The compact muon solenoid detector in the Large Hadron Collider. Credit: CERN
This week ten years ago, two international collaborations from groups of scientists, including a large contingent of Caltech, confirmed that they had found conclusive evidence of the Higgs boson, an elusive elementary particle, first predicted in a series of articles. published in the mid-1960s, which is believed to give mass to elementary particles.
Fifty years earlier, when theoretical physicists struggled to understand the so-called electroweak theory, which describes both electromagnetism and weak nuclear force (involved in radioactive decay), Peter Higgs, who worked in the United Kingdom, and so independent of François Englert, it became apparent. and Robert Brout, in Belgium, as well as the American physicist Gerald Guralnik and others, who needed an previously unidentified field that filled the universe to explain the behavior of the elementary particles that make up matter. This field, the Higgs field, would result in a particle with zero spin, too large, and would have the ability to spontaneously break the symmetry of the earliest universe, allowing the universe to materialize. This particle became known as the Higgs boson.
Over the following decades, experimental physicists first devised and then developed the instruments and methods needed to detect the Higgs boson. The most ambitious of these projects was the Large Hadron Collider (LHC), which is operated by the European Organization for Nuclear Research, or CERN. Since LHC planning in the late 1980s, the U.S. Department of Energy and the National Science Foundation have worked in collaboration with CERN to provide funding and technology expertise, and to support thousands of scientists helping to search for Higgs.
Credit: (c) 2022 CERN
The LHC is a 27-kilometer-long underground ring through which superconducting magnets accelerate protons at a speed just below the speed of light. Two beams of protons traveling in opposite directions are focused and directed to collide with each other at specific points where detectors can observe the particles produced by these collisions. The use of major detector installations with different designs, mainly the Compact Muon Solenoid (CMS) and the A Toroidal LHC ApparatuS (ATLAS), allows scientists to conduct a variety of experiments to test predictions. of the standard model whose Higgs boson. it is a part, to look for new particles and interactions that are beyond the standard model, and to verify the results of the others. The detection of the Higgs boson, announced on July 4, 2012, was based on the analysis of an unprecedented amount of data collected by CMS and ATLAS.
Harvey Newman, physics professor Marvin L. Goldberger at Caltech and one of the leaders of the Caltech team, which is part of the CMS collaboration, calls the discovery of the Higgs boson “a milestone in human history. “that” has changed permanently “. how we see the universe “.
Humorously called the “particle of God” in 1993 in a book of the same name by Leon Lederman and Dick Teresi, the Higgs boson plays a crucial role in the standard model of physics: it provides the mechanism by which elementary particles acquire too. As the particles traverse the Higgs field and interact with the Higgs bosons, some slide across the surface without changing at all. But others get caught up in the weeds, so to speak, and earn too much.
The standard model has yet to adequately explain dark matter or gravitation, but its predictions have been experimentally confirmed one after the other. “It is a surprising, and astonishing result, that through the analysis of increasing amounts of data, with increasingly sensitive methods, the agreement with the Standard Model has continued to improve in all its details, even the first hints of what’s beyond, in terms of new particles and new interactions, has continued to elude us, ”says Newman.
Any deviation from the results predicted by the standard model suggests the presence of other particles or dynamics that may one day provide the basis for a new, more global physics model.
Collisions that produce Higgs bosons are very rare. For every billion proton-proton collisions, only one Higgs boson is created. To further complicate this picture, Higgs bosons disintegrate very rapidly into other particles, and only by measuring the characteristics of these particles can the previous existence of the Higgs boson be inferred. Caltech’s Maria Spiropulu, Shang-Yi Ch’en physics professor and the other leader of Caltech’s original team of researchers who helped detect Higgs, describes him as the “proverbial needle of the barn problem.” .
Technological improvements to the LHC and its detectors have allowed for greater energy and accuracy in the colliders and their detectors. Since the discovery of the Higgs boson in 2012, experiments at the LHC have revealed more information about the Higgs boson and its processes of mass and disintegration. For example, in 2018, Newman, Spiropulu, and other Caltech researchers worked with an international team that produced evidence showing that the Higgs boson decomposes into pairs of fundamental particles called background quarks, a work Spiropulu described in that moment as a “Herculean work.” Prior to this discovery, the CMS team made the first observation of the Higgs boson that was coupled directly to the heaviest standard model particle, the upper quark.
In 2020, Spiropulu and colleagues documented a rare process of disintegration of the Higgs boson that results in two muons. “Examining the properties of the Higgs boson is tantamount to looking for a new physics that we know exists,” Spiropulu said.
“I just graduated from high school when I learned about the discovery of Higgs at the LHC,” says Caltech graduate student and CMS team member Irene Dutta (MS ’20), who worked in muon research. “It’s humble to know how well the standard model can describe elementary particles and their interactions so accurately.”
More recently, a team of Caltech-led researchers working on the CMS experiment has used neural network-based machine learning algorithms to develop a new method for hunting what may be an even more elusive prey than the Higgs itself: an extremely rare “.pair” of interactive Higgs bosons that, according to theory, could occur during proton collisions.
After a three-year hiatus to further upgrade the LHC’s accelerator and experiments, the LHC began final preparations for a third test (Run 3) in early 2022. Run 3, which is expected to continue until the end of 2025, will take place on July 5, producing the first collisions with the new 13.6 tera-electron-volt energy.
“The discovery of Higgs is a milestone a long way ahead,” says Barry Barish, Ronald and Maxine Linde, professor emeritus of Caltech Physics, a former leader of Caltech’s high-energy physics group (and co-winner of the Nobel Prize of Physics). in 2017 for his work on another large-scale physics project, the Laser Interferometer Gravitational-wave Observatory, or LIGO, which made the first detection of ripples in space and time known as gravitational waves in 2016) . “Particle physics is advancing considering that the standard model describes only a small fraction of what we know exists and there are more unanswered questions than unanswered ones; yes, we have a great deal of simple parameterization in the standard model, but the real origin. Breaking electroweak symmetry is unknown. We have a lot more work ahead of us, “says Barish.
Reflecting on a decade of exploring the Higgs boson, Newman notes that research “continues to motivate us to think more and design improved detectors and accelerator enhancements that allow us to greatly expand our reach now and over the next two decades.” . This includes the second major phase of the LHC program, known as the High-Brightness LHC, scheduled to run from 2029 to 2040. It will provide substantial upgrades to the accelerator and detector complex that will result in a projected increase in data collected by a factor of 20 in relation to what CMS and ATLAS currently have.
The Caltech team also includes Si Xie, an assistant professor of physics research, as well as research scientists Adi Bornheim and Ren-Yuan Zhu, all of whom have spent decades studying and discovering the Higgs boson. The Caltech Group is leading new upgrades of ultra-precision timing detectors for the high-brightness LHC and developing new AI-based data analysis approaches that will allow accelerated discovery in near real time. The group has produced more than a dozen doctorates. thesis and allowed approximately 100 undergraduate students and fellows to participate in analytical, instrumentation, and computational research since the discovery of Higgs.
ATLAS and CMS publish the results of the most comprehensive studies to date on the properties of the Higgs boson.