New ATLAS Analysis Reveals Strong Evidence of Higgs Boson Decay Into Muon-Antimuon Pair
Higgs Boson Research Advances With New 3.4σ Findings
Evidence Builds at CERN's Large Hadron Collider
Although theorized for decades, the Higgs boson was only confirmed in 2012 at CERN's Large Hadron Collider (LHC). Since that landmark discovery, scientists have continued to scrutinize it at the LHC. A new analysis from CERN researchers now brings together data from the last two runs of ATLAS—one of the LHC's main detectors—to present evidence that the Higgs boson can decay into a muon-antimuon pair.
Published in Physical Review Letters, the study reports a combined significance of 3.4 standard deviations above background noise, surpassing the previous 3.0 standard-deviation result from CMS.
The Higgs Mechanism and Particle Mass
Understanding Mass Through Higgs Field Interactions
Physicists have strong motivation for pursuing this specific Higgs boson decay. In the peculiar realm of particle physics—where subatomic entities are defined by their "flavours" and particles annihilate upon meeting their antiparticles—a particle's mass emerges through its interaction with the Higgs field.
Under the Standard Model, the Higgs boson represents an excitation of this field and, through interaction, effectively "grants" mass to particles. While the underlying process is more nuanced, observing such interactions reinforces the idea that the Higgs mechanism governs particle mass and confirms the validity of the Standard Model.
The strength of these interactions scales with the particle's mass: heavier particles couple more strongly, while massless ones do not interact with the Higgs field at all.
In earlier experiments, physicists successfully identified interactions involving heavier subatomic particles—the so-called "third-generation fermions," such as top quarks, bottom quarks and tau leptons—via processes known as Yukawa couplings.
"The Yukawa interactions between the Higgs boson and third-generation charged fermions are now well established. Its coupling to second-generation fermions, however, remains to be definitively proven," the researchers note.
Higgs Boson Coupling to Second-Generation Fermions
ATLAS Data Strengthens Rare Decay Observation
Muons belong to the family of second-generation fermions lighter than those in the third generation, yet still heavier than first-generation particles such as electrons.
Observing a Higgs boson decay into a muon-antimuon pair is therefore a crucial test of the Higgs mechanism for this generation of particles, and the proton-proton collisions recorded by ATLAS offer an ideal environment in which to spot it.
Although this decay is exceptionally uncommon, the latest ATLAS results provide fresh support for its occurrence, with findings consistent with Standard Model predictions. Future runs at ATLAS and CMS could strengthen this evidence further and perhaps enable studies of Higgs couplings to even lighter particles.


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