Measurement of the $Z(\rightarrow\ell^+\ell^-)\gamma$ production cross-section in $pp$ collisions at $\sqrt{s} =13$ TeV with the ATLAS detector

TL;DR

This paper reports a precise measurement of the production cross-section of a Z boson with an associated photon in proton-proton collisions at 13 TeV, using ATLAS data, and compares results with advanced theoretical predictions.

Contribution

It provides the first high-precision measurement of Z+photon production cross-section at 13 TeV, including differential distributions and electroweak correction considerations.

Findings

Fiducial cross-section measured with 2.9% precision

Differential cross-sections as functions of six kinematic variables

Data agree with NLO and NNLO QCD predictions within uncertainties

Abstract

The production of a prompt photon in association with a $Z$ boson is studied in proton-proton collisions at a centre-of-mass energy $\sqrt{s} =$ 13 TeV. The analysis uses a data sample with an integrated luminosity of 139 fb$^{-1}$ collected by the ATLAS detector at the LHC from 2015 to 2018. The production cross-section for the process $pp \rightarrow \ell^+\ell^-\gamma+X$ ($\ell = e, \mu$) is measured within a fiducial phase-space region defined by kinematic requirements on the photon and the leptons, and by isolation requirements on the photon. An experimental precision of 2.9% is achieved for the fiducial cross-section. Differential cross-sections are measured as a function of each of six kinematic variables characterising the $\ell^+\ell^-\gamma$ system. The data are compared with theoretical predictions based on next-to-leading-order and next-to-next-to-leading-order perturbative QCD calculations. The impact of next-to-leading-order electroweak corrections is also considered.