Measurement of the Z$γ$ production cross section and search for anomalous neutral triple gauge couplings in pp collisions at √s = 13 TeV

A measurement of the fiducial cross section of the associated production of a Z boson and a high-pT photon, where the Z decays to two neutrinos, and a search for anomalous triple gauge couplings are reported. The results are based on data collected by the CMS experiment at the LHC in proton-proton collisions at √s = 13 TeV during 2016$-$2018, corresponding to an integrated luminosity of 138 fb⁻¹. The fiducial Z$γ$ cross section, where a photon with a pT greater than 225 GeV is produced in association with a Z, and the Z decays to a νν pair (Z(νν)$γ$), is measured to be 23.3+1.4-1.3 fb, in agreement, within uncertainties, with the standard model prediction. The differential cross section as a function of the photon pT has been measured and compared with standard model predictions computed at next-to-leading and at next-to-next-to-leading order in perturbative quantum chromodynamics. Constraints have been placed on the presence of anomalous couplings that affect the ZZ$γ$ and Z$γγ$ vertex using the pT spectrum of the photons. The observed 95% confidence level intervals for $CP$-conserving h₃^γ and h₄^γ are determined to be ($-$3.4, 3.5) × 10⁻⁴ and ($-$6.8, 6.8) × 10⁻⁷, and for h₃Z and h₄Z they are ($-$2.2, 2.2) × 10⁻⁴ and ($-$4.1, 4.2) × 10⁻⁷, respectively. These are the strictest limits to date on h₃^γ, h₃Z and h₄Z.