Evidence for Higgs boson decays to a low-mass dilepton system and a photon in pp collisions at $\sqrt{s} =$ 13 TeV with the ATLAS detector

TL;DR

This paper reports evidence for Higgs boson decays into a low-mass dilepton pair and a photon using 13 TeV proton-proton collision data, with a significance of 3.2 sigma, indicating a potential new decay channel.

Contribution

First evidence for the Higgs decay into a dilepton pair and a photon at the LHC, with detailed measurement of the signal strength and production cross-section.

Findings

Evidence for $H ightarrow ext{dilepton} + ext{photon}$ decay with 3.2 sigma significance.

Measured signal strength parameter $ = 1.5  0.5.

Higgs production cross-section times branching ratio is 8.7 +2.8/-2.7 fb.

Abstract

A search for the Higgs boson decaying into a photon and a pair of electrons or muons with an invariant mass $m_{\ell\ell} < 30$ GeV is presented. The analysis is performed using 139 fb$^{-1}$ of proton-proton collision data, produced by the LHC at a centre-of-mass energy of 13 TeV and collected by the ATLAS experiment. Evidence for the $H \rightarrow \ell \ell \gamma$ process is found with a significance of 3.2$\sigma$ over the background-only hypothesis, compared to an expected significance of 2.1$\sigma$. The best-fit value of the signal strength parameter, defined as the ratio of the observed signal yield to the one expected in the Standard Model, is $\mu = 1.5 \pm 0.5$. The Higgs boson production cross-section times the $H \rightarrow\ell\ell\gamma$ branching ratio for $m_{\ell\ell} <$ 30 GeV is determined to be 8.7 $^{+2.8}_{-2.7}$ fb.