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

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 $pp$ collision data are recorded by the ATLAS detector at the CERN Large Hadron Collider with 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 production cross section times the decay branching ratio to $Z\gamma$. 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 via gluon-gluon fusion (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.