The Higgs Discovery at 10

Ten years ago, on July 4, 2012, the ATLAS and CMS collaborations at the Large Hadron Collider (LHC) announced the discovery of a new particle with features consistent with those of the Higgs boson predicted by the Standard Model of particle physics. . The discovery was a milestone in the history of science and caught the attention of the world. A year later, François Englert and Peter Higgs won the Nobel Prize in Physics for their prediction made decades earlier, along with the late Robert Brout, of a new fundamental field, known as the Higgs field, permeating and manifesting the universe. as the Higgs boson and gives mass to the elementary particles.

“The discovery of the Higgs boson was a monumental milestone in particle physics. It marked both the end of a decades-long journey of discovery and the beginning of a new era of studies of this very special particle,” said Fabiola Gianotti, Director General of CERN and the project leader (‘spokesperson’) of the ATLAS experiment at the time. of the discovery. “I remember with emotion the day of the announcement, a day of immense joy for the global particle physics community and for all the people who have worked tirelessly for decades to make this discovery possible.”

The search for the Higgs boson was an international effort, involving scientists from researchers around the world, including UC Santa Barbara. Physics professors Claudio Campagnari, Joe Incandela, Jeffrey Richman and David Stuart — members of UCSB’s High Energy Physics Group — along with their teams of students, postdocs and engineers were among the scientists who ushered in the discovery of the Higgs boson. Incandela was also the project lead for the CMS collaboration at the time of the discovery.

In just a decade, physicists have made huge strides in our understanding of the universe, not only confirming early on that the particle discovered in 2012 is indeed the Higgs boson, but also enabling researchers to image to form of how the ubiquitous presence of a Higgs boson field in the universe was brought about one-tenth of a billionth of a second after the Big Bang.

The new journey so far
The new particle discovered in 2012 by the international ATLAS and CMS collaborations was very similar to the Higgs boson predicted by the Standard Model. But was it actually that long-sought particle? Once the discovery was made, ATLAS and CMS set out to investigate in detail whether the properties of the particle they had discovered really matched those predicted by the Standard Model. Using data from the disintegration, or “decay,” of the new particle into two photons, the carriers of the electromagnetic force, the experiments showed that the new particle has no intrinsic angular momentum or quantum spin — just like the Higgs particle predicted by the Standard Model. In contrast, all other known elementary particles have spin: the matter particles, such as the ‘up’ and ‘down’ quarks that form protons and neutrons, and the force-carrying particles, such as the W and Z bosons.

By observing that the Higgs bosons are produced from and decay into pairs of W or Z bosons, ATLAS and CMS confirmed that these gain their mass through their interactions with the Higgs field, as predicted by the Standard Model. The strength of these interactions explains the short range of the weak force, which is responsible for a form of radioactivity and initiates the nuclear fusion reaction that powers the sun.

The experiments also showed that the top quark, bottom quark and tau-lepton — the heaviest fermions — obtain their mass from their interactions with the Higgs field, again as predicted by the Standard Model. They did this by observing, in the case of the top quark, that the Higgs boson is produced along with pairs of top quarks, and in the case of the bottom quark and tau lepton, the decay of the boson in pairs. of bottom quarks and tau leptons, respectively. † These observations confirmed the existence of an interaction, or force, called the Yukawa interaction, which is part of the Standard Model but unlike any other force in the Standard Model: it is mediated by the Higgs boson and its strength. is not quantified, that is, it does not come in multiples of a particular unit.

ATLAS and CMS measured the mass of the Higgs boson at 125 billion electron volts (GeV), with an impressive precision of nearly one per mil. The mass of the Higgs boson is a fundamental constant of nature that is not predicted by the Standard Model. In addition, the mass of the Higgs boson, along with the mass of the heaviest known elementary particle, the top quark and other parameters, can determine the stability of the universe’s vacuum.

These are just some of the concrete results of ten years of exploration of the Higgs boson at the world’s largest and most powerful collider – the only place in the world where this unique particle can be produced and studied in detail.

“The large data samples provided by the LHC, the exceptional performance of the ATLAS and CMS detectors, and new analysis techniques have enabled both collaborations to extend the sensitivity of their Higgs boson measurements beyond what was thought possible. when the experiments were designed,” said ATLAS spokesman Andreas Hoecker.

In addition, since the LHC began colliding with protons of record energy in 2010, the LHC collaborations have discovered more than 60 composite particles predicted by the Standard Model, some of which are exotic, thanks to the unprecedented sensitivity and precision of the four main experiments. ‘tetraquarks’ and ‘pentaquarks’. The experiments also uncovered a series of intriguing hints of deviations from the Standard Model that warrant further investigation, and have studied the quark-gluon plasma that filled the universe in its early moments in unprecedented detail. They have also observed many processes of rare particles, made increasingly accurate measurements of the Standard Model phenomena, and pioneered the search for new particles beyond those predicted by the Standard Model, including those that could make up the dark matter that responsible for most of the mass of the universe.

The results of these searches add important pieces to our understanding of fundamental physics. “Discoveries in particle physics don’t have to mean new particles,” said Joachim Mnich, CERN’s director of research and computer science. “The LHC results obtained over a decade of using the machine have allowed us to spread a much larger network in our searches, setting strong limits on possible extensions of the Standard Model and new search and data capabilities. devise analysis techniques. †

Remarkably, all LHC results obtained to date are based on only 5% of the total amount of data the accelerator will provide during its lifetime. “With this ‘tiny’ sample, the LHC has enabled major advances in our understanding of elementary particles and their interactions,” said CERN theorist Michelangelo Mangano. “And while all the results obtained so far are consistent with the Standard Model, there is still plenty of room for new phenomena beyond what is predicted by this theory.”

“The Higgs boson itself may point to new phenomena, including some that may be responsible for the dark matter in the universe,” said CMS spokesman Luca Malgeri. “ATLAS and CMS are conducting many searches to investigate all forms of unexpected processes related to the Higgs boson.”

The journey ahead of us
What is there left to learn about the Higgs field and the Higgs boson ten years later? A lot. Does the Higgs field also give mass to the lighter fermions or could there be another mechanism at play? Is the Higgs boson an elementary or composite particle? Could it interact with dark matter and reveal the nature of this mysterious form of matter? What generates the mass and self-interaction of the Higgs boson? Does it have twins or family?

Finding the answers to these and other intriguing questions will not only expand our understanding of the universe at its smallest scales, but may also help unravel some of the greatest mysteries of the universe as a whole, such as how it came to be the way it was. is and what his ultimate fate might be. In particular, the Higgs boson’s self-interaction could hold the key to a better understanding of the matter-antimatter imbalance and the stability of the universe’s vacuum.

Since the discovery of the Higgs boson ten years ago, members of the UCSB High Energy Physics group have been studying some of the properties of this particle, such as its lifetime and its interactions with top and charmed quarks. They have also used Higgs bosons to search for new physical phenomena. The effort at UCSB is broad, with many postdocs, graduate students and undergraduates involved in the efforts to build, operate and upgrade the detector, develop the software algorithms, analyze the data, and publish the results. UCSB’s effort has been funded during the time of Higgs’ discovery and since then by the United States Department of Energy, Office of Science and the National Science Foundation.

While answers to some of the new questions may be provided by data from the forthcoming third run of the LHC or from the major upgrade of the accelerator, the high-brightness LHC, as of 2029, answers to other conundrums are believed to be beyond the reach of the LHC, which will require a future ‘Higgs factory’. For this reason, CERN and its international partners are investigating the technical and financial feasibility of a much larger and more powerful machine, the Future Circular Collider, in response to a recommendation in the latest update of the European Strategy for Particle Physics.

“High-energy accelerators remain the most powerful microscope at our disposal to explore nature at the smallest scale and discover the fundamental laws that govern the universe,” said Gian Giudice, Head of Theory Department at CERN. “In addition, these machines also have enormous social benefits.”

Historically, the accelerator, detector and computing technologies associated with high-energy accelerators have had a major positive impact on society, with inventions such as the World Wide Web, the detector developments leading to the PET (Positron Emission Tomography) scanner and the design of accelerators for hadron therapy in cancer treatment. In addition, the design, construction and operation of particle physics adjusters and experiments have resulted in the training of new generations of scientists and professionals in other fields, and in a unique model of international collaboration.

-Sarah Charley, CERN

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