Search for the $Z\gamma$ decay mode of new high-mass resonances in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

TL;DR

This paper reports a search for high-mass $Z\,\gamma$ resonances in proton-proton collisions at 13 TeV using ATLAS data, setting upper limits on production cross sections and finding no significant excess over the Standard Model background.

Contribution

It provides the first comprehensive search for $Z\gamma$ resonances at high mass with the full Run 2 ATLAS dataset, establishing new upper limits across a wide mass range.

Findings

No significant excess observed over background.

Set upper limits on resonance production cross sections.

Excluded certain resonance models at high masses.

Abstract

This letter presents a search for narrow, high-mass resonances in the $Z\gamma$ final state with the $Z$ boson decaying into a pair of electrons or muons. The $\sqrt{s}=13$ TeV $pp$ collision data were recorded by the ATLAS detector at the CERN Large Hadron Collider and have an integrated luminosity of 140 fb$^{-1}$. The data are found to be in agreement with the Standard Model background expectation. Upper limits are set on the resonance production cross section times the decay branching ratio into $Z\gamma$. For spin-0 resonances produced via gluon-gluon fusion, the observed limits at 95% confidence level vary between 65.5 fb and 0.6 fb, while for spin-2 resonances produced via gluon-gluon fusion (or quark-antiquark initial states) limits vary between 77.4 (76.1) fb and 0.6 (0.5) fb, for the mass range from 220 GeV to 3400 GeV.