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The discovery of the Higgs boson at CERN’s Large Hadron Collider (LHC) in 2012 marked a milestone in particle physics. Since then, the ATLAS and CMS collaborations have diligently studied the properties of this unique particle and sought to establish the different ways in which it is produced and decays into other particles.
At the Large Hadron Collider Physics (LHCP) conference this week, ATLAS and CMS report how they teamed up to find the first evidence for the rare process in which the Higgs boson decays into a Z boson, the electrically neutral carrier of the weak force, and a photon, carrier of the electromagnetic force. This decay of the Higgs boson could provide indirect evidence for the existence of particles beyond those predicted by the Standard Model of particle physics.
The decay of the Higgs boson into a Z boson and a photon is similar to that of a decay into two photons. In these processes, the Higgs boson does not decay directly into these pairs of particles. Instead, decays proceed through an intermediate “loop” of “virtual” particles that appear and disappear and cannot be directly detected. These virtual particles could include new, yet unknown particles that interact with the Higgs boson.
The Standard Model predicts that if the Higgs boson has a mass of about 125 billion electron volts, about 0.15% of the Higgs bosons will decay into a Z boson and a photon. But some theories that extend the Standard Model predict a different rate of decay. Measuring the decay rate therefore provides valuable insight into physics beyond the Standard Model and into the nature of the Higgs boson.
Previously, using data from proton-proton collisions at the LHC, ATLAS and CMS independently conducted extensive research into the decay of the Higgs boson into a Z boson and a photon. Both searches used similar strategies, identifying the Z boson by its decays into electron pairs or muons, heavier versions of electrons. These Z boson decays occur in about 6.6% of cases.
In these searches, the collision events associated with this Higgs boson decay (the signal) would be identified as a narrow peak, against a smooth background of events, in the distribution of the combined mass of the decay products. To improve decay sensitivity, ATLAS and CMS exploited the most frequent modes of Higgs boson production and classified events according to the characteristics of these production processes. They also used advanced machine learning techniques to better distinguish between signal and background events.
In a new study, ATLAS and CMS have now joined forces to maximize the outcome of their research. By combining the datasets collected by the two experiments during the LHC’s Run 2, which ran between 2015 and 2018, the collaborations have significantly increased the statistical accuracy and scope of their research.
This collaborative effort resulted in the first evidence for the decay of the Higgs boson into a Z boson and a photon. The result has a statistical significance of 3.4 standard deviations, which is less than the conventional requirement of 5 standard deviations to claim an observation. The measured signal rate is 1.9 standard deviations above the Standard Model prediction.
“Each particle has a special relationship with the Higgs boson, which makes the search for rare Higgs decays a high priority,” explains Pamela Ferrari, ATLAS Physics Coordinator. “Thanks to a meticulous combination of individual results from ATLAS and CMS, we have taken a step towards solving another Higgs boson puzzle.”
“The existence of new particles could have very significant effects on rare Higgs decay modes,” says CMS physics coordinator Florencia Canelli. “This study is a powerful test of the Standard Model. With the ongoing third LHC run and the future High-Luminosity LHC, we will be able to improve the accuracy of this test and probe increasingly rare Higgs decays.