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

2024    CERN İşbirliği

This Letter presents the first study of the energy dependence of diboson polarization fractions in $WZ\to \ell \nu {\ell }^{\prime }{\ell }^{\prime }\left(\ell ,{\ell }^{\prime }=e,\mu \right)$ production. The dataset used corresponds to an integrated luminosity of $140\text{}\text{}{\mathrm{fb}}^{-1}$ 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 $100<{p}_{T}^{Z}\le 200\text{}\text{}\mathrm{GeV}$ and 1.6 standard deviations in the region with ${p}_{T}^{Z}>200\text{}\text{}\mathrm{GeV}$, 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 $\Delta Y\left({\ell }_{W}Z\right)$ and $\Delta Y\left(WZ\right)$ 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 $\Delta Y\left({\ell }_{W}Z\right)$ and $\Delta Y\left(WZ\right)$ distributions are also measured and compared to theoretical predictions.