technology. The new detector will exhibit improved sensitivity, with over 2 billion sensor channels — up from 80 million. USCMS is also replacing the central part of the detector so that, when charged particles fly through it, the upgraded device will take readings of their momenta at an astounding 40 million times per second, a first for hadron colliders. They’re implementing an innovative design for the detector, measuring the energy of particles using very precise silicon sensors. The upgraded CMS will also have a breakthrough component to take higher-resolution, more precisely timed images of complex particle interactions. Scientists are introducing a system using machine learning on electronic circuits called FPGAs to more efficiently select which of the billions of particle events that CMS processes every 25 nanoseconds might signal new physics.
“The successful completion of the CD-1 review is a reflection of the competence, commitment and dedication of a very large team of Fermilab scientists and university colleagues,” said Fermilab scientist Steve Nahn, U.S. project manager for the CMS detector upgrade.
Now USCMS will refine the plan, getting it ready to serve as the project baseline.
“With these improvements, we’ll be able to explore uncharted territories and might discover new phenomena that revolutionize our description of nature,” Canepa said.
Fermi National Accelerator Laboratory (Fermilab), located just outside Batavia, Illinois, near Chicago, is a US Department of Energy national laboratory specializing in high-energy particle physics. Fermilab is America’s premier laboratory for particle physics and accelerator research, funded by the U.S. Department of Energy. Thousands of scientists from universities and laboratories around the world
collaborate at Fermilab on experiments at the frontiers of discovery.
One of the best strategies for searching for new physics in the TeV regime is to look for the decays of new particles. The CMS collaboration has searched in the dilepton channel for particles with masses above a few hundred GeV since the start of LHC data taking. Thanks to newly developed triggers, the searches are now being extended to the more difficult lower range of masses. A promising possible addition to the Standard Model (SM) that could exist in this mass range is the dark photon (Zd). Its coupling with SM particles and production rate depend on the value of a kinetic mixing coefficient ε, and the resulting strength of the interaction of the Zd with ordinary matter may be several orders of magnitude weaker than the electroweak interaction.
The CMS collaboration has recently presented results of a search for a narrow resonance decaying to a pair of muons in the mass range from 11.5 to 200 GeV. This search looks for a strikingly sharp peak on top of a smooth dimuon mass spectrum that arises mainly from the Drell–Yan process. At masses below approximately 40 GeV, conventional triggers are the main limitation for this analysis as the thresholds on the muon transverse momenta (pT), which are applied online to reduce the rate of events saved for offline analysis, introduce a significant kinematic acceptance loss, as evident from the red curve in figure 1.
Fig. 1. Dimuon invariant-mass distributions obtained from data collected by the standard dimuon triggers (red) and the dimuon scouting triggers (green).
A dedicated set of high-rate dimuon “scouting” triggers, with some additional kinematic constraints on the dimuon system and significantly lower muon pT thresholds, was deployed during Run 2 to overcome this limitation. Only a minimal amount of high-level information from the online reconstruction is stored for the selected events. The reduced event size allows significantly higher trigger rates, up to two orders of magnitude higher than the standard muon triggers. The green curve in figure 1 shows the dimuon invariant mass distribution obtained from data collected with the scouting triggers. The increase in kinematic acceptance for low masses can be well appreciated.
The full data sets collected with the muon scouting and standard dimuon triggers during Run 2 are used to probe masses below 45 GeV, and between 45 and 200 GeV, respectively, excluding the mass range from 75 to 110 GeV where Z-boson production dominates. No significant resonant peaks are observed, and limits are set on ε2 at 90% confidence as a function of the ZD mass (figure 2). These are among the world’s most stringent constraints on dark photons in this mass range.
Fig. 2. Upper limits on ε2 as a function of the ZD mass. Results obtained with data collected by the dimuon scouting triggers are to the left of the dashed line. Constraints from measurement of the electroweak observables are shown in light blue.
A candidate event for a top quark–antiquark pair recorded by the CMS detector. Such an event is expected to produce an electron (green), a muon (red) of opposite charge, two high-energy “jets” of particles (orange) and a large amount of missing energy (purple) (Image: CMS/CERN)
For the first time, CMS physicists have investigated an effect called the “running” of the top quark mass, a fundamental quantum effect predicted by the Standard Model.
Mass is one of the most complex concepts in fundamental physics, which went through a long history of conceptual developments. Mass was first understood in classical mechanics as a measure of inertia and was later interpreted in the theory of special relativity as a form of energy. Mass has a similar meaning in modern quantum field theories that describe the subatomic world. The Standard Model of particle physics is such a quantum field theory, and it can describe the interaction of all known fundamental particles at the energies of the Large Hadron Collider.
Quantum Chromodynamics is the part of the Standard Model that describes the interactions of fundamental constituents of nuclear matter: quarks and gluons. The strength of the interaction between these particles depends on a fundamental parameter called the strong coupling constant. According to Quantum Chromodynamics, the strong coupling constant rapidly decreases at higher energy scales. This effect is called asymptotic freedom, and the scale evolution is referred to as the “running of the coupling constant.” The same is also true for the masses of the quarks, which can themselves be understood as fundamental couplings, for example, in connection with the interaction with the Higgs field. In Quantum Chromodynamics, the running of the strong coupling constant and of the quark masses can be predicted, and these predictions can be experimentally tested.
The experimental verification of the running mass is an essential test of the validity of Quantum Chromodynamics. At the energies probed by the Large Hadron Collider, the effects of physics beyond the Standard Model could lead to modifications of the running of mass. Therefore, a measurement of this effect is also a search for unknown physics. Over the past decades, the running of the strong coupling constant has been experimentally verified for a wide range of scales. Also, evidence was found for the running of the masses of the charm and beauty quarks.
Figure 1: Display of an LHC collision detected by the CMS detector that contains a reconstructed top quark-antiquark pair. The display shows an electron (green) and a muon (red) of opposite charge, two highly energetic jets (orange) and a large amount of missing energy (purple).
With a new measurement, the CMS Collaboration investigates for the first time the running of the mass of the heaviest of the quarks: the top quark. The production rate of top quark pairs (a quantity that depends on the top quark mass) was measured at different energy scales. From this measurement, the top quark mass is extracted at those energy scales using theory predictions that predict the rate at which top quark-antiquark pairs are produced.
Figure 2: The running of the top quark mass determined from the data (black points) compared to the theoretical prediction (red line). As the absolute scale of the top quark mass is not relevant for this measurement, the values have been normalised to the second data point.
Experimentally, interesting top quark pair collisions are selected by searching for the specific decay products of a top quark-antiquark pair. In the overwhelming majority of cases, top quarks decay into an energetic jet and a W boson, which in turn can decay into a lepton and a neutrino. Jets and leptons can be identified and measured with high precision by the CMS detector, while neutrinos escape undetected and reveal themselves as missing energy. A collision that is likely the production of a top quark-antiquark pair as it is seen in the CMS detector is shown in Figure 1. Such a collision is expected to contain an electron, a muon, two energetic jets, and a large amount of missing energy.
The measured running of the top quark mass is shown in Figure 2. The markers correspond to the measured points, while the red line represents the theoretical prediction according to Quantum Chromodynamics. The result provides the first indication of the validity of the fundamental quantum effect of the running of the top quark mass and opens a new window to test our understanding of the strong interaction. While a lot more data will be collected in the future LHC runs starting with Run 3 in 2021, this particular CMS result is mostly sensitive to uncertainties coming from the theoretical knowledge of the top quark in Quantum Chromodynamics. To witness the top quark mass running with even higher precision and maybe unveil signs of new physics, theory developments and experimental efforts will both be necessary. In the meantime, watch the top quark run!
3D cut of the LHC dipole CERN LHC underground tunnel and tube. 
The European Organization for Nuclear Research [La Organización Europea para la Investigación Nuclear][ Organization européenne pour la recherche nucléaire] [Europäische Organization für Kernforschung](CH)[CERN] BASE: Baryon Antibaryon Symmetry Experiment. 
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