The Compact Muon Solenoid Detector at the Large Hadron Collider. Credit: CERN
Ten years ago this week, two international collaborations of groups of scientists, including a large contingent from Caltech, confirmed that they had found compelling evidence for the Higgs boson, an elusive elementary particle first predicted in a series of papers published in the mid-1960s. , which is thought to give elementary particles mass.
Fifty years ago, while theoretical physicists were trying to understand the so-called electroweak theory, which describes both electromagnetism and the weak nuclear force (involved in radioactive decay), it became clear to Peter Higgs, working in the UK, and independently to François Englert and Robert Braut in Belgium, as well as American physicist Gerald Guralnik and others, that a previously unidentified field that filled the universe was needed 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, significant mass, and 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 created and then developed the tools 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 the planning of the LHC in the late 1980s, the US Department of Energy and the National Science Foundation have worked in collaboration with CERN to provide funding and technological know-how and support thousands of scientists helping to search for the Higgs .
Credit: (c) 2022 CERN
The LHC is a 27km underground ring through which protons are accelerated by superconducting magnets to just below the speed of light. Two beams of protons traveling in opposite directions are focused and guided to collide with each other at specific points where detectors can observe the particles produced by these collisions. The use of large detector facilities of different designs—mainly the Compact Muon Solenoid (CMS) and the Toroidal LHC Instrument (ATLAS)—allows scientists to conduct a wide variety of experiments to test the predictions of the Standard Model, of which the Higgs boson is a part, for to look for new particles and interactions that are outside the standard model and cross-check their results. The discovery of the Higgs boson, announced on July 4, 2012, is based on the analysis of an unprecedented amount of data collected by CMS and ATLAS.
Harvey Newman, the Marvin L. Goldberger Professor of Physics at Caltech and one of the leaders of the Caltech team that is part of the CMS collaboration, calls the discovery of the Higgs boson “a milestone in human history” that “is permanently changed the way we see the universe.”
Humorously dubbed the “God Particle” in a 1993 book of the same name by authors 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 mass. As the particles cross the Higgs field and interact with the Higgs bosons, some slide along the surface without changing at all. But others are caught in the weeds, so to speak, and are gaining ground.
The Standard Model has not yet adequately explained dark matter or gravity, but its predictions have been repeatedly confirmed experimentally. “A striking and surprising result is that through the analysis of increasing amounts of data, with increasingly sensitive methods, the agreement with the Standard Model continues to improve in all its details, even as the first signs of what lies beyond, in terms of of new particles and new interactions continues 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 comprehensive model of physics.
Collisions that produce Higgs bosons are very rare. For every billion collisions between protons, only one Higgs boson is created. To further complicate this picture, Higgs bosons decay very rapidly into other particles, and only by measuring the characteristics of these particles can the former existence of the Higgs boson be inferred. Caltech’s Maria Spiropoulou, the Shang-Yi Ch’en Professor of Physics and the other leader of the original team of Caltech researchers who helped discover the Higgs, describes it as “the proverbial needle in a haystack problem.”
Technological improvements to the LHC and its detectors allowed for higher energy and greater precision in the colliders and their detectors. Since the discovery of the Higgs boson in 2012, experiments at the LHC have revealed additional information about the Higgs boson and its mass and decay processes. For example, in 2018, Newman, Spiropoulou and other Caltech researchers worked with an international team that presented evidence showing that the Higgs boson decays into pairs of fundamental particles called bottom quarks, work that Spiropoulou described at the time as a “Herculean “. Prior to this discovery, the CMS team made the first observation of the Higgs boson, which binds directly to the heaviest standard model particle, the top quark.
In 2020, Spiropoulou and her colleagues documented a rare Higgs boson decay process that results in two muons. “Exploring the properties of the Higgs boson is tantamount to searching for new physics that we know must exist,” Spiropoulou said.
“I was just graduating high school when I heard about the Higgs discovery at the LHC,” says Caltech graduate student and CMS team member Irene Dutta (MS ’20), who worked on the muon study. “It is humbling to know how well the Standard Model can describe elementary particles and their interactions with such accuracy.”
Most recently, a Caltech-led team of researchers working on the CMS experiment used machine learning algorithms based on neural networks to develop a new method to pursue what may be even more elusive prey than the Higgs itself: an extremely a rare “pair” of interacting Higgs bosons that, according to theory, could be produced during proton collisions.
After a three-year shutdown for further upgrades of the LHC accelerator and experiments, the LHC began final preparations for a third run (Run 3) in early 2022. The start of Run 3, scheduled to last until the end of 2025, will take place on July 5, triggering the first collisions at the new energy of 13.6 tera-electron-volts.
“The discovery of the Higgs is a milestone in a long road ahead,” says Barry Barish, Caltech’s Ronald and Maxine Linde Professor of Physics, Professor Emeritus, former head of Caltech’s High Energy Physics Group (and co-winner of the Nobel Prize in 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 the ripples in space and time known as gravitational waves in 2016). “Particle physics is moving forward, given that the Standard Model describes only a fraction of what we know is out there, and more questions remain unanswered than answered; yes, we do have a great simple parameterization in the Standard Model, but the actual origin of electroweak symmetry breaking is unknown. We still have a lot of work to do,” says Barish.
Reflecting on a decade of Higgs boson research, Newman noted that the research “continues to motivate us to think harder and design improved detectors and accelerator improvements, allowing 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 Luminosity LHC, scheduled to operate from 2029 to 2040. It will provide significant improvements to the accelerator complex and detectors, leading to a projected increase in data collected by a factor of 20 over that which CMS and ATLAS have today.
The Caltech team also includes Si Xie, assistant professor of physics, and researchers Adi Bornheim and Ren-Yuan Zhu, all of whom have devoted decades of research to discover and understand the Higgs boson. The Caltech group is leading new ultra-precise timing detector upgrades for the High Luminosity LHC and developing new AI-based data analysis approaches that will enable accelerated near-real-time detection. The group has produced more than a dozen Ph.D. dissertations and enabled approximately 100 students and interns to engage in analysis, instrumentation, and computational research following the discovery of the Higgs.
ATLAS and CMS publish results of most comprehensive study of Higgs boson properties to date Provided by…
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