Higgs Boson Muons

Physicists from the ATLAS (A Toroidal LHC ApparatuS) and CMS (Compact-Muon-Solenoid) collaborations at CERN’s Large Hadron Collider have reported strong evidence for the Higgs boson decay to a pair of muons.

Candidate event displays of a Higgs boson decaying into two muons as recorded by CMS (left) and ATLAS (right). Image credit: CERN.

In 2012, CERN physicists reported the discovery of a new boson with a mass near 125 GeV and properties compatible with those expected for the Higgs boson, a fundamental particle first proposed in 1964.

In the Standard Model, the Higgs boson is a spin-zero particle predicted to arise from the Higgs field which is responsible for electroweak symmetry breaking.

So far, the physicists have observed the Higgs boson decays into different types of gauge bosons such as the W and Z bosons and photons; heavier fermions such as tau leptons; and interactions with the heaviest quarks, the top and bottom.

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 Large Hadron Collider (LHC).

The physicists from the CMS Collaboration achieved evidence of the Higgs boson decay to a muon pair with 3 sigma, which means that the chance of seeing this decay from statistical fluctuation is less than one in 700.

ATLAS Collaboration’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.

What makes these experiments challenging is that 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.

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.

In addition, the sensitivity of the analyses was improved through methods such as sophisticated background modeling 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.

“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,” said Dr. Roberto Carlin, spokesperson for the CMS Collaboration.

“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.”

“The measurements of the Higgs boson’s properties have reached a new stage in precision and rare decay modes can be addressed,” said Dr. Karl Jakobs, spokesperson for the ATLAS Collaboration.

“These achievements rely on the large LHC dataset, the outstanding efficiency and performance of the ATLAS detector and the use of novel analysis techniques.”

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CMS Collaboration. 2020. Measurement of Higgs boson decay to a pair of muons in proton-proton collisions at s√=13TeV. CENR, report # CMS-PAS-HIG-19-006

ATLAS Collaboration. 2020. A search for the dimuon decay of the Standard Model Higgs boson with the ATLAS detector. Phys. Lett. B, in press; arXiv: 2007.07830