Large Hadron Collider restarts after major upgrades

UC Santa Cruz physicists have been at the forefront of U.S. participation in the LHC and contributed to a major upgrade of the ATLAS detector during the shutdown

Carolyn Gee is among the UCSC graduate students doing their thesis research at CERN, the international physics lab in Geneva, Switzerland, that operates the Large Hadron Collider. (Photo by Carolyn Gee)
UCSC postdoctoral scholar Andrea Sciandra working in the ATLAS Control Room at CERN in October. (Photo by Manuella Vincter)
Close-up view of a “beam splash” event, used to test the instruments, that was recorded by the ATLAS experiment on Saturday, May 7, 2022. (Image: ATLAS Collaboration/CERN)

The Large Hadron Collider (LHC) restarted in April and is gearing up to probe new realms of physics during a third run, which will be its most powerful yet. Run 3 of the LHC will feature higher-energy particle beams and higher collision rates, allowing physicists to collect more data than in the previous two runs combined.

Based at CERN, the international particle physics lab in Geneva, Switzerland, the LHC has been in a long shutdown for maintenance and upgrades since December 2018. UC Santa Cruz physicists at the Santa Cruz Institute for Particle Physics (SCIPP) have been working on upgrades of the detector components in the ATLAS experiment, the largest of the particle detectors at the LHC.

Michael Hance, associate professor of physics at UCSC, will be the U.S. ATLAS physics support manager for Run 3.

“Run 3 is exciting for me not only because it will give us more data at higher collision energies, which we always love, but also because the detector is the best it’s ever been,” he said. “The collaboration has been busy improving all aspects of the experiment over the past few years, and we’ll be able to extract more information than ever out of the new data, on top of increasing the size of the dataset and the energy of the collisions. Putting all of those together gives us a really exciting opportunity to find new and unexpected things.”

UCSC physicists have been involved in the ATLAS project since the 1990s, said SCIPP Director Jason Nielsen. The detector has multiple layers of devices to track and measure the jets of subatomic particles generated by high-energy collisions of proton beams within the LHC. SCIPP’s work has focused on the two innermost tracking layers, including the silicon sensors and readout electronics.

“The detector we built is still in there, and it’s a bit of a challenge to keep everything going even as components get radiation damage,” he said. “We have to make sure it’s still reading out signals and tracking particles accurately. At the same time, we’re also building the next-generation detector, which won’t be installed until the next shutdown in 2026.”

ATLAS is a general purpose detector designed to explore the widest possible range of physics phenomena. More than 5,500 scientists from 245 institutes in 42 countries work on the ATLAS experiment, investigating a variety of unanswered questions about the universe. In 2012, the ATLAS and CMS experiments confirmed the existence of the Higgs boson, which is crucial to understanding the origin of mass, and efforts continue to measure its properties with ever higher precision.

“I’m excited about getting Higgs measurements at high momentum, which opens up a new avenue where we may be able to sense the presence of new particles through indirect effects, even though we can’t see them directly,” Nielsen said.

Marco Battaglia, adjunct professor of physics, has driven much of the work at SCIPP on Higgs measurements at high momentum, not only in the ATLAS analysis but also in phenomenological studies with a team of theorists to provide tools for an optimal interpretation of past and future results.

“The anticipated Run 3 data will almost triple the total data set on which our analyses are based,” Battaglia said. “This is a major step forward, in particular for those studies probing the high energy frontier, such as new particle searches and, probably more importantly, measurements of Higgs boson properties that are basically limited by the number of available data events.”

Postdoctoral scholar Andrea Sciandra worked with Battaglia on new tracking algorithms that will improve the measurements of Higgs bosons, and he has been working in the ATLAS control room during start-up and commissioning for Run 3. Battaglia has also been responsible for tuning the ATLAS pixel detector readout to compensate for the radiation damage incurred during the previous runs.

“The emphasis on large data sets brings major challenges to mitigate the radiation damage effects on the detectors, in particular the silicon pixels installed closest to the colliding beams,” Battaglia said. “Run 3 is an important opportunity to develop procedures to operate these detectors in ways that minimize the impact of radiation damage effects on the ATLAS physics performance.”

Another area of research UCSC physicists are involved in is the search for new particles that could make up dark matter. Hance’s group is looking for “supersymmetric” particles, which are among the leading candidates for dark matter. “Our work will really benefit from the increased dataset as well as the detector improvements,” Hance said.

The UCSC ATLAS group includes nine senior physicists, along with many postdoctoral researchers and graduate students. Carolyn Gee is among the UCSC graduate students doing their thesis research at CERN. Gee worked during the shutdown to upgrade the semiconductor tracker control system in the ATLAS experimental cavern, and she developed new user interfaces for the control panel.

“CERN is emerging from its pandemic restrictions, which means that our students and postdocs are able to travel there to work on the experiment,” Hance said. “Spending time at CERN is a unique experience, one that several of our students have almost missed entirely during the pandemic. I can’t wait to see what they learn and accomplish while they’re there!”