“Particle at the End of the Universe” –LHC Unveils Phenomenon of Pivotal Importance for Fundamental Physics

Higgs Boson


The detection of the “Higgs” in 2012  -described by Caltech physicist Sean Carroll as “the particle at the end of the universe” –was the physics equivalent of the discovery of DNA. Higgs bosons have the capacity to share space because they are more like a force than a thing in the way we normally think of “things” or “particles”. It’s a vibration in the Higgs field, just as a photon of light is a vibration in the electromagnetic field. Bosons are entities that have effects; they carry the forces (strong, weak, gravitational or electromagnetic) described by the Standard Model in physics, making them what physicists call force-carrying particles.

“Hidden Symmetries in Underlying Laws of Physics”

The four famous forces of nature arise from symmetries sometimes hidden and therefore invisible to us– one of the startling insights of twentieth-century physics. Physicists often say that hidden symmetries are “broken,” writes Carroll in The Particle At the End of the Universe, “but they’re still there in the underlying laws of physics. The weak nuclear force, in particular, is based on a certain kind of symmetry. If that symmetry were unbroken, it would be impossible for elementary particles to have mass. They would all zip around at the speed of light. Therefore, the symmetry of the weak interactions must be broken. When space is completely empty, most fields are turned off, set to zero. If a field is not zero in empty space, it can break a symmetry. In the case of the weak interactions, that’s the job of the Higgs field. Without it,” Carroll concludes, “the universe would be an utterly different place.”

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“An Elegant Mechanism”

On the surface, observes Carroll, “the weak interactions are a mess: The force-carrying bosons have different masses and charges, and different interaction strengths for different particles. Then we dig deeper, and an elegant mechanism emerges: a broken symmetry, hidden from our view by a field pervading space.”

Physicists at CERN have been studying the Higgs in order to probe the properties of this very special particle. The boson, produced from proton collisions at the Large Hadron Collider, disintegrates—referred to as decay—almost instantaneously into other particles. One of the main methods of studying the Higgs’ properties, reports CERN, is by analyzing how it decays into the various fundamental particles and the rate of disintegration.

New Results of Pivotal Importance for Fundamental Physics

At the 40th ICHEP conference, the ATLAS and CMS experiments announced new results which show that the Higgs boson decays into two muons. The muon is a heavier copy of the electron, one of the elementary particles that constitute the matter content of the Universe. While electrons are classified as a first-generation particle, muons belong to the second generation.

The physics process of the Higgs boson decaying into muons is a rare phenomenon as only about one Higgs boson in 5000 decays into muons. These new results have pivotal importance for fundamental physics because they indicate for the first time that the Higgs boson interacts with second-generation elementary particles.

CMS achieved evidence of this decay with 3 sigma, which means that the chance of seeing the Higgs boson decaying into a muon pair from statistical fluctuation is less than one in 700. ATLAS’s two-sigma result means the chances are one in 40. The combination of both results would increase the significance well above 3 sigma and provides strong evidence for the Higgs boson decay to two muons.

“CMS is proud to have achieved this sensitivity to the decay of Higgs bosons to muons, and to show the first experimental evidence for this process. The Higgs boson seems to interact also with second-generation particles in agreement with the prediction of the Standard Model, a result that will be further refined with the data we expect to collect in the next run,” said Roberto Carlin, spokesperson for the CMS experiment.

The Higgs boson, reports CERN, “is the quantum manifestation of the Higgs field, which gives mass to elementary particles it interacts with, via the Brout-Englert-Higgs mechanism. By measuring the rate at which the Higgs boson decays into different particles, physicists can infer the strength of their interaction with the Higgs field: the higher the rate of decay into a given particle, the stronger its interaction with the field. So far, the ATLAS and CMS experiments have observed the Higgs boson decays into different types of bosons such as W and Z, and heavier fermions such as tau leptons. The interaction with the heaviest quarks, the top and bottom, was measured in 2018. Muons are much lighter in comparison and their interaction with the Higgs field is weaker. Interactions between the Higgs boson and muons had, therefore, not previously been seen at the LHC.”

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Reach a New Stage in Precision

“This evidence of Higgs boson decays to second-generation matter particles complements a highly successful Run 2 Higgs physics program. The measurements of the Higgs boson’s properties have reached a new stage in precision and rare decay modes can be addressed. These achievements rely on the large LHC dataset, the outstanding efficiency and performance of the ATLAS detector and the use of novel analysis techniques,” said Karl Jakobs, ATLAS spokesperson.

Cosmic Mimicry

What makes these studies even more challenging is that, at the LHC, for every predicted Higgs boson decaying to two muons, there are thousands of muon pairs produced through other processes that mimic the expected experimental signature. The characteristic signature of the Higgs boson’s decay to muons is a small excess of events that cluster near a muon-pair mass of 125 GeV, which is the mass of the Higgs boson. Isolating the Higgs boson to muon-pair interactions is no easy feat. To do so, both experiments measure the energy, momentum and angles of muon candidates from the Higgs boson’s decay.

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In addition, the sensitivity of the analyses was improved through methods such as sophisticated background modelling strategies and other advanced techniques such as machine-learning algorithms. CMS combined four separate analyses, each optimized to categorize physics events with possible signals of a specific Higgs boson production mode. ATLAS divided their events into 20 categories that targeted specific Higgs boson production modes.

Consistent with Standard Model Predictions

The results, which are so far consistent with the Standard Model predictions, used the full data set collected from the second run of the LHC. With more data to be recorded from the particle accelerator’s next run and with the High-Luminosity LHC, the ATLAS and CMS collaborations expect to reach the sensitivity (5 sigma) needed to establish the discovery of the Higgs boson decay to two muons and constrain possible theories of physics beyond the Standard Model that would affect this decay mode of the Higgs boson.

Source: Measurement of Higgs boson decay to a pair of muons in proton-proton collisions at s√=13TeV: cds.cern.ch/record/2725423
A search for the dimuon decay of the Standard Model Higgs boson with the ATLAS detector: arxiv.org/abs/2007.07830

The Daily Galaxy, curated and editied by Sam Cabot, via CERN

Image credit: Shutterstock License

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