Studies of the Energy Dependence of Diboson Polarization Fractions and the Radiation-Amplitude-Zero Effect in WZ Production with the ATLAS Detector

This Letter presents the first study of the energy dependence of diboson polarization fractions in production. The dataset used corresponds to an integrated luminosity of 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 and 1.6 standard deviations in the region with , where is the transverse momentum of the boson. This Letter also reports the first study of the radiation-amplitude-zero effect. Events with two transversely polarized bosons are analyzed for the and distributions defined respectively as the rapidity difference between the lepton from the boson decay and the boson and the rapidity difference between the boson and the boson. Significant suppression of events near zero is observed in both distributions. Unfolded and distributions are also measured and compared to theoretical predictions.