A Search for Heavy Resonances Decaying to \(HH \to b\bar{b}b\bar{b}\) with the ATLAS Detector

\hspace{10mm}A search for Higgs boson pairs produced in the decay of high mass exotic resonances is presented. The search uses the \bbbb final state, analyzing 139~\ifb of proton--proton collision data at \(\sqrt{s} = \SI{13}{TeV}\) collected with the ATLAS detector. Spin-0 and spin-2 benchmark signal models are considered and no significant deviation from the Standard Model prediction is observed. The search is combined with a complementary analysis for lower mass resonances to set upper limits on the production cross-section times \(HH\) branching ratio of new resonances in the mass range of \SI{251}{GeV} to \SI{5}{TeV}. \hspace{10mm}In addition, the methodology for the in-situ calibration of a novel double-\btag algorithm (\XbbTag) using gluon to \(b\bar{b}\) decays is presented. Preliminary scale factors for Monte-Carlo simulation are computed using 139~\ifb of \(\sqrt{s} = \SI{13}{TeV}\) \(pp\) collision data collected with the ATLAS detector. The completed calibration will allow the \XbbTag algorithm to be used in future ATLAS searches for \Hbb decays.