Studies of the energy dependence of diboson polarization fractions and the radiation-amplitude-zero effect in WZ production with the ATLAS detector

his Letter presents the first study of the energy dependence of diboson polarization fractions in WZ $\to \ell \nu \ell '\ell '(\ell, \ell ' = e, \mu)$ 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 $\Delta Y(\ell_{W}Z)$ 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 $\Delta Y(\ell_{W}Z)$ and $\Delta Y(WZ)$ distributions are also measured and compared to theoretical predictions.