Measurements of rare electroweak processes including vector boson scattering and triboson production with ATLAS

Measurements of rare processes in the electroweak sector provide unprecedented constraints on the SM theory, and unique sensitivity to study the electroweak symmetry breaking and the quartic boson self-couplings. This proceeding presents recent ATLAS measurements of electroweak di-boson production in the vector boson scattering process (VBS) and tri-boson $VVZ$ production using 140 fb$^{-1}$ of data collected at LHC in Run 2 at $\sqrt{s}=13$ TeV. In addition to cross-section measurements, studies of boson polarisation states in the VBS process are being actively pursued to bring further sensitivity to new phenomena. First evidence for longitudinally polarised $W$ bosons in the VBS production of the same-sign $W$ boson pairs is presented, also based on the 140 fb$^{-1}$ of data collected by ATLAS in Run 2 at LHC.