In 2012, the discovery of the Higgs boson at CERN’s Large Hadron Collider (LHC) marked a major achievement in particle physics. Since then, the ATLAS and CMS collaborations have been diligently studying this unique particle, investigating its properties, and searching for different ways it decays into other particles.
During this week’s Large Hadron Collider Physics (LHCP) conference, ATLAS and CMS announced their joint effort to find the first evidence of a rare process involving the decay of the Higgs boson. They observed the decay where the Higgs boson transforms into a Z boson (an electrically neutral particle carrying the weak force) and a photon (the carrier of the electromagnetic force). This particular decay of the Higgs boson could indirectly indicate the existence of particles beyond those predicted by the Standard Model of particle physics.
The decay process of the Higgs boson into a Z boson and a photon is similar to its decay into two photons. However, instead of directly decaying into these particle pairs, the Higgs boson undergoes an intermediate “loop” involving “virtual” particles. These virtual particles appear and disappear rapidly and cannot be directly detected. Among these virtual particles, there might be undiscovered particles that interact with the Higgs boson.
According to the Standard Model, if the Higgs boson has a mass of around 125 billion electronvolts, approximately 0.15% of Higgs bosons should decay into a Z boson and a photon. However, alternative theories extending the Standard Model predict different decay rates. Measuring the decay rate is thus crucial for gaining insights into physics beyond the Standard Model and understanding the nature of the Higgs boson.
Previously, both the ATLAS and CMS experiments independently conducted extensive searches for the decay of the Higgs boson into a Z boson and a photon using proton-proton collision data from the LHC. The searches employed similar strategies, identifying the Z boson through its decays into pairs of electrons or muons (heavier counterparts of electrons). These Z boson decays occurred in approximately 6.6% of the cases.
In these searches, collision events associated with the Higgs boson decay under investigation (the signal) would be identified as a narrow peak in the combined mass distribution of the decay products, which is superimposed on a smooth background of events. To enhance the sensitivity to the decay, both ATLAS and CMS categorized events based on the characteristics of the Higgs boson’s production processes, which occur most frequently. They also employed advanced machine-learning techniques to further distinguish between signal and background events.
In a new study, the ATLAS and CMS collaborations joined forces to maximize the potential of their search. By combining the datasets collected by both experiments during the LHC’s second run (2015-2018), the collaborations significantly improved the statistical precision and reach of their searches.
This collaborative effort led to the first evidence of the Higgs boson decaying into a Z boson and a photon. The result has a statistical significance of 3.4 standard deviations, falling short of the conventional requirement of 5 standard deviations for claiming an observation. However, the measured signal rate is 1.9 standard deviations above the prediction of the Standard Model.
Pamela Ferrari, the physics coordinator of ATLAS, emphasizes the importance of exploring rare Higgs decays, noting that each particle has a special relationship with the Higgs boson. By meticulously combining the individual results of ATLAS and CMS, they have taken a step forward in unraveling another mystery of the Higgs boson.
Florencia Canelli, the physics coordinator of CMS, highlights that the existence of new particles could have significant implications for rare Higgs decay modes.