ATLAS and competitor CMS together find sufficient clue to rare Higgs decay

17 July 2023

A minute fraction of Higgs particles produced at the LHC accelerator at CERN appears to decay into a Z particle and a photon. Clues to this come from a new combination of measurements from ATLAS and the competing CMS experiment.

Earlier, both experiments at the LHC accelerator reported separate measurements of the extremely rare decay of the Higgs particle to two electrons or muons (created from a Z particle) and a photon.

Those measurements were not yet statistically strong enough for firm conclusions. Combining all the data from ATLAS and CMS now yields a result with 3.4 sigma. For physicists, that is enough to speak of a clue.

Candidate event in the ATLAS detector in which two muons (red) and a photon (green) are created from a Higgs particle.

The measurements used are from the second measurement period of the LHC accelerator, so-called Run-2 between 2015-2018. It is expected that with even more data from the current LHC Run-3, the decay of Higgs to Z and a photon will be conclusively seen.

Those analyses are already being worked on, said Nicolo de Groot of Radboud University and Nikhef, who was involved in the work with a PhD student. The earlier ATLAS result had 2.2 sigma relevance.

Measurements of a rare higgs decay to Z and a photon has spearheaded experiments at CERN since the higgs particle was found there in 2012. The Higgs particle is intimately involved in the mechanism that gives elementary particles their mass according to the Standard Model.

Rare decay processes such as to Z and a photon are important because they can reveal any deviations from the Standard Model. This is because they can only occur through subtle processes, which are sensitive to any unknown particles or forces.

According to the Standard Model, about 0.15 percent of the Higgs particles decay to Z and a photon. This makes the decay very difficult to measure. In the data, it would create a narrow peak against a smooth background of electrons, muons and photons.

The measurements are important, though, says physics coordinator at ATLAS and Nikhef researcher Pamela Ferrari in a commentary in this month’s CERN Courier. Some theoretical extensions of the Standard Model predict other decay rates, for example, if the Higgs were composed of even smaller elements.

There is no evidence of that as yet. The new combined measurements seem mostly consistent with the predicted value according to the Standard Model.