Search for Higgs boson decays to a Z boson and a photon in proton-proton collisions at √s = 13 TeV

A bstract Results are presented from a search for the Higgs boson decay H → Zγ, where Z → ℓ + ℓ − with ℓ = e or μ. The search is performed using a sample of proton-proton (pp) collision data at a center-of-mass energy of 13 TeV, recorded by the CMS experiment at the LHC, corresponding to an integrated luminosity of 138 fb − 1 . Events are assigned to mutually exclusive categories, which exploit differences in both event topology and kinematics of distinct Higgs production mechanisms to enhance signal sensitivity. The signal strength μ , defined as the product of the cross section and the branching fraction [σ (pp→ H)B(H→ Z )] σpp→HBH→Zγ relative to the standard model prediction, is extracted from a simultaneous fit to the ℓ + ℓ − γ invariant mass distributions in all categories and is measured to be μ = 2 . 4 ± 0 . 9 for a Higgs boson mass of 125.38 GeV. The statistical significance of the observed excess of events is 2.7 standard deviations. This measurement corresponds to [σ (pp→ H)B(H→ Z )]=0.21± 0.08 σpp→HBH→Zγ=0.21±0.08 pb. The observed (expected) upper limit at 95% confidence level on μ is 4.1 (1.8), where the expected limit is calculated under the background-only hypothesis. The ratio of branching fractions B(H→ Z )/B(H→ ) BH→Zγ/BH→γγ is measured to be 1.5-0.6+0.7 1.5−0.6+0.7 , which agrees with the standard model prediction of 0 . 69 ± 0 . 04 at the 1.5 standard deviation level.