Abstract

This article presents searches for the Zγ decay of the Higgs boson and for narrow high-mass resonances decaying to Zγ, exploiting Z boson decays to pairs of electrons or muons. The data analysis uses 36.1 fb−1 of pp collisions at sqrt{s}=13 recorded by the ATLAS detector at the CERN Large Hadron Collider. The data are found to be consistent with the expected Standard Model background. The observed (expected — assuming Standard Model pp → H → Zγ production and decay) upper limit on the production cross section times the branching ratio for pp → H → Zγ is 6.6. (5.2) times the Standard Model prediction at the 95% confidence level for a Higgs boson mass of 125.09 GeV. In addition, upper limits are set on the production cross section times the branching ratio as a function of the mass of a narrow resonance between 250 GeV and 2.4 TeV, assuming spin-0 resonances produced via gluon-gluon fusion, and spin-2 resonances produced via gluon-gluon or quark-antiquark initial states. For high-mass spin-0 resonances, the observed (expected) limits vary between 88 fb (61 fb) and 2.8 fb (2.7 fb) for the mass range from 250 GeV to 2.4 TeV at the 95% confidence level.

Highlights

  • IntroductionThe background is mainly composed of non-resonant production of a Z boson in association with a photon (irreducible background), and of inclusive Z bosons in association with jets (Z+jets) events where a jet is misidentified as a photon (reducible background), and the relative contributions are determined using data as described below

  • Background modellingThe background is mainly composed of non-resonant production of a Z boson in association with a photon, and of inclusive Z bosons in association with jets (Z+jets) events where a jet is misidentified as a photon, and the relative contributions are determined using data as described below

  • The background is mainly composed of non-resonant production of a Z boson in association with a photon, and of inclusive Z+jets events where a jet is misidentified as a photon, and the relative contributions are determined using data as described below

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Summary

Introduction

The background is mainly composed of non-resonant production of a Z boson in association with a photon (irreducible background), and of inclusive Z+jets events where a jet is misidentified as a photon (reducible background), and the relative contributions are determined using data as described below. The estimated background composition is used to construct simulated background samples with the same composition as the data background. These samples are used in the optimisation of the selection criteria, the choice of analytical model of the background shape, and the estimation of the related systematic uncertainties. The searches rely only indirectly on the measured background composition since the background shape parameters are determined from the data

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