Measurement of the Z$\gamma$ production cross section and search for anomalous neutral triple gauge couplings in pp collisions at $\sqrt{s}$ = 13 TeV

TL;DR

This paper measures the Zγ production cross section at 13 TeV, compares it with Standard Model predictions, and sets new limits on anomalous neutral triple gauge couplings using CMS data from 2016-2018.

Contribution

First measurement of the Zγ cross section with Z decaying to neutrinos at 13 TeV, and the most stringent limits to date on certain anomalous gauge couplings.

Findings

Measured Zγ cross section: 23.3 fb with uncertainties.

Differential cross section matches SM predictions.

Set new limits on anomalous couplings h3γ, h4γ, h3Z, h4Z.

Abstract

A measurement of the fiducial cross section of the associated production of a Z boson and a high-$p_\mathrm{T}$ 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 $\sqrt{s}$ = 13 TeV during 2016$-$2018, corresponding to an integrated luminosity of 138 fb$^{-1}$. The fiducial Z$\gamma$ cross section, where a photon with a $p_\mathrm{T}$ greater than 225 GeV is produced in association with a Z, and the Z decays to a $\nu\bar{\nu}$ pair (Z($\nu\bar{\nu}$)$\gamma$), 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 $p_\mathrm{T}$ 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$\gamma$ and Z$\gamma\gamma$ vertex using the $p_\mathrm{T}$ spectrum of the photons. The observed 95% confidence level intervals for $CP$-conserving $h_3^{\gamma}$ and $h_4^{\gamma}$ are determined to be ($-$3.4, 3.5) $\times$ 10$^{-4}$ and ($-$6.8, 6.8) $\times$ 10$^{-7}$, and for $h_3^\mathrm{Z}$ and $h_4^\mathrm{Z}$ they are ($-$2.2, 2.2) $\times$ 10$^{-4}$ and ($-$4.1, 4.2) $\times$ 10$^{-7}$, respectively. These are the strictest limits to date on $h_3^{\gamma}$, $h_3^\mathrm{Z}$ and $h_4^\mathrm{Z}$.