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, predicted for the first time in a series of articles published in the middle of the 20th century. 1960s, which is supposed to endow elementary particles with mass.

Fifty years ago, as theoretical physicists struggled to understand the so-called electroweak theory, which describes both electromagnetism and the weak nuclear force (involved in radioactive decay), it became apparent 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 that filled the universe was needed to explain the behavior of particles elements that make up matter. This field, the Higgs field, would lead to a particle of spin zero, large mass, and the ability to spontaneously break the symmetry of the oldest universe, allowing the universe to materialize. This particle became known as the Higgs boson.

Over the decades that followed, experimental physicists first designed 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 the planning of the LHC in the late 1980s, the US Department of Energy and the National Science Foundation have worked with CERN to provide funding and technological know-how, and to support thousands of scientists who participate 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 under 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 detection facilities of different designs, primarily the Compact Muon Solenoid (CMS) and the LHC Toroidal Apparatus A (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, to search for new particles and interactions that lie beyond the standard model, and to verify the results of the other. 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, 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 ” has changed permanently. the way we see the universe.”

Humorously called the “God particle” in 1993 in a 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 gain mass. As the particles pass through the Higgs field and interact with the Higgs bosons, some glide across the surface, without changing at all. But others get caught in the weeds, so to speak, and gain mass.

The Standard Model has yet to adequately explain dark matter or gravity, but time and time again its predictions have been confirmed experimentally. “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 steadily improved in all its details. , even as the first signs of what lies beyond, in terms of new particles and new interactions, have 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 could one day provide the basis for a new, more global model of physics.

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

Technological improvements to the LHC and its detectors have increased the energy and precision of the colliders and their detectors. Since the discovery of the Higgs boson in 2012, experiments at the LHC have revealed new information about the Higgs boson and its mass and decay processes. For example, in 2018, Newman, Spiropulu and other Caltech researchers worked with an international team that produced evidence showing that the Higgs boson decays into pairs of fundamental particles called lower quarks, work that Spiropulu described to the time as a “Herculean task”. Prior to this discovery, the CMS team made the first observation of the Higgs boson coupling directly to the heaviest Standard Model particle, the top quark.

In 2020, Spiropulu and his colleagues documented a rare Higgs boson decay process that results in two muons. “Probing the properties of the Higgs boson is like searching for new physics that we know must exist,” Spiropulu said.

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

More 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 for hunting what may be even more elusive prey than the Higgs. -even: an extremely rare ‘pair’ of interacting Higgs bosons that the theory suggests could be produced in proton collisions.

After a three-year shutdown to continue upgrading the LHC accelerator and experiments, the LHC began final preparations for a third run (Run 3) in early 2022. The launch of Run 3, which should to continue until the end of 2025, will take place on July 5, producing the first collisions at the new energy of 13.6 tera-electron-volts.

“The discovery of Higgs is an important step on a long road ahead,” says Barry Barish, Ronald and Maxine Linde, Caltech Professor of Physics 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 achieved the first detection of ripples in space and time called gravitational waves in 2016 ). “Particle physics progresses keeping in mind that the Standard Model only describes a tiny fraction of what we know and that there are more unanswered questions than answers; yes, we have a large simple parametrization in the Standard Model, but the actual origin of the electroweak symmetry breaking is unknown. We still have a lot of work to do,” says Barish.

Reflecting on a decade of Higgs boson exploration, Newman notes that the research “continues to motivate us to think harder and design improved detectors and accelerator upgrades that allow us to greatly extend our reach now and for next two decades”. This includes the second major phase of the LHC programme, known as the High-Luminosity LHC, scheduled to run from 2029 to 2040. It will deliver substantial improvements to the accelerator complex and detectors that will lead to an expected increase in data collected by a factor of 20 compared to what CMS and ATLAS have today.

The Caltech team also includes Si Xie, Assistant Research Professor of Physics, and 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-precise detector upgrades for the High-Luminosity LHC and developing new AI-based data analysis approaches that will enable accelerated discovery in near real-time. The group has produced more than a dozen doctoral theses and has enabled about 100 undergraduates and trainees to engage in research in analysis, instrumentation and computation since the discovery of the Higgs.