Research, teaching and outreach in Physics at UW–Madison
CMS
New Chair to continue department’s strengths, commit to diversity and inclusion
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Prof. Kevin Black
The department of physics is pleased to announce that Prof. Kevin Black has been named new department chair. His three-year term began July 1, 2024, succeeding Prof. Mark Eriksson. Black says he is looking to continue the department’s excellence in its mission of research, teaching, and outreach, and to continue developing an intentional commitment to diversity.
“Under Prof. Eriksson’s leadership, our department has attained near-record highs of faculty members as well as graduate and undergraduate students, which will lead to significant successes in our research program,” Black says. “Now, we need to continue to focus on making a commitment to diversity an active component of what we do as a department.”
Two pillars of the department’s mission have always been research and teaching, and Black wants to sustain successes in those areas. He begins his term with over a dozen faculty members who have joined the department in the previous three years, bringing the total number of professors to 56. These faculty members represent a range of seniority levels and a breadth of research fields. He also begins at a time when more students than ever are being taught in department courses.
“Research and education are the core values of a research university,” Black says. “We want to do excellent, cutting-edge research and we want to teach the next generation of scientists.”
Black’s focus on diversity and climate efforts represents a continuing effort from leadership before him. The need to add diversity as a pillar of the department’s mission became evident to him when he saw the list of department chairs who came before him, and he noted that he was the 33rd white male chair out of 35. He acknowledges the challenge that the broader field of physics faces, and specifically at UW–Madison: both lack adequate representation of students from marginalized groups.
“We need to improve diversity at all levels in this department,” Black says. “There’s no magic wand. It takes a concerted, sustained effort and we need to make it a priority going forward.”
Lastly, Black also believes that the department’s commitment to educational outreach is critical to fulfilling the Wisconsin Idea, the idea that education should influence people’s lives beyond the boundaries of the university. The department has a long-standing tradition of engaging in outreach, including over 100 years of running the Physics Museum and over four decades of running The Wonders of Physics outreach program.
“Physics outreach can inspire the next generation to think about the natural universe and think about how things work,” Black says. “In a world which is increasingly driven by soundbites and nonsense on the internet, it’s crucial to encourage and guide young students to think rationally about science and formulate questions and opinions.”
Black joined the faculty as a full professor in 2018 and works with the high energy experiment group on the Compact Muon Solenoid (CMS) experiment at CERN. He had previously been a professor at Boston University. Black earned a bachelor’s degree at Wesleyan University where he worked in an atomic physics lab. He has a doctorate in physics from Boston University, and much of his thesis work was completed on the Tevatron at Fermilab. He was then a postdoc and research scientist at Harvard University, where his work transitioned to the Large Hadron Collider at CERN.
Department of Energy grant to train students at the interface of high energy physics and computer science
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To truly understand our physical world, scientists look to the very small, subatomic particles that make up everything. Particle physics generally falls under the discipline of high energy physics (HEP), where higher and higher energy collisions — tens of teraelectronvolts, or about ten trillion times the energy of visible light — lead to the detection and characterization of particles and how they interact.
These collisions also lead to the accumulation of inordinate amounts of data, and HEP is increasingly becoming a field where researchers must be experts in both particle physics and advanced computing technologies. HEP graduate students, however, rarely enter graduate school with backgrounds in both fields.
Physicists from UW–Madison, Princeton University, and the University of Massachusetts-Amherst are looking to address the science goals of the HEP experiments by training the next generation of software and computing experts with a 5-year, ~$4 million grant from the U.S. Department of Energy (DOE) Office of Science, known as Training to Advance Computational High Energy Physics in the Exascale Era, or TAC-HEP.
“The exascale era is upon us in HEP and the complexity, computational needs and data volumes of current and future HEP experiments will increase dramatically over the next few years. A paradigm shift in software and computing is needed to tackle the data onslaught,” says Tulika Bose, a physics professor at UW–Madison and TAC-HEP principal investigator. “TAC-HEP will help train a new generation of software and computing experts who can take on this challenge head-on and help maximize the physics reach of the experiments.”
Tulika Bose
In total, DOE announced $10 million in funding today for three projects providing classroom training and research opportunities in computational high energy physics to train the next generation of computational scientists and engineers needed to deliver scientific discoveries.
At UW–Madison, TAC-HEP will annually fund four-to-six two-year training positions for graduate students working on a computational HEP research project with Bose or physics professors Keith Bechtol, Kevin Black, Kyle Cranmer, Sridhara Dasu, or Brian Rebel. Their research must broadly fit into the categories of high-performance software and algorithms, collaborative software infrastructure, or hardware-software co-design.
Bose’s research group, for example, focuses on proton-proton collisions in the Compact Muon Solenoid (CMS) at the CERN Large Hadron Collider (LHC). The high luminosity run of the LHC, starting in 2029, will bring unprecedented physics opportunities — and computing challenges, challenges that TAC-HEP graduate students will tackle firsthand.
“The annual data volume will increase by 30 times while the event reconstruction time will increase by nearly 25 times, requiring modernization of the software and computing infrastructure to handle the demands of the experiments,” Bose says. “Novel algorithms using modern hardware and accelerators, such as Graphics Processing Units, or GPUs, will need to be exploited together with a transformation of the data analysis process.”
TAC-HEP will incorporate targeted coursework and specialized training modules that will enable the design and development of coherent hardware and software systems, collaborative software infrastructure, and high-performance software and algorithms. Structured R&D projects, undertaken in collaboration with DOE laboratories (Fermilab and Brookhaven National Lab) and integrated within the program, will provide students from all three participating universities with hands-on experience with cutting-edge computational tools, software and technology.
The training program will also include student professional development including oral and written science communication and cohort-building activities. These components are expected to help build a cohort of students with the goal of increasing recruitment and retention of a diverse group of graduate students.
“Future high energy physics discoveries will require large accurate simulations and efficient collaborative software,” said Regina Rameika, DOE Associate Director of Science for High Energy Physics. “These traineeships will educate the scientists and engineers necessary to design, develop, deploy, and maintain the software and computing infrastructure essential for the future of high energy physics.
Higgs @ Ten: UW–Madison physicists’ past and future roles
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Ten years ago, on July 4, 2012, the CMS and ATLAS collaborations at the Large Hadron Collider (LHC) at CERN — including many current and former UW–Madison physicists — announced they had discovered a particle that was consistent with predictions of the Higgs boson.
In the ten years since, scientists have confirmed the finding was the Higgs boson, but its discovery opened more avenues of discovery than it closed. Now, with the LHC back up and running, delivering proton collisions at unprecedented energies, high energy physicists are ready to investigate even more properties of the particle.
The Higgs discovery was published in Physics Letters B and received the cover
“The Higgs plays an incredibly important role in particle physics,” says Kevin Black, who previously worked on ATLAS before joining the UW–Madison physics department and is now part of CMS. “But for being such a fundamental particle, for giving mass to all elementary particles, for being deeply connected to flavor physics and why we have different generations of matter particles — we know a relatively small amount about it.”
Finding the Higgs particle had been one of the main goals of the LHC. The particle was first theorized by physicist Peter Higgs (amongst others, but his name was forever associated with it) in the 1960s.
“The basic idea was that if you just had electromagnetic and strong interactions, then the theory would have been fine if you just put a mass in by hand for the elementary particles,” explains Black. “The weak interaction spoils that, and it was a big question at the time of whether or not the whole structure of particle physics and of quantum field theory were actually going to be consistent.”
Higgs and others realized that there was a way to make it happen if they introduced a new field, which then became the Higgs field and the Higgs particle, that can interact with all other matter and give particles their mass. The Higgs particle, however, eluded experimental observation, leaving a gap in the Standard Model. In retrospect, one of the difficulties was that the mass of the Higgs — around 125 GeV — was much larger than the technology at the time could reach experimentally.
In earlier generations of experiments, UW–Madison physicist Sau Lan Wu participated in searches using the ALEPH experiment that placed a strong lower bound on the mass of the Higgs boson. Also at UW–Madison, Duncan Carlsmith, Matthew Herndon and their groups participated in searches at the CDF experiment that placed an upper bound on the mass of the Higgs boson and saw evidence of Higgs production in the region of mass where it was finally discovered.
Wesley Smith shows the electronics of the trigger system which led to the discovery of the Higgs Boson. Smith led the team that designed and developed the trigger system.
This research set the stage for the experiments that were perfectly designed to discover the Higgs boson: the world’s most powerful hadron collider, the LHC, and the most capable pair of high energy collider experiments ever built, CMS and ATLAS.
The UW–Madison CMS group had three major projects: the trigger project led by Wesley Smith (now emeritus faculty), and the end cap muon system led by Don Reeder (now emeritus faculty) and Dick Loveless (now emeritus scientist), and a computing project led by Sridhara Dasu, who is current head of the group. Having made essential detector contributions, the UW–Madison CMS group, including Herndon, moved on to Higgs hunting and the discoveries. The group, now bolstered by the addition of Black and Tulika Bose to the physics department faculty, continues the work of understanding the Higgs Boson thoroughly.
The UW–Madison ATLAS group, founded and led by Wu, is an important leader of Higgs physics. The group is fortunate to attract another important leader of ATLAS, Higgs physicist Kyle Cranmer, who recently joined UW–Madison as physics department faculty and the director of the American Family Data Science Institute.
Both CMS and ATLAS announced the discovery, made separately but concurrently, in 2012. When it was first discovered, it conformed to expected energies and momentum of the Higgs, but finding it in this rare decay mode was unexpected, so LHC scientists called it the Higgs-like particle for a while.
The UW–Madison ATLAS group at CERN at the time of the Higgs discovery all celebrated with printouts of the data confirming 5sigma. | Provided by Sau Lan Wu
At 3:00pm [on June 25, 2012], there was a commotion in the Wisconsin corridor on the ground floor of CERN Building 32. My graduate student Haichen Wang was saying loudly, ‘Haoshuang is going to announce the Higgs discovery!’ Our first reaction was that it was a joke; thus when we entered Haoshuang’s office, we all had smiles on our faces. Those smiles suddenly became much bigger when we got to look at the result of Haoshuang’s combination: It showed the 5.08s close to the Higgs mass of 125GeV/c2. Pretty soon, cheers were ringing down the Wisconsin corridor.
ATLAS had a discovery!”
The Higgs-like announcement from ten years ago has since been confirmed to be the Higgs particle. Several years later, Dasu’s group’s work saw the Higgs decay into the tau, and provided the first evidence of the particle coupling to matter particles, not just to bosons.
Sau Lan Wu and other Higgs scientists were featured on the cover of the New York Times for a story about the chase for the Higgs boson.
On the ten-year anniversary, both ATLAS and CMS collaborations published summaries of their findings to date and future directions. Experimental questions still being addressed include continuing to measure higher-precision interactions between the Higgs and particles it has already been observed to interact with, and detecting previously-unobserved interactions between the Higgs and other particles.
“One big question that immediately comes to my mind is the mass problem. The breakthrough generated by the Higgs discovery was that elementary particles acquire their masses through the Higgs particle,” Wu writes in her Physics Today essay. “A deeper question that needs to be answered is how to explain the values of the individual masses of the elementary particles. In my mind, this mass problem remains a big topic to be explored in the years to come.”
“Another one of the big things that we’re looking for in future data is to understand Higgs potential,” Black says. “Right now, by measuring the mass, we’ve only measured right around its ground state, and that has great implications for the stability of our universe.”
Also on the ten-year anniversary, CERN announced that the LHC — which had been shut down for three years to work on upgrades — was ready to again start delivering proton collisions at an unprecedented energy of 13.6 TeV in its third round of runs. It is expected that the ATLAS and CMS detectors will record more collisions in this upcoming run than in the previous two combined.
The LHC program is scheduled to run through 2040, and the UW–Madison scientists who are part of the CMS and ATLAS collaborations will almost certainly continue to play key roles in future discoveries.
UW–Madison’s current CMS collaboration members include Kevin Black, Tulika Bose, Sridhara Dasu, and Matthew Herndon, and their research groups. Current ATLAS collaboration members include Kyle Cranmer and Sau Lan Wu and their research groups.
Does the behavior of the Higgs boson match the expectations?
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Note: This story has been modified slightly from the original, which was published by the CMS Collaboration. Their version has some nice interactive graphics to check out, too!
The standard model of particle physics is our current best theory to describe the most basic building blocks of the universe, the elementary particles, and the interactions among them. At the heart of the standard model is a hypothesis describing how all the elementary particles acquire mass. Importantly, this scheme also envisages the existence of a new type of particle, called the Higgs boson. It took nearly 50 years, since its postulation, to observe the Higgs boson at the LHC experiments at CERN. It is strongly believed that the Higgs boson, the only scalar particle known to date, is a key to answer some of the questions that standard model cannot answer. Thus a detailed study of the properties of the Higgs boson is the order of the day. Often, specially at the LHC, one of the essential observables concerns the probability that a certain unstable particle is produced momentarily, albeit obeying the laws of nature. In experiments this production cross section is estimated using a specific decay final state of this transient particle in terms of the number of events over a given amount of time. The standard model predicts the cross section for the Higgs boson production as well as the decay rates very precisely. The frequency distribution of a given type of event, as a function of some of the measured variables in the experiment, helps us understand better various aspects of the interactions involved; they are typically lost in the summed or total cross section. Hence measurement of this differential cross section is a powerful tool to vindicate the standard model; also any deviation from the standard model predictions in data would indicate presence of a New Physics.
The Higgs boson is roughly about 125 times more massive than a proton and decays to lighter particles including cascade processes in some cases. Physicists typically use the signatures of stable particles in the detector to trace back suitable decay chains of the Higgs boson. The tau lepton is the heaviest lepton known so far, and as such it is the lepton with strongest ties to the Higgs boson. The probability of a Higgs boson decaying to a pair of tau leptons is reasonably high (about 6%), when compared, for example, to a pair of muons (about 0.02%). But the tau lepton is also an unstable particle and decays quickly to lighter particles always accompanied by its partner, the tau neutrino. Often the decay products from the tau lepton are hadrons producing a shower of particles or jet in the calorimeter system. The tau neutrino goes undetected affecting the accuracy of measurement of the tau lepton energy. It is interesting to study the detailed characteristics of the Higgs boson events using the decay to tau leptons which possess a rest mass of only about 1.4% that of the parent.
Andrew Loeliger
A recent study from the CMS Collaboration, focuses on the events where the Higgs boson decays into a pair of tau leptons using data collected by the experiment between 2016 and 2018. The analysis measures the Higgs boson production cross section as a function of three key variables: the Higgs boson momentum in the direction transverse to the beam, the number of jets produced along with the Higgs boson, and the transverse momentum of the leading jet. New Physics could manifest in excess of events in the frequency distribution of these variables when compared with the standard model predictions.
Says Andrew Loeliger, a UW–Madison physics grad student and one of the lead authors on the study:
The Higgs Boson is the most recent addition to the standard model of particle physics, discovered jointly between the CMS and ATLAS collaborations in 2012, so a big goal of the High Energy Physics field is to make very detailed measurements of its properties, to understand if our predictions are all confirmed, or if there is some kind of new physics or strange properties that might foreshadow or necessitate further discoveries. This work provides, what amounts to, a very fine grained consistency check (alternatively, a search for deviations in the amount) that the Higgs Boson is produced with the amounts/strengths we would expect when categorizing alongside some second interesting property (the transverse momentum of the Higgs Boson is a big one). This type of analysis had not been performed before using the particles we used, so it may open the door for far more precise measurements in places we may not have been able to do before, and a better overall confirmation of the Higgs Boson’s properties.
Other UW–Madison researchers involved in the study include former postdoc Cecile Caillol and Profs. Tulika Bose and Sridhara Dasu.
The analysis employs deep neural networks to exploit simultaneously a variety of tau lepton properties for identifying them with high efficiency. Eventually, to ensure that the selected tau lepton pair is produced from the decay of the Higgs boson and discard those from other processes, such as Z boson decay, the mass of the selected tau pair (m𝝉𝝉 ) is scrutinized. Reconstruction of m𝝉𝝉 , after taking into account the neutrinos involved in the decay as mentioned earlier, required a dedicated algorithm which computes, for each event, a likelihood function P(m𝝉𝝉) to quantify the level of compatibility of a Higgs boson process.
Higgs boson produced in vector boson fusion and decay to tau pair | credit: CMS Collaboration
The Higgs boson typically has more transverse momentum or boost when produced in conjunction with jet(s), compared to the case when it is produced singly. One such event, collected by the CMS detector in 2018 and shown in Figure 1, could correspond to such a boosted Higgs boson decaying to two tau leptons which, in turn, decay hadronically. However, several other less interesting processes could also be the cause of such an event and pose as backgrounds. Such contributions have been measured mostly from the data itself by carefully studying the properties of the jets. Figure 2 shows the good agreement in the m𝝉𝝉 distribution between the prediction and data collected by the CMS experiment for the events with the transverse momentum of the Higgs boson below 45 GeV. The contribution from the Higgs boson process is hardly noticeable due to the overwhelming background. On the other hand, Figure 3 presents m𝝉𝝉 distribution for the events with highly boosted Higgs boson, when its transverse momentum is above 450 GeV. Selecting only events with high boost reduces a lot the total number of available events, but the fraction of the signal events in the collected sample is significantly improved. The data agrees with the sum of predicted contributions from the Higgs boson and all the standard model background processes.
This CMS result presents the first-ever measurement of the differential cross sections for the Higgs boson production decaying to a pair of tau leptons. Run 2 data is allowing us to scrutinize the Higgs boson in the tau lepton decay channel which was only observed a few years back. Future comparison and combination of all Higgs boson decay modes will offer better insights on the interactions of the Higgs boson to different standard model particles. But the story does not end here! The Run 3 of the LHC machine is just around the corner and looking into the future, the high luminosity operation (the HL-LHC) will offer a huge increase in data volume. That could perhaps provide hints of the question if the discovered Higgs boson is the one as predicted by the standard model or if there is any new interaction depicting another fundamental particle contributing to such measurements. That will indeed point to New Physics!
CMS Group publishes new study on Lepton flavor in Higgs boson decays
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Neutrinos mix and transform from one flavor to the other. So do quarks. However, electron and its heavier cousins, the muon and the tau, seem to conserve their flavor identity. This accidental conservation of charged lepton flavor must have a profound reason, or low-levels of violation of that conservation principle should occur at high energy scales. However, evidence for any charged lepton flavor violation remains elusive.
The CMS group recently published a new study on Lepton flavor in Higgs boson decays. At UW–Madison, the effort was led by Sridhara Dasu and postdoctoral researcher Varun Sharma, building off of work done by former postdoctoral researcher Maria Cepeda and former graduate student Aaron Levine.
The international CMS collaboration recently published a news story about this new study. Please read the full story here.
An event similar to the lepton flavor violating decay of the Higgs boson, produced with the gluon fusion production mechanism. The red track corresponds to a muon, while the red cone along with its corresponding calorimeter deposits is the tau lepton. | CMS Collaboration
Kevin Black named co-coordinator of LHC Physics Center at Fermilab
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Professor Kevin Black has been named one of the next co-coordinators of the LHC (Large Hadron Collider) Physics Center at Fermilab (LPC at FNAL), LPC announced recently. His initial appointment starts on September 1st, 2020 and lasts for two years.
Prof. Kevin Black
As co-coordinator, Black’s roles will include leading the several hundred physicists who are residents or visit the LPC for research on CMS, managing the distinguished research program, and leading the training of students and young physicists at FNAL.
According to their website, LPC at FNAL is a regional center of the Compact Muon Solenoid (CMS) Collaboration. It serves as a resource and physics analysis hub primarily for the seven hundred US physicists in the CMS collaboration. The LPC offers a vibrant community of CMS scientists from the US and overseas who play leading roles in analysis of data, in the definition and refinement of physics objects, in detector commissioning, and in the design and development of the detector upgrade.
Black joined the CMS experiment in 2018 when he joined the UW–Madison physics faculty after 13 years on CMS’s companion experiment, ATLAS. Since that time, he has been involved in the forward muon upgrade project — which will install GEM (Gas Electron Multiplier) detectors — as manager of the U.S. component of the electronic readout project. He has also served as deputy run coordinator of the GEM system, and his group is focusing on the data-acquisition development for that system. Additionally, his students and post-docs are working on a variety of physics analysis ranging from searches for new physics with the top quark, flavor anomalies in bottom quark decays, and searches for pair-production of Higgs bosons.
“I am excited for this important leadership opportunity to play a crucial role in facilitating U.S. participation in cutting edge particle physics research at a unique facility,” Black says. It will allow me to continue the excellent tradition of the LPC and bring my own ideas and initiatives to the center.”
As LPC at FNAL co-coodinator, Black will also serve as co-Chair of the LPC Management Board. He will be working with Dr. Sergo Jindariani, a senior scientist at FNAL, and succeed Prof. Cecilia Gerber from the University of Illinois at Chicago.
Particle collider experiment CMS — and UW physicists who contribute — celebrate 1000th publication
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In June 2020, The Compact Muon Solenoid (CMS) collaboration announced the submission of its 1000th scientific publication since the experiment began a decade ago. With multiple University of Wisconsin–Madison physics faculty involved in CMS over the years, the physics department wanted to use this milestone to celebrate their achievements.
CMS is an international collaboration of over 4000 scientists at CERN’s Large Hadron Collider, which churns out data that have contributed immensely to our understanding of particle physics and pointing directions to moving beyond the Standard Model. Amongst its achievements, CMS announced in 2012 the discovery of the Higgs boson, along with ATLAS collaboration.
Image from CERN. The original image can be found at https://cms.cern/news/cms-collaboration-celebrates-1000th-paper
“It’s a proud moment for CMS in general and for the UW CMS group to see our work over the years culminate in this historic milestone!” says Bose, who currently serves as the Deputy U.S. CMS Software and Computing Operations program manager. “We are looking forward to more with the upcoming run and with the High-Luminosity LHC upgrade.”
Of the current UW–Madison physics faculty involved:
Sridhara Dasu currently leads the UW–Madison High Energy Physics group. On CMS, his focus is in better understanding the Higgs boson, searching for its partners, and possible new physics connections, especially to dark matter. He helped design the CMS calorimeter trigger system and continues to dabble in designing its upgrades.
Matthew Herndon is involved in the ongoing upgrade of the CSC (cathode strip chamber) forward muon system and well as detailed studies of the performance of the CSC system. He studies the physics of multiple gauge boson interactions and associated new physics phenomena involving multiple gauge bosons.
Tulika Bose previously served as the Physics Co-coordinator (PC) of the CMS Experiment during 2017-2019 and as the CMS Trigger Co-coordinator (2014-2016). In addition to her current program manager role, she is involved in physics studies that cover both precision measurements of Standard Model processes as well as direct searches for new physics including dark matter and top quark partners.
Kevin Black joined CMS when he joined the UW–Madison physics department in 2018, after 13 years on the CMS companion experiment ATLAS. Since then, he has been involved in the forward muon upgrade project — which will install GEM (Gas Electron Multiplier) detectors — as manager of the U.S. component of the electronic readout project and as deputy run coordinator of the GEM system. His group is focusing on the data-acquisition development for that system.
“I am especially proud of our eighteen PhD graduates who have contributed about two papers each to this set of thousand; one on a search for new physics channel and another on a carefully made measurement,” Dasu says.
Adds Herndon, “It’s an amazing milestone and a testament to the scientific productivity of the CMS experiment! UW personnel, especially our students, have been a major part of that achievement contributing to nearly 100 of those papers.”
In collaboration with the Physical Sciences Laboratory, the UW Physics team helped design the steel structures and other mechanical systems of the CMS experiment, especially leading the installation, commissioning and operations of the endcap muon system. The UW Physics team has also helped design, build, install and operate the electronics and data acquisition systems, in particular the calorimeter trigger system, and began collecting data from day one of LHC operations. They also collaborated with the HT Condor group of the Department of Computer Science to design and build the Worldwide LHC Computing Grid (WLCG), hosting one of the productive Tier-2 computing centers in Chamberlin.
The UW–Madison group was a key player in the discovery of Higgs boson in 4-lepton decay mode and establishing its coupling to fermions. The group has also searched for new physics especially looking for evidence of beyond the standard model in the form of heavy Higgs bosons that decay to tau-pairs. The group also upgraded the calorimeter trigger system and completed the endcap muon chamber system for the second higher energy run of the LHC. Searches continue for new Higgs partners, rare decays of the SM-like Higgs boson, and searches for new particles. They have added to our repertoire a series of searches for anomalous production of single high energy objects that are indicative of dark matter production in the LHC collisions.
The abundant production of papers proclaiming discoveries or the best measurements to date were possible in large part because of numerous UW–Madison electronics and computing personnel.
“The publication of the 1000th paper of the CMS collaboration is a significant milestone capping the achievement of thousands of physicists worldwide on a wide range of topics that can only be made at this unique instrument and facility,” Black says.
