Orateur
Description
The scattering of longitudinally polarised electroweak gauge bosons provides a direct probe of the mechanism of electroweak symmetry breaking and of the structure of the Standard Model at high energies. This talk presents the first measurement of longitudinal polarisation states in electroweak $\mathrm{W}^{\pm}\mathrm{Z}$ production in association with two jets ($\mathrm{WZjj}$-EW), using 140 fb$^{-1}$ of ATLAS Run-2 data at $\sqrt{s}=13$ TeV and 164 fb$^{-1}$ of Run-3 data at $\sqrt{s}=13.6$ TeV.
The $\mathrm{WZjj}$-EW signal is isolated from backgrounds and the polarisation states are disentangled using deep neural networks, whose outputs are combined into an optimised two-dimensional discriminant for a binned maximum-likelihood fit. The fraction of events containing a longitudinally polarised $\mathrm{Z}$ boson is measured to be $f_{\mathrm{X0}} = 0.324^{+0.092}_{-0.085}$, with an observed (expected) significance of 4.0 (2.9) standard deviations, providing the first evidence for longitudinally polarised $\mathrm{Z}$ bosons in this process. Upper limits are also obtained on the fractions of events with a longitudinally polarised $\mathrm{W}$ boson and with both bosons longitudinally polarised. The $\mathrm{WZjj}$-EW fiducial cross-section at 13.6 TeV is measured for the first time and is found to be consistent with Standard Model predictions.
These measurements provide a new experimental probe of electroweak symmetry breaking through vector-boson scattering at the LHC.