The discovery of the Higgs at 10 years old

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 characteristics consistent with those of the Higgs boson predicted by the standard model of particle physics. . The discovery marked a turning point in the history of science and captured the world’s attention. A year later, it won François Englert and Peter Higgs the Nobel Prize in Physics for their prediction, made decades earlier, with the late Robert Brout, of a new fundamental field, known as the Higgs field, which pervades the universe, manifests as the Higgs boson and gives mass to elementary particles.

“The discovery of the Higgs boson was a monumental milestone in particle physics. It marked both the end of a decades-long exploration journey and the beginning of a new era of studies on this very special particle,” said Fabiola Gianotti, CERN Director General and Project Leader ( “spokesperson”) of the ATLAS experiment. at the time of discovery. “I fondly remember 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, with the participation of 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 – and their teams of students, postdocs and engineers were among the scientists who inaugurated the discovery of the Higgs boson. Incandela also served as project manager for the CMS collaboration at the time of the discovery.

In just ten years, physicists have made enormous progress in our understanding of the universe, not only confirming early on that the particle discovered in 2012 is indeed the Higgs boson, but also allowing researchers to begin to get a sense of how the ubiquitous presence of a Higgs field boson throughout the universe became established a tenth of a billionth of a second after the Big Bang.

The New Journey So Far
The new particle discovered by the international ATLAS and CMS collaborations in 2012 looked a lot like the Higgs boson predicted by the Standard Model. But was it really this much sought-after particle? As soon as the discovery was made, ATLAS and CMS set out to study in detail whether the properties of the particle they had discovered really matched those predicted by the Standard Model. Using data from the decay, or “decay”, of the new particle into two photons, carriers of the electromagnetic force, the experiments demonstrated that the new particle has no intrinsic angular momentum, or quantum spin – just like the Higgs boson. provided by the standard model. In contrast, all other known elementary particles have spin: particles of matter, such as “up” and “down” quarks that form protons and neutrons, and force-carrying particles, such as W bosons and Z.

By observing the production and decay of Higgs bosons from pairs of W or Z bosons, ATLAS and CMS have confirmed that these acquire 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 demonstrated that the top quark, bottom quark and tau lepton – which are the heaviest fermions – derive 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, the Higgs boson being produced with pairs of top quarks, and in the case of the bottom quark and the tau lepton, the decay of the boson into pairs of bottom quarks and of 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 which is different from all the other forces of the standard model: it is mediated by the Higgs boson and its force is not quantized, i.e. they are not multiples of a certain unit.

ATLAS and CMS have measured the mass of the Higgs boson at 125 billion electron volts (GeV), with an impressive accuracy of nearly one in a thousand. The mass of the Higgs boson is a fundamental constant in nature that is not predicted by the Standard Model. Moreover, together with the mass of the heaviest known elementary particle, the top quark, and other parameters, the mass of the Higgs boson can determine the vacuum stability of the universe.

These are just some of the concrete results of ten years of exploration of the Higgs boson in 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 outstanding 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 the was thought possible when designing the experiments. said ATLAS spokesman Andreas Hoecker.

Moreover, since the LHC began colliding protons at record energies in 2010, and thanks to the unprecedented sensitivity and precision of the four main experiments, the LHC collaborations have discovered more than 60 predicted composite particles by the standard model, some of which are exotic “tetraquarks” and “pentaquarks”. The experiments also revealed a series of intriguing hints of deviations from the Standard Model that require further investigation, and investigated the quark-gluon plasma that filled the early universe with unprecedented detail. They also observed many rare particle processes, made increasingly precise measurements of Standard Model phenomena, and broke new ground in the search for new particles beyond those predicted by the Standard Model, including particles that may constitute dark matter which makes up most of the mass of the universe.

The results of this research add important elements to our understanding of fundamental physics. ‘Discoveries in particle physics do not necessarily mean new particles,’ said CERN Director of Research and Computing, Joachim Mnich. “The LHC results obtained over a decade of machine operation have allowed us to extend a much wider network in our research, setting firm limits to the possible extensions of the Standard Model, and proposing new research and data analysis techniques. ”

Remarkably, all LHC results achieved so far are based on only 5% of the total amount of data the collider will provide over its lifetime. “With this ‘small’ sample, the LHC has made great strides in our understanding of elementary particles and their interactions,” said CERN theorist Michelangelo Mangano. “And although all the results obtained so far are consistent with the Standard Model, there is still a lot of room for new phenomena beyond what is predicted by this theory.”

“The Higgs boson itself may indicate new phenomena, some of which may be responsible for dark matter in the universe,” CMS spokesman Luca Malgeri said. “ATLAS and CMS are doing a lot of research to probe all forms of unexpected processes involving the Higgs boson.”

The journey that still awaits us
What remains to be learned about the Higgs field and the Higgs boson ten years later? A lot. Does the Higgs field also give mass to lighter fermions or could another mechanism be at play? Is the Higgs boson an elementary or composite particle? Can 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 he have twins or parents?

Finding the answers to these and other intriguing questions will not only deepen our understanding of the universe on the smallest scale, but can also help unravel some of the greatest mysteries of the universe as a whole, like how he became as he is. and what his ultimate fate might be. The self-interaction of the Higgs boson, in particular, could hold the keys to a better understanding of the imbalance between matter and antimatter and the stability of the vacuum in the universe.

Since the discovery of the Higgs boson a decade ago, members of UCSB’s 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 also used Higgs bosons as a tool to search for new physical phenomena. The effort at UCSB is broad, with many postdocs, graduate students, and undergraduates involved in the effort to build the detector, operate and upgrade it, develop the software algorithms, data analysis and publication of results. The UCSB effort was funded throughout the discovery of the Higgs, and since then, by the US Department of Energy’s Office of Science and the National Science Foundation.

While the answers to some of the new questions could be provided by data from the imminent third LHC run or the major collider upgrade, the High-Luminosity LHC, from 2029, the answers to other puzzles are considered beyond the reach of the LHC, requiring a future “Higgs factory”. This is why CERN and its international partners are studying the technical and financial feasibility of a much larger and more powerful machine, the future circular collider, in response to a recommendation made in the latest update of the European strategy for particle physics.

“High-energy colliders remain the most powerful microscope at our disposal for exploring nature at the smallest scales and uncovering the fundamental laws that govern the universe,” said Gian Giudice, Head of CERN’s Theory Department. “In addition, these machines also bring huge societal benefits.”

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

-Sarah Charley, CERN

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