Studies of the Energy Dependence of Diboson Polarization Fractions and the Radiation-Amplitude Effect in <i>wz</I> Production With the Atlas Detector

Abstract

This Letter presents the first study of the energy dependence of diboson polarization fractions in WZ -> lvl ' l '(l, l ' = e, mu) production. The dataset used corresponds to an integrated luminosity of 140 fb(-1) of proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector. Two fiducial regions with an enhanced presence of events featuring two longitudinally polarized bosons are defined. A nonzero fraction of events with two longitudinally polarized bosons is measured with an observed significance of 5.3 standard deviations in the region with 100 < p(T)(Z) <= 200 GeV and 1.6 standard deviations in the region with p(T)(Z) > 200 GeV, where p(T)(Z) is the transverse momentum of the Z boson. This Letter also reports the first study of the radiation-amplitude-zero effect. Events with two transversely polarized bosons are analyzed for the Delta Y(l(W)Z) and Delta Y(WZ) distributions defined respectively as the rapidity difference between the lepton from the W boson decay and the Z boson and the rapidity difference between the W boson and the Z boson. Significant suppression of events near zero is observed in both distributions. Unfolded Delta Y(l(W)Z) and Delta Y(WZ) distributions are also measured and compared to theoretical predictions.