Atlas Collaboration Narrows Search for Rare Higgs Decays to Explore New Physics

The ATLAS collaboration presented analyses at the 2025 European Physical Society Conference on High Energy Physics, combining data from LHC Runs 2 and 3 to investigate exceptionally rare Higgs-boson decays. Researchers focused on the Higgs decaying into muon pairs and a Z boson with a photon, processes occurring in approximately 1 in 5000 Higgs decays and with the Z boson decaying into electrons or muons roughly 6% of the time, respectively. Previous analyses using Run 2 data indicated a potential signal at 2 standard deviations for ATLAS and 3 standard deviations (with an expected 2.5) for the CMS collaboration; further analysis of the combined dataset is now in progress.

Rare Higgs Decays Under Scrutiny

The ATLAS collaboration has focused on two exceptionally rare Higgs-boson decays to probe fundamental aspects of particle physics and potentially reveal physics beyond the Standard Model. One analysis examined the decay of the Higgs boson into a pair of muons (H), an event occurring in approximately one in every 5000 Higgs decays, allowing researchers to study the interaction of the Higgs boson with second-generation fermions and contribute to understanding the origin of mass across particle generations. Currently, observed interactions between the Higgs boson and matter particles are limited to particles from the heaviest, third generation: the tau lepton, and the top and bottom quarks.

The second area of investigation centred on the Higgs-boson decay into a Z boson and a photon (HZ), where the Z boson subsequently decays into electron or muon pairs. This decay is of particular interest as it proceeds via a loop of virtual particles, meaning the presence of previously unknown particles contributing to this loop could provide evidence for physics beyond the Standard Model. Identifying these rare decays is challenging, particularly given the difficulty in isolating the signals from the substantial background noise generated by other particle interactions.

To improve the sensitivity of their searches, ATLAS physicists combined data from the first three years of LHC Run3 with the complete LHC Run 2 dataset. This combined dataset was analysed alongside improvements in modelling background processes, categorisation of events by specific Higgs-production modes, and refinement of event-selection techniques. Previous analyses using the complete Run 2 data indicated a potential signal for the H decay at the level of 2 standard deviations for ATLAS, while the CMS collaboration reported a significance of 3 standard deviations with an expected value of 2.5 standard deviations; further analysis of the combined dataset is currently in progress as part of ongoing Higgs boson research.

Precision Measurements and Data Combination

The difficulty in isolating the HZ decay signal stemmed from the Z boson decaying into electrons or muons only approximately 6% of the time, and photons frequently being mimicked by particle jets. To enhance the sensitivity of their searches, ATLAS physicists combined the first three years of LHC Run3 data with the full LHC Run 2 data. Refinements were also implemented to better model background processes, categorise recorded events by specific Higgs-production modes, and refine event-selection techniques. Previous searches for H, utilising the full Run 2 data set, had indicated a potential signal at the level of 2 standard deviations for the ATLAS collaboration, while the CMS collaboration reached a significance of 3 standard deviations with an expected value of 2.5 standard deviations. Further analysis is currently underway with the combined Run 2 and Run 3 data, contributing to ongoing Higgs boson research.

Challenges in Signal Isolation

Identifying these rare decays presents considerable challenges, as the H decay signal requires discerning a small excess of events clustered near a muon-pair mass of 125 GeV from the thousands of muon pairs produced through other processes. The HZ decay proved even more difficult to isolate, with the Z boson decaying into electrons or muons only approximately 6% of the time and photons frequently mimicked by particle jets.

To enhance the sensitivity of their searches, ATLAS physicists combined the first three years of LHC Run3 data with the full LHC Run 2 data, alongside implementing a sophisticated method to better model background processes. Recorded events were categorised by specific Higgs-production modes, and event-selection techniques were refined to improve signal isolation. Previous searches for H, utilising the full Run 2 data set, had indicated a potential signal at the level of 2 standard deviations for the ATLAS collaboration, while the CMS collaboration reached a significance of 3 standard deviations with an expected value of 2.5 standard deviations. Now, with the combined Run 2 and Run 3 data, further analysis is underway, contributing to ongoing Higgs boson research.