Search for Higgs boson decays to a Z boson and a photon in proton-proton collisions at $\sqrt{s}$ = 13 TeV

TL;DR

This study searches for the Higgs boson decay to Z gamma using 13 TeV proton-proton collision data, finding a slight excess with a 2.7 sigma significance and measuring the decay rate relative to the standard model.

Contribution

First measurement of Higgs to Z gamma decay rate at 13 TeV, with detailed categorization and analysis of signal strength and branching ratio.

Findings

Signal strength μ = 2.4 ± 0.9 with 2.7 sigma significance

Measured cross section times branching ratio: 0.21 ± 0.08 pb

Branching ratio ratio B(H→Zγ)/B(H→γγ) = 1.5^{+0.7}_{-0.6}

Abstract

Results are presented from a search for the Higgs boson decay H $\to$ Z$\gamma$, where Z $\to$ $\ell^+\ell^-$ with $\ell$ = e or $\mu$. 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 $\mu$, defined as the product of the cross section and the branching fraction [$\sigma($pp $\to$ H$)\mathcal{B}($H $\to$ Z$\gamma)$] relative to the standard model prediction, is extracted from a simultaneous fit to the $\ell^+\ell^-\gamma$ invariant mass distributions in all categories and is found to be $\mu$ = 2.4 $\pm$ 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 $\sigma($pp $\to$ H$)\mathcal{B}($H $\to$ Z$\gamma)$ = 0.21 $\pm$ 0.08 pb. The observed (expected) upper limit at 95% confidence level on $\mu$ is 4.1 (1.8). The ratio of branching fractions $\mathcal{B}($H $\to$ Z$\gamma) / \mathcal{B}($H $\to$ $\gamma\gamma)$ is measured to be 1.5 $^{+0.7}_{-0.6}$, which agrees with the standard model prediction of 0.69 $\pm$ 0.04 at the 1.5 standard deviation level.