Search for diboson resonances in hadronic final states in 79.8 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

Narrow resonances decaying into $WW$, $WZ$ or $ZZ$ boson pairs are searched for in 79.8 fb$^{-1}$ of proton-proton data at a centre-of-mass energy of $\sqrt{s}=13$ TeV recorded with the ATLAS detector at the Large Hadron Collider from 2015 to 2017. The diboson system is reconstructed using pairs of high transverse momentum, large-radius jets build from a combination of calorimeter- and tracker-inputs tagged as compatible with the hadronic decay of a boosted $W$ or $Z$ boson, using jet mass and substructure properties. The search covers diboson resonances with masses in the range 1.2 TeV - 5.0 TeV. No significant deviations from the background expectations are seen. Exclusion limits at the $95$% confidence level are set on the production cross section times branching ratio to dibosons for a range of theories beyond the Standard Model, with the highest lower limit on the mass of a new gauge boson at 4.15 TeV in the context of mass-degenerate resonances that couple predominantly to bosons.