First evidence of rare Higgs boson decay at CERN’s Large Hadron Collider

For the first time since the discovery of the Higgs boson made international headlines in 2012, physicists have witnessed the Higgs boson decay into a Z boson and a photon.

The observations, part of the ATLAS and CMS experiments at CERN’s Large Hadron Collider (LHC) on the French-Swiss border, may provide indirect evidence for particles beyond those predicted by the Standard Model of particle physics.

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Higgs bosons are the quantum excitation of the Higgs field which interacts with other elementary particles to give them their mass. Z bosons are the electrically neutral carrier of the weak force (responsible for radioactive decay) and photons (light particles) carry the electromagnetic force.

The decay process was announced at the Large Hadron Collider Physics conference in Belgrade.

Like a Higgs boson decay into two photons, the newly-observed decay process is not direct. The decay proceeds via an intermediate “loop” of “virtual” particles that come into and out of existence and cannot be directly detected.

Some of these virtual particles may be new and unaccounted for in the Standard Model.

2016 measurements of the Higgs boson’s mass placed it at 125.35 GeV (nearly 250 billion times heavier than an electron) with a precision of 0.1 GeV, an uncertainty of just 0.1 percent.

For this mass, which sits nicely in predictions made by the Standard Model, approximately 0.15 percent of Higgs bosons will decay into a Z boson and a photon. But theories that go beyond the Standard Model give a different decay rate.

Previous proton-proton collision experiments at the LHC have independently shown that these Z boson decays occur in about 6.6 percent of cases.

“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 the individual results of ATLAS and CMS, we have made a step forward towards unravelling yet another riddle of the Higgs boson.”

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“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 run of the LHC and the future High-Luminosity LHC, we will be able to improve the precision of this test and probe ever rarer Higgs decays.”