One of the most prestigious awards for scientific research, the Breakthrough Prize in Fundamental Physics recognizes those who have made significant advances in improving humanity’s understanding of the universe at the deepest level. In 2025, the award recognized four collaborations that contributed to the measurement of the Higgs boson particle first theorized in the 1960s. The experiments took place over roughly a decade at CERN, and among the thousands of scientific contributors are Livermore researchers who have held leadership roles within CERN’s research community.
Straddling the French–Swiss border, CERN’s Large Hadron Collider (LHC) is one of the foremost destinations for high-energy particle physics experiments. Inside the LHC’s 27-kilometer-long accelerator beamline buried 100 meters underground, researchers propel opposing streams of particles to relativistic speeds and collide them at one of four designated large, general purpose detectors: ATLAS (a toroidal LHC apparatus), CMS (compact muon solenoid), ALICE (a large ion collider experiment), and LHCb (large hadron collider beauty). Each detector, related to specific experiments, is outfitted with sophisticated diagnostics that measure collision products. Livermore scientists have primarily contributed to research at the largest experiments, ATLAS and CMS, to address the most tantalizing open questions in physics.
Meaningful Near-Misses
One of the first Livermore scientists to participate in LHC experiments was physicist Douglas Wright, who joined Lawrence Livermore in the early 1990s. Wright was designing scientific instrumentation destined for the U.S. Superconducting Super Collider (SSC), a research facility planned for construction in Texas that would have made the country home to the world’s highest-energy accelerator. However, construction on the SSC halted before it was complete, and as European nations continued developing the CERN site, LHC assumed this title. Wright petitioned CERN administrators for Lawrence Livermore to contribute to the CMS experiment at LHC, and upon acceptance in 2005, Livermore researchers received funding through the Laboratory Directed Research and Development (LDRD) Program to participate.
CERN members constructed LHC to detect new fundamental particles and their means of interaction, and they saw discovering the Higgs particle as the ultimate payoff. For decades until its discovery in 2012, the Higgs boson (an excitation of the Higgs field) and the mechanism by which it gives elementary particles their rest mass were theoretically proposed yet experimentally unconfirmed parts of the Standard Model of particle physics. The Standard Model describes the interaction of electromagnetism, the strong nuclear force, and the weak nuclear force in terms of elementary particles and their force carriers. Despite its successes, scientists recognize the Standard Model fails to account for some observations such as dark matter, so for decades, they have hunted for signs of other particles to address its blind spots.
When scientists collide particle beams at LHC, specialized detectors capture the ensuing spray of elementary particles as terabytes of experimental data. Scientists must comb through this data for specific particle signatures. “We could observe hundreds if not thousands of potential interactions in a collision that are well motivated by theory. Although we can predict which signatures to look for, plenty of interactions exist that scientists haven’t theorized,” says Wright, who focused on proton collisions at CMS. If two protons graze past other yet remain intact and undergo ultraperipheral collisions, they may exchange extremely high-energy photons in a process called photon–photon fusion. Through this interaction, two quasi-massless particles can produce massive particles (whether documented or newly discovered) or modify described interactions with new physics. Wright explains, “In a way, near misses of protons turned the accelerator into a photon collider, another type of accelerator that Livermore researchers wanted to use.” Wright and colleagues reasoned they could study the rare instances in which photon–photon fusion probes for signals of the Higgs boson and other new particles with more clarity than head-on collisions. Since scientists can calculate the probability of producing different particles through photon–photon fusion—namely W and Z bosons responsible for mediating the weak nuclear force. Any perturbations to the expected distribution would hint at the presence of unknown particles or reaction mechanisms.
To perform photon–photon fusion experiments at CMS, researchers accelerate opposing bunches of protons to the enormously energetic state of 7 teraelectronvolts (TeV). When the beams cross paths, protons involved in a photon–photon fusion reaction lose a small degree of energy and respond differently to the magnetic field that directs the beam. The challenge becomes successfully tracking those protons whose energy is slightly off from the rest, a process known as tagging. While non-interacting, 7-TeV protons remain in the middle of the particle beam, protons that emit a photon steer radially outward, signaling that photon–photon fusion occurred. Wright served as a technical design lead for the Precision Proton Spectrometer (PPS), which performs proton tagging at CMS. PPS’s silicon pixel detectors sit mere centimeters away from the beamline and maintain a 500-meter line-of-sight parallel to the particle stream to map trajectories of radially separated protons. The timing precision of PPS is on the order of tens of picoseconds (1012 seconds), providing millimeter-scale accuracy over the 500-meter sightline.
“In the early days of photon–photon fusion experiments, we thought we had the magic bullet; this mechanism would be a different, unambiguous way to discover the Higgs particle. However, during experiments, the signal associated with the Higgs was too small for the hardware to see. Fortunately, the setup’s high sensitivity made it sensitive to other particles, too, so we could continue the search for new physics,” says Wright. In 2012, Finn Rebassoo, who had worked on CMS experiments as a graduate student, joined the Livermore team. Rebassoo led the analysis effort that published the first experimental evidence for photon–photon fusion producing a W boson. In subsequent measurements made possible by PPS, researchers could analyze these events in greater detail. Their team is now working to achieve higher gain at the facility and meet the energy criteria necessary for testing other theories of particle interaction.
Pictures of Plasmas
On another experimental platform at CERN, Livermore scientists are testing the Standard Model by colliding ions to mimic the environment of the universe’s first microseconds. When heavy ions, such as lead, collide at relativistic speeds, their masses briefly meld into a hot, dense soup of elementary particles called quark– gluon plasma (QGP). Cosmological theory suggests practically all matter was in this state immediately following the Big Bang. QGP can only form at higher temperatures where the quarks and gluons become deconfined. At lower temperatures, the quarks and gluons are confined to composite particles such as protons and neutrons. QGP is analogous to the plasmas that form in stars and lightning bolts where intense heating separates electrons from atoms, and each interacts as individual particles. Scientists aim to better understand the distribution of quarks and gluons in atomic nuclei and their deconfined interactions in QGPs. Livermore physicist Dhanush Hangal says, “We use models of quark–gluon plasmas formed at CERN to inform our models of the early universe and its evolution. We still have much to discover about the strong force and quantum chromodynamics, the mathematical lens through which we describe this force.” For instance, although the Standard Model holds quarks and gluons to be elementary particles, research has yet to show whether they are truly indivisible building blocks of nature or contain smaller structures.
At CERN’s ATLAS experiment, Hangal and Livermore scientist Benjamin Gilbert work to characterize QGPs, and both individuals have served as subconveners for the ATLAS Heavy-Ion Jet Group. Complicating their search, laboratory-produced QGPs are impossible to observe directly. During heavy-ion collisions, researchers have a trillionth of a trillionth of a second to characterize QGPs—too brief to snap an x-ray image. Instead, scientists look for how QGPs alter the particle distribution expected from each collision. Early in heavy-ion collisions, some quarks and gluons may scatter off of their counterparts in the oncoming nucleus, releasing one or more conical jets of energetic elementary particles. As they radiate outward, these particles recombine into protons, neutrons, and other hadrons—particles made up of quarks and gluons. Hangal’s work investigates how jets can provide information about the structure of QGPs.
Jets come in many shapes and sizes. They vary with respect to total energy released, opening angle, and, in cases of multijet events, their relative orientation. Researchers have discovered that jet properties affect their interaction with QGPs, meaning jets can serve as a diagnostic tool to capture the internal state of QGPs on an event-by-event basis. Recent studies show jets produced from lead–lead collisions lose on average half of their energy while passing through a QGP—a process called quenching—whereas jets from proton– proton collisions experience negligible quenching due to lack of QGP formation. How jets experience quenching and change heading through interactions with the QGP reflects the plasma’s transitory properties such as density, temperature, and transport efficiency.
Along with dozens of researchers from the ATLAS Collaboration, Hangal and Gilbert published the first seminal measurement of jet quenching in QGPs by correlating how jet energy loss relates to the structure of the QGP it passes through. Using lead-ion collision data from ATLAS, they showed that when individual segments (prongs) of a jet are separated by a small angle, the jet loses a small amount of energy to quenching when passing through QGP. However, at wider angles of separation, jet quenching is pronounced, suggesting a critical angle exists past which QGPs treat these jets differently. Gilbert explains, “I picture this effect by imagining the QGP as a tank of water and the jet of particles as a spear. A single spear tip will pass almost effortlessly through the water, but if the spearhead has two or more prongs, the angle between the tips causes it to slow down. We want to determine the angle by which these metaphorical spear tips must be separated to interact differently with the medium,” says Gilbert.
Looking forward, Hangal plans to conduct similar analysis at Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC). Compared to LHC, RHIC is a smaller facility with lower acceleration potential, but the research conducted there plays an important role in capturing heavy-ion collisions. With LDRD support, Hangal’s team is investigating how lower-energy initial conditions affect the formation of QGPs and their effects on jet quenching. Since ultrahigh-energy collision experiments at CERN replicate the first instances of the Big Bang, performing similar experiments at lower energy could offer insight into how, as the universe cooled, the once-omnipresent QGP gave way to composite particles including protons and neutrons, paving the way for the first atoms.
Given the complexity of experiments at CMS, ATLAS, and elsewhere at CERN, Livermore scientists spend several weeks each year at the site collecting data and troubleshooting instrumentation to ensure experiments will run smoothly. Those in leadership positions help design new experiments, determining the data that needs to be collected to inform scientific conclusions and managing the publication of findings through sizeable organizational structures. Gilbert says, “Being at CERN feels like being a little fish in a big pond. I’ve benefitted from spending time in a subgroup that brings together people with similar physics interests, but simultaneously, I also get to interact with the broader ATLAS group, which boasts thousands of researchers. Seeing how scientific projects of this scale operate is enlightening and rewarding.”
—Elliot Jaffe
For further information contact Douglas Wright (925) 423-2347 (wright20 [at] llnl.gov (wright20[at]llnl[dot]gov)).
