First evidence of a rare Higgs boson decay

2023-05-29 11:13:03

ATLAS (left) and CMS (right) candidate events for a Higgs boson decaying into a Z boson and a photon, with the Z boson decaying into a muon pair. – CERN

MADRID, 29 May. (EUROPA PRESS) –

Physicists from the ATLAS and CMS collaborations at CERN’s LHC accelerator have found the first evidence of a rare Higgs boson decay. The particle that confers mass to matter.

It is a rare process in which the Higgs boson decays into a Z boson, the electrically neutral carrier of the weak force, and a photon, the 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, reports CERN it’s a statement.

The decay of the Higgs boson into a Z boson and one photon is similar to the decay into two photons. In these processes, the Higgs boson does not directly decay 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, as yet undiscovered 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 decay rate. Therefore, to measure the rate of decay it provides valuable information both on physics beyond the Standard Model and on the nature of the Higgs boson.

Previously, using data from proton-proton collisions at the LHC, ATLAS and CMS independently conducted comprehensive searches for the decay of the Higgs boson into a Z boson and a photon. Both searches used similar strategies, identifying the Z boson through 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 uniform background of events, in the combined mass distribution of the decay products. To improve sensitivity to decay, ATLAS and CMS exploited the most frequent modes in which the Higgs boson is produced and classified the events based on 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 your search results. By combining the data sets collected by both experiments during the second LHC activation, which took place between 2015 and 2018, the collaborations have significantly increased the statistical precision and scope of their searches.

This collaborative effort resulted in the first evidence for the decay of the Higgs boson into a Z boson and a photon. The result is statistically significant at 3.4 standard deviations, which is below 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, making the search for rare Higgs decays a high priority,” says ATLAS physics coordinator Pamela Ferrari. “Through a meticulous combination of individual ATLAS and CMS results, we have taken a step forward in unraveling another Higgs boson puzzle.”

“The existence of new particles could have very significant effects on the rare Higgs decay modes“, says CMS physics coordinator, Florencia Canelli. “This study is a powerful test of the standard model. With the ongoing third run of the LHC and the future high-luminosity LHC, we will be able to improve the precision of this test and probe increasingly rare Higgs decays.”