HL-LHC prospects for diboson resonance searches and electroweak vector boson scattering in the $WW/WZ\to\ell\nu qq$ final state

This note presents the prospects of searches for new heavy resonances decaying to dibosons ($WW/WZ$) and measurements of electroweak $WW/WZ$ production via vector boson scattering in association with a high-mass dijet system in the $\ell\nu qq$ final states. The prospects are presented for an integrated luminosities of 300 and 3000 fb$^{-1}$ of proton-proton collisions at $\sqrt{s}$= 14 TeV to be recorded with the ATLAS detector at the high-luminosity LHC assuming the average number of $pp$ interactions per bunch crossing to be 200. The cross-section for electroweak $WW/WZ$ production in vector boson scattering processes is expected to have an observation greater than 5$\sigma$ at 300 fb$^{-1}$ and to be measured to within 6.5% at 3000 fb$^{-1}$. The diboson resonance searches are interpreted for sensitivity to a heavy scalar singlet, a simplified phenomenological model with a heavy gauge boson and a Randall-Sundrum model with a spin-2 graviton. With 3000 fb$^{-1}$ of $pp$ data, the exclusion limit reach for the new resonance is extended to 4.9 TeV in the heavy gauge boson model and 3.3 TeV in the Randall-Sundrum model. These are improvements of approximately 1 TeV in comparison to existing mass limits. With 3000 fb$^{-1}$ of $pp$ data, the 5$\sigma$ discovery reach is 3.3 TeV for the heavy gauge boson model and 1.7 TeV for the Randall-Sundrum model.