Measurement prospects of the pair production and self-coupling of the Higgs boson with the ATLAS experiment at the HL-LHC

A prospect study for the search for non-resonant Higgs-boson-pair production, using a statistical combination of the $b\bar{b}b\bar{b}$, $b\bar{b}\gamma\gamma$ and $b\bar{b}\tau^{+}\tau^{-}$ final states, is performed assuming 3000 fb$^{-1}$ of $pp$ collisions at a centre-of-mass energy of 14 TeV in the ATLAS detector at the High Luminosity LHC (HL-LHC). When systematic uncertainties are neglected, the expected signal significance is found to be $3.5\sigma$ while the signal strength relative to the Standard Model prediction is expected to be measured with an accuracy of 31%. The Higgs boson self-coupling can be constrained to $-0.1\leq\lambda_{HHH}/\lambda_{HHH}^{SM}\leq2.7\cup5.5\leq\lambda_{HHH}/\lambda_{HHH}SM\leq6.9$, at 95% confidence level, and the measured value of $\lambda_{HHH}/\lambda_{HHH}^{SM}$ is expected to be $1.0^{+0.7}_{-0.6}$. If systematic uncertainties are included, the signal significance is found to be $3.0\sigma$ while the signal strength relative to the Standard Model expectation is expected to be measured with an accuracy of 40%. In that case, the Higgs boson self-coupling can be constrained to $-0.4\leq\lambda_{HHH}/\lambda_{HHH}^{SM}\leq7.3$, at 95% confidence level, and the measured value of $\lambda_{HHH}/\lambda_{HHH}^{SM}$ is expected to be $1.0^{+0.9}_{-0.8}$. The $b\bar{b}\gamma\gamma$ and $b\bar{b}\tau^{+}\tau^{-}$ final states are further extrapolated to provide first estimates of the prospects at the High Energy LHC, assuming 15 ab$^{-1}$ of data at a centre-of-mass collision energy of 27 TeV.