Observation of gauge boson joint-polarisation states in $W^{\pm}Z$ production from $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

TL;DR

This paper reports the first measurement of joint polarization states of W and Z bosons in proton-proton collisions at 13 TeV, confirming Standard Model predictions with high significance using ATLAS data.

Contribution

It presents the first measurement of joint polarization states of W and Z bosons in $W^{ ext{±}}Z$ production at the LHC, including the simultaneous observation of longitudinally polarized pairs.

Findings

First observation of longitudinally polarized WZ pairs with 7.1 sigma significance

Measured joint helicity fractions agree with Standard Model predictions

Provided differential cross sections sensitive to polarization effects

Abstract

Measurements of joint-polarisation states of $W$ and $Z$ gauge bosons in $W^{\pm}Z$ production are presented. The data set used corresponds to an integrated luminosity of $139$ fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of $13$ TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. The $W^{\pm}Z$ candidate events are reconstructed using leptonic decay modes of the gauge bosons into electrons and muons. The simultaneous pair-production of longitudinally polarised vector bosons is measured for the first time with a significance of $7.1$ standard deviations. The measured joint helicity fractions integrated over the fiducial region are $f_{\mathrm{00}} = 0.067 \pm 0.010$, $f_{\mathrm{0T}} = 0.110 \pm 0.029$, $f_{\mathrm{T0}} = 0.179 \pm 0.023$ and $f_{\mathrm{TT}} = 0.644 \pm 0.032$, in agreement with the next-to-leading-order Standard Model predictions. Individual helicity fractions of the $W$ and $Z$ bosons are also measured and found to be consistent with joint helicity fractions within the expected amount of correlations. Both the joint and individual helicity fractions are also measured separately in $W^+Z$ and $W^-Z$ events. Inclusive and differential cross sections for several kinematic observables sensitive to polarisation are presented.