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 𝑊⁢𝑍→ℓ⁡𝜈⁢ℓ′⁡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 𝑝𝑍𝑇 >200  GeV, where 𝑝𝑍𝑇 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 Δ⁢𝑌⁡(ℓ𝑊⁢𝑍) and Δ⁢𝑌⁡(𝑊⁢𝑍) 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 Δ⁢𝑌⁡(ℓ𝑊⁢𝑍) and Δ⁢𝑌⁡(𝑊⁢𝑍) distributions are also measured and compared to theoretical predictions.