Since the discovery of the Higgs boson in 2012, physicists have made considerable progress in exploring its properties. Does this mean the subject is closed? Far from it!
In new results presented at the European Physical Society Conference on High Energy Physics (EPS-HEP) in Marseille this year, the ATLAS collaboration focused on an exceptionally rare decay of the Higgs boson, using data collected during phase 3 (Run 3) of the Large Hadron Collider (LHC). This study provides in-depth insights into how the behavior of the Higgs boson aligns with the predictions of the Standard Model.
The ATLAS detector at the Large Hadron Collider (LHC).
Credit: CERN
The ATLAS team at Irfu has long experience in Higgs boson research and made a significant contribution to the discovery of the Higgs boson and the measurement of its properties. The team is heavily involved in this analysis of Higgs boson decay into two muons, thanks to its major contribution and leading role in muon reconstruction and identification, ensuring the quality of data collected in Run 3, and providing guidelines for physics analysis.
This contribution enabled the use of Run 3 data (the three-year period 2022-2024) and, by combining it with Run 2 data (2015-2018 period), led to the observation of this rare decay.
The process studied is the decay of the Higgs boson into a pair of muons (H→μμ). Despite its rarity—occurring in only about one in 5000 Higgs decays—this process offers the best opportunity to study the Higgs interaction with second-generation fermions and shed light on the origin of mass across different generations.
Searching for needles in a haystack
Identifying this rare decay is a real challenge. For H→μμ, researchers looked for a small excess of events clustering near a muon pair mass of 125 GeV (the mass of the Higgs boson). This signal can easily be hidden behind the thousands of muon pairs produced by other processes ("background noise").
To increase the sensitivity of their search, ATLAS physicists combined the first three years of Run 3 data (165 fb
-1, collected between 2022 and 2024) with the full Run 2 dataset (140 fb
-1, from 2015 to 2018). They also developed a sophisticated method to better model background processes, classify recorded events according to specific Higgs boson production modes, and improve their event selection techniques to maximize the likelihood of spotting genuine signals.
The figure below shows the mass distribution of muon pairs obtained from data collected between 2022 and 2024 and combined for all categories.
Invariant mass spectrum of two muons of opposite charges observed in Run 3 data, for all analysis categories combined. The probability density functions (pdf) of the background and signal are obtained from the combined fit of all categories to Run 3 data, corresponding to a signal strength of μ = 1.6 ±0.6. The lower panel shows the fitted signal pdf, normalized to the best-fit value of the signal, and the difference between the observed data and the background model. The error bars represent the statistical uncertainties of the data.
Image: ATLAS Collaboration/CERN Highlighting
In the previous search for H→μμ using the full Run 2 dataset, the ATLAS collaboration saw its first hint of this process at the level of 2 standard deviations (i.e., the hypothesis that the observed result is due only to a background fluctuation and not to the presence of H→μμ signal is rejected at two standard deviations, corresponding to about a 1 in 22 chance). The comparable CMS result achieved an observed (expected) significance of 3 (2.5) standard deviations.
Today, with the combined Run 2 and Run 3 datasets, the ATLAS collaboration has observed the H→μμ process with an observed (expected) significance relative to the background-only hypothesis of 3.4 (2.5) standard deviations. This means that the probability of the result being a statistical fluctuation is less than 1 in 1500!
These results were made possible by the excellent and extensive dataset provided by the LHC, the efficiency and outstanding performance of the ATLAS experiment, and the use of new analysis techniques. With more data on the horizon, the exploration journey continues!