HL-LHC prospects for the measurement of Higgs boson pair production in the $b\bar{b}b\bar{b}$ final state and combination with the $b\bar{b}\gamma\gamma$ and $b\bar{b}\tau^+\tau^-$ final states at the ATLAS experiment

Projection studies for the non-resonant Higgs boson pair production in the $b\bar{b}b\bar{b}$ final state with the ATLAS detector at the HL-LHC are presented. They are based on extrapolated results of the latest Run 2 analysis which targeted both the gluon-gluon fusion and vector-boson fusion Higgs boson pair production modes and exploited $126\;\text{fb}^{-1}$ of $\sqrt{s} = 13\;\text{TeV}$ data. The projection assumes an integrated luminosity of $3000\;\text{fb}^{-1}$ for HL-LHC proton-proton collisions at $\sqrt{s} = 14\;\text{TeV}$. Various systematic uncertainty scenarios are explored. With (without) systematic uncertainties, the discovery significance of the Standard Model Higgs boson pair production is found to be $1.0\sigma$ ($1.8\sigma$), while the 95% confidence-level upper limit on the HH signal strength is set at $2.0$ ($1.1$). The allowed 68% confidence-interval for the HHH coupling modifier $\kappa_\lambda$ is found to be $[-0.5, 6.1]$ ($[0.1, 2.6]$), while the allowed 68% confidence-interval for the HHVV coupling modifier $\kappa_{2V}$ is $[0.7, 1.4]$ ($[0.7. 1.3]$). A statistical combination of these $b\bar{b}b\bar{b}$ projections with the $b\bar{b}\gamma\gamma$ and $b\bar{b}\tau^+\tau^-$ channels is also presented. This results in a discovery significance of $3.4\sigma$ ($4.9\sigma$), the 95% confidence-level upper limit on the Standard Model HH signal strength in set at $0.55$ ($0.39$). The $\kappa_\lambda$ coupling modifier is expected to be constrained to $[0.5, 1.6]$ $([0.7, 1.4])$ at 68% confidence-level.