What comes after the Higgs boson?

Ten years ago this week, two international collaborations of groups of scientists, including a large contingent from Caltech, confirmed that they had found conclusive evidence for the Higgs boson, an elusive elementary particle first predicted in a series of papers published in the middle of the year. 1960s, which is thought to give mass to elementary particles.

Fifty years earlier, when theoretical physicists tried 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 for François Englert and Robert Brout. , in Belgium, as well as American physicist Gerald Guralnik and others, that a previously unidentified field filling the universe was needed to explain the behavior of the elementary particles that make up matter. This field, the Higgs field, would lead to a particle with no spin, significant mass, and the ability to spontaneously break the symmetry of the earliest universe, allowing the universe to materialize. That particle became known as the Higgs boson.

In the decades that followed, experimental physicists first invented and 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 planning for the LHC in the late 1980s, the U.S. Department of Energy and the National Science Foundation have partnered with CERN to provide funding, technological know-how and support thousands of scientists in the search for the Higgs.

The LHC is a 27 kilometer long 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 directed to collide with each other at specific points where detectors can detect the particles produced by these collisions. The use of large detector facilities of various designs – mainly the Compact Muon Solenoid (CMS) and the A Toroidal LHC ApparatuS (ATLAS) – allows scientists to perform a wide variety of experiments to test the predictions of the Standard Model, of which the Higgs- particle is a part, to look for new particles and interactions outside the Standard Model, and to verify each other’s results. The detection of the Higgs boson, announced on July 4, 2012was 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, which is part of the CMS collaboration, calls the discovery of the Higgs boson “a milestone in human history” that ” forever has changed the way we view the universe.”

The Higgs boson, humorously referred to as the “God particle” in 1993 in a book of the same name by authors Leon Lederman and Dick Teresi, plays a vital role in the Standard Model of physics: it provides the mechanism by which elementary particles gain mass. As particles traverse the Higgs field and interact with Higgs bosons, some slide across the surface and don’t change at all. But others are, as it were, caught in the weeds and gain mass.

The Standard Model has yet to adequately explain dark matter or gravity, but time and again its predictions have been experimentally confirmed. “It is a striking and surprising 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 if the first signs of what lies beyond, in terms of new particles and new interactions, has always eluded 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 form the basis for a new, more comprehensive physical model.

Collisions that produce Higgs bosons are very rare. For every billion collisions between protons and protons, only one Higgs boson is created. To complicate this picture, Higgs bosons decay very quickly into other particles, and it is only by measuring the characteristics of these particles that the Higgs boson’s previous existence can be inferred. Caltech’s Maria Spiropulu, the Shang-Yi Ch’en professor of physics and the other leader of the original team of Caltech researchers who helped detect the Higgs, describes it as the “proverbial needle in a haystack.”

Technological improvements to the LHC and its detectors have enabled higher energy and greater precision in the accelerators and their detectors. Since the discovery of the Higgs boson in 2012, experiments at the LHC have provided more information about the Higgs boson and its mass and decay processes. For example, in 2018, Newman, Spiropulu and other Caltech researchers collaborated with an international team that produced evidence showing the Higgs boson. decays into pairs of fundamental particles called bottom quarks, work that Spiropulu described at the time as a ‘Hercules labour’. Prior to that discovery, the CMS team made the first observation of the Higgs boson, which is directly linked to the heaviest Standard Model particle, the top quark.

In 2020, Spiropulu and her colleagues documented: a rare decay process for the Higgs boson resulting in two muons. “Exploring the properties of the Higgs boson comes down to looking for new physics that we know must be out there,” Spiropulu said.

“I had just graduated from high school when I heard about the Higgs discovery at the LHC,” said Caltech student and CMS team member Irene Dutta (MS ’20), who worked on the muon study. “It’s humbling to know how well the Standard Model can describe elementary particles and their interactions with such precision.”

Most recently, a Caltech-led team of researchers working on the CMS experiment used machine learning algorithms based on neural networks to a new method to hunt for what is perhaps even more elusive prey than the Higgs themselves: an extraordinarily rare “pair” of interacting Higgs bosons that, the theory says, could be produced during proton collisions.

After a three-year shutdown to further upgrade the LHC accelerator and experiments, the LHC began final preparations for a third run (Run 3) in early 2022. The start of Run 3, slated to continue through the end of 2025, will take place on July 5, producing the first collisions with the new energy of 13.6 tera-electron-volts.

“The discovery of Higgs is a milestone on a long way forward,” said Barry Barish, Ronald and Maxine Linde of Caltech, professor of physics emeritus, former leader 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 continues, bearing in mind that the Standard Model describes only a small part of what we know there is and there are more questions unanswered than answered; yes, we have a great simple parameterization in the Standard Model, but the real origin of the electroweak symmetry breaking is unknown, we have a lot more work ahead of us,” says Barish.

Looking back on a decade of exploration of the Higgs boson, Newman notes that the research “continues to motivate us to think harder and design improved detectors and accelerator enhancements, vastly expanding our reach now and for the next two decades.” ” This includes the second major phase of the LHC program, known as the High Luminosity LHC, which will run from 2029 to 2040. It will provide substantial upgrades to the accelerator complex and detectors leading to an expected increase in collected data by a factor of 20 over what CMS and ATLAS have today.

The Caltech team also includes Si Xie, research assistant professor of physics, as well as research scientists Adi Bornheim and Ren-Yuan Zhu, all of whom have devoted decades of study to discovering and understanding the Higgs boson. The Caltech group is leading new ultra-precision timing detector upgrades for the High Luminosity LHC and developing new AI-based approaches to data analysis that enable accelerated near-real-time discovery. The group has produced more than a dozen dissertations and enabled about 100 undergraduate students and interns to participate in analysis, instrumentation and computational research since the discovery of the Higgs.

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