CMS reports hint of rare Higgs decay to Z boson and photon

The CMS collaboration has reported a new search for the elusive Higgs boson decay into a Z boson and a photon. The analysis uses proton-proton collision data from the Large Hadron Collider (LHC) at center-of-mass energies of 13 and 13.6 TeV.

This decay, denoted H → Zγ, is a rare, loop-induced process predicted by the Standard Model of particle physics. Observing it provides a important test of the Higgs boson's properties and potential interactions with new particles. The CMS team analyzed data corresponding to a total integrated luminosity of 200 inverse femtobarns, recorded by the experiment.

The search specifically looked for events where the Z boson subsequently decays into a pair of electrons or muons. The analysis was designed to separately target and optimize sensitivity for Higgs bosons produced through three distinct mechanisms: gluon-gluon fusion, vector boson fusion, and associated production with other particles.

Signal Extraction and Results

The signal was extracted using a simultaneous statistical fit to the invariant mass distributions of the lepton-pair and photon system across the different production channels and event categories. The key result is the measured signal strength, represented by the parameter μ.

This parameter is the ratio of the observed product of the Higgs production cross section and the H → Zγ decay branching fraction to the value predicted by the Standard Model. For a Higgs boson mass of 125.38 GeV, the observed signal strength was μ = 1.10, with an uncertainty range from +0.52 to -0.61. The expected signal strength, based on the Standard Model and the experiment's sensitivity, was 1.00 with an uncertainty of +0.49 to -0.46.

The observed result is consistent with the Standard Model expectation of μ = 1. The statistical significance of the observed signal was found to be 1.9 standard deviations. The analysis had an expected sensitivity of 2.3 standard deviations.

Implications of the Findings

A significance of 5 standard deviations is the gold standard required to claim a discovery in particle physics. The result of 1.9 standard deviations reported by CMS does not reach this threshold. Therefore, the collaboration states that the findings are compatible with both the background-only hypothesis and the presence of a signal at the level predicted by the Standard Model.

The measurement provides the most sensitive constraint to date on this particular Higgs boson decay channel. The paper notes that the analysis separately considered the different Higgs production modes to maximize the search's sensitivity. The observed signal strength and its associated uncertainty offer a valuable data point for global fits of Higgs boson couplings, which test for deviations from the Standard Model that could hint at new physics.

The search continues as the LHC collects more data, with future runs expected to improve the precision of this measurement. The CMS collaboration's paper is publicly available on the arXiv preprint server.