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 WZ → ℓνℓ′ℓ′(ℓ,ℓ′ = e; μ) 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^Z_T ≤ 200 GeV and 1.6 standard deviations in the region with p^Z_T > 200 GeV, where p^Z_T 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 ΔY(ℓ_WZ) and ΔYℓWZ) 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 ΔY(ℓ_WZ) and ΔY(WZ) distributions are also measured and compared to theoretical predictions.