Since its revelation in 2012, the Higgs boson has been the subject of intense scrutiny by physicists. New findings presented at the 2025 European physical Society conference on High Energy physics (EPS-HEP) by the ATLAS Collaboration detail the observation of two extremely rare decay modes of the Higgs boson, utilizing data from Run 3 of the Large Hadron Collider (LHC). These investigations provide valuable insights into the Higgs boson’s behavior and its alignment with the Standard Model.

One focus was the Higgs-boson decay into a pair of muons (H→μμ). Though rare, occurring in approximately 1 out of every 5000 Higgs decays, this process is invaluable for studying the Higgs interaction with second-generation fermions and understanding the origin of mass across different generations. The second process examined was the Higgs-boson decay into a Z boson and a photon (H→Zγ), where the Z boson subsequently decays into electron or muon pairs.This decay is particularly interesting because it occurs via an intermediate “loop” of virtual particles, potentially revealing hints of physics beyond the Standard Model if new particles contribute to this loop.

The Hunt for Rare Decays

Identifying these rare decays poses a considerable challenge.For H→μμ, researchers searched for a slight excess of events concentrated around a muon-pair mass of 125 GeV (the mass of the Higgs boson). This signal is easily obscured by the numerous muon pairs produced through other processes, creating significant background noise.

The H→Zγ decay is even more arduous to isolate, compounded by the fact that the Z boson decays into detectable leptons onyl about 6% of the time. The operational conditions of LHC Run 3,wich involve more overlapping collisions,further complicate the search,as particle jets can mimic real photons.

To enhance the sensitivity of their searches, ATLAS physicists combined data from the first three years of Run-3 (165 fb^-1 collected between 2022-2024) with the complete Run-2 dataset (140 fb^-1 from 2015-2018). They also refined their methods for modeling background processes, categorized recorded events by specific Higgs-production modes, and improved their event-selection techniques to maximize the detection of genuine signals.

Evidence and Enhanced Sensitivity

In the previous search for H→μμ using the full Run-2 dataset, the ATLAS Collaboration observed an initial indication of this process at the level of 2 standard deviations. The CMS result showed a comparable observed (expected) significance of 3 (2.5) standard deviations.Now, with the combined Run-2 and run-3 datasets, the ATLAS Collaboration has found evidence for H→μμ with an observed (expected) significance over the background-only hypothesis of 3.4 (2.5) standard deviations.

Regarding the H→Zγ process, a prior combined analysis by ATLAS and CMS using Run-2 data found evidence of this decay mode, reporting an observed (expected) excess over the background-only hypothesis of 3.4 (1.6) standard deviations. The latest ATLAS result, incorporating Run-2 and Run-3 data, reported an observed (expected) excess over the background-only hypothesis of 2.5 (1.9) standard deviations, representing the most stringent expected sensitivity to date for