Projected sensitivity of Higgs boson pair production in the $bb\tau\tau$ final state using proton-proton collisions at HL-LHC with the ATLAS detector

A study of the projected sensitivity to non-resonant Higgs boson pair production in the $b\bar{b}\tau^{+}\tau^{-}$ final state using the ATLAS detector at the High Luminosity LHC is presented, assuming 3000 fb$^{-1}$ of $pp$ collisions at a centre-of-mass energy of 14 TeV. The estimated 95% confidence level upper limit on the Standard Model Higgs boson pair production rate with (without) systematic uncertainties is $0.71$ ($0.49$) times the Standard Model expectation with respect to the background-only hypothesis, corresponding to a signal significance of $2.8\sigma$ ($4.0\sigma$). Assuming Standard Model couplings, the modifier $\kappa_\lambda$ of the trilinear Higgs boson self-coupling is estimated to be constrained to the $1\sigma$ confidence interval $[0.3, 1.9]\cup[5.2, 6.7]$ ($[0.5, 1.6]$). In the absence of a Higgs boson pair production signal, values of $\kappa_\lambda$ outside of $[1.7, 5.4]$ ([2.4, 4.5]) are projected to be excluded at the 95% confidence level.