Observations of the Radiation Amplitude Zero effect and of the Diboson Longitudinal-Longitudinal Interactions in high $p_T^Z$ phase space using $W^{\pm}Z$ Production with the ATLAS Detector

In the standard model of particle physics, the spontaneous symmetry breaking of the complex Higgs field gives rise to the massive Higgs boson and three Goldstone bosons. These Goldstone bosons give the longitudinal degree of freedom to the W and Z bosons. In the production of WZ from the proton-proton collision in the ATLAS experiment of the Large Hadron Collider, it is critical to study the longitudinal-longitudinal interactions of W and Z. This analysis studies diboson polarization states in $WZ$ production. The dominant contribution of both bosons being transversely polarized nearly vanishes at tree-level if the bosons are produced centrally, which effectively enhances the longitudinal-longitudinal $WZ$ contribution. As high jet multiplicity skews this Radiation Amplitude Zero (RAZ) effect, only events with lower $p_T^{WZ}$ ($<20, 40, 70$ GeV) are selected. We measure RAZ as the depth in the central region of the distributions of the rapidity differences between the $W$ lepton and the $Z$ boson and between the $W$ boson and the $Z$ boson. A high $p_T^Z$ cut also enhances the $W_0Z_0$ contribution. A BDT variable is trained to distinguish different diboson polarization states in two high $p_T^Z$ exclusive regions: $100 < p_T^Z \leq 200$ GeV and $p_T^Z > 200$ GeV. A maximum log-likelihood fit is then executed, yielding an observation of a non-zero longitudinal-longitudinal polarization fraction ($f_{00}$). Notably, this analysis marks the first observations of the Radiation Amplitude Zero Effect and of the longitudinal-longitudinal $WZ$ production in the high-$p_T^Z$ phase space.