This week in history, ten years ago, two international groups of scientists from different countries, including large numbers from Caltech, announced that they had discovered conclusive evidence of the Higgs boson. It is an elusive elementary particle that was first predicted in a series articles published in mid-1960s.
As theoretical physicists tried to understand electroweak theory (which describes both electromagnetism as well as the weak nuclear force in radioactive decay), fifty years ago, Peter Higgs in the UK and independent to Francois Englert, Robert Brout, and Gerald Guralnik, in the U.S., realized that there was a field that filled the universe and could explain the behavior and composition of elementary particles. The Higgs field would result in a particle with significant mass and zero spin. It also has the ability to break the symmetry of early universes, which allows the universe to materialize. This particle was known as the Higgs boson.
In the decades that followed the discovery of the Higgs boson, experimental physicists developed and first created the instruments and methods necessary to detect it. Large Hadron Collider (LHC) was the most ambitious of all these projects. It is currently operated by CERN, the European Organization for Nuclear Research. The U.S. Department of Energy has worked with CERN since the early 1980s to fund the LHC and provide technology expertise to thousands of scientists who are searching for the Higgs.
The LHC is a underground ring that runs 27 kilometers long through which protons accelerate by superconducting magnetics to speeds just below the speed of light. The LHC is a 27-kilometer-long underground ring that focuses protons in opposite directions. These beams collide at points where detectors can see the particles created by collisions. Scientists can use major detector facilities of different designs, namely the Compact Muon Solnoid (CMS), and the A Toroidal LHC ApparatuS(ATLAS), to perform a variety of experiments. These experiments allow scientists to check the predictions of The Standard Model, of which the Higgs Boson is a component, search for new particles, interactions, and verify each other's results. The unprecedented amount of data that ATLAS and CMS collected was used to confirm the Higgs boson's detection. It was announced July 4, 2012.
Harvey Newman, Marvin L. Goldberger Professor in Physics at Caltech, and one of Caltech's leaders, describes the discovery of Higgs boson as "a landmark in human history" that has "permanently changed the way we see the universe."
The Higgs boson, which was nicknamed the "God particle" by Leon Lederman and Dick Teresi in 1993, plays a key role in the Standard Model of Physics. It is the mechanism that elementary particles gain mass. Some particles glide across the Higgs field, while others interact with Higgs bosons. Others, however, are caught in the Higgs field and become mass.
Although the Standard Model cannot yet adequately explain dark matter and gravitation, experimental confirmations of its predictions continue to be made. Newman states, "It's a striking, surprising result that through the analysis and use of increasingly sensitive methods, we have continued to improve the agreement with Standard Model in all its details, even though the first signs of what lies beyond in terms of new particles or new interactions has continued to eludes us."
Any deviation from the Standard Model results suggests the existence of other particles and dynamics that could one day be the foundation for a new, more comprehensive model of physics.
Higgs bosons are rare. One Higgs boson can be created for every billion proton–proton collisions. This picture is further complicated by the fact that Higgs bosons are very fast decaying into other particles. It is only through measuring the characteristics of these particles can we infer the existence of the Higgs Boson. Maria Spiropulu from Caltech, the Shang Yi Ch'en Professor in Physics and the other leader among the original Caltech team that discovered the Higgs boson, refers to it as the "proverbial needle problem".
Technological advances to the LHC have allowed for higher energy and more precision in colliders, and their detectors. Experiments at the LHC since 2012's discovery of the Higgs Boson have provided additional information about the Higgs and its mass and decay. In 2018, Newman, Spiropulu and other Caltech researchers collaborated with an international team to produce evidence that the Higgs boson is decaying into fundamental particles called bottom quearks. This work was described by Spiropulu as "Herculean labor." The CMS team had already made the first observation that the Higgs boson was directly coupled to the heaviest standard particle, the top quark, prior to this discovery.
Spiropulu and her collaborators discovered a rare decay process that produces two muons for the Higgs boson in 2020. Spiropulu stated, "Probing Higgs boson properties is like searching for new Physics that we know must exist."
Irene Dutta, a Caltech graduate student and CMS team member who worked on the muon research, said that she was only graduating high school when I first heard about the Higgs discovery at LHC. It is amazing to see how accurately the Standard Model can describe elementary particles, and their interactions with such precision.
Recent research by a Caltech-led group of researchers on the CMS experiment used machine-learning algorithms based upon neural networks to create a new method of hunting for what might be even more difficult prey than the Higgs: an extremely rare pair of interconnected Higgs bosons. These bosons could, theoretically, be created during proton collisions.
The LHC started final preparations for Run 3 in 2022, after a three year shutdown to upgrade the LHC accelerator. Run 3 will begin on July 5, and the first collisions will occur at the new energy of 13.6 Tera-electronvolts.
"The Higgs discovery marks a significant milestone in a long road ahead," said Caltech's Barry Barish. He is the Ronald and Maxine Linne Professor of Physics Emeritus and was the former leader of Caltech’s high-energy Physics group. In 2017, he also won the Nobel Prize in Physics for his work on the Laser Interferometer Gravitational Wave Observatory (or LIGO), which detected the ripples in time and space known as gravitational wave. Particle Physics is moving forward, keeping in mind that the Standard Model only describes a small fraction of what is out there, and more questions remain unanswered than answers. Yes, the Standard Model does have a simple parametrization, but it is not clear where the electroweak, symmetry-breaking came from. Barish says that there is still much to be done.
Newman reflects on a decade spent exploring the Higgs boson and notes that it "continues to drive us to think harder" and to design improved detectors and accelerators, which will allow us to greatly expand our reach for the future. This includes the High Luminosity LHC Phase II, which is the second phase of the LHC program. It will run from 2029-2040. This will see major upgrades to the accelerator complex as well as detectors. It is expected to lead to an increase of data collected by 20 times the amount of CMS and ATLAS.
Si Xie is a research assistant professor in physics. Ren-Yuan Zhu and Adi Bornheim are both research scientists. All of them have spent decades studying the Higgs boson. The Caltech group is leading novel ultraprecision-timing detector upgrades for the High Luminosity LHC and developing novel AI-based data-analysis approaches that will allow for accelerated near-real-time discovery. Since the discovery of Higgs, the group has produced over a dozen Ph.D. theses, as well as allowing approximately 100 undergraduate and intern students to participate in analysis, instrumentation and computation research.