Higgs @ Ten: UW–Madison physicists’ past and future roles

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.

cover of an issue of Physics Letters B, with data plots of the Higgs discovery in the foreground and a background aerial shot of CERN
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 holds a large electronics board full of circuits and wires
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.

a group of very happy scientists pose for a shot, all holding a printout of the same graph
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

Wu recalls her and her group’s involvement in a recent essay published in Physics Today:

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.

a screenshot of a newspaper front page, with an artistically-rendered photo of 5 key scientists involved in the Higgs discovery
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.

Particle collider experiment CMS — and UW physicists who contribute — celebrate 1000th publication

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.

UW–Madison’s Professor Emeritus Don Reeder, Professor Emeritus Wesley H. Smith, Emeritus Distinguished Scientist Richard Loveless and Professor and current department chair Sridhara Dasu are amongst the founding members of CMS. The team later expanded to include Professor Matt Herndon and Senior Scientists Dr. Sascha Savin and Dr. Armando Lanaro. In 2018, Professor Kevin Black and Professor Tulika Bose joined the physics department.

1000 boxes laid out to make an image of the CMS detector at CERN, and spells "1000 papers"
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.