Prospects for Observing $t \bar t HH$ Production with the ATLAS Experiment at the HL-LHC

Measuring the self-coupling of the Higgs boson is one of the primary goals of the High Luminosity-LHC. In this note, the prospects for the study of $t \bar t H H$ production, with both Higgs bosons decaying to $H\rightarrow b\bar b$, using $3000\;\mathrm{fb}^{-1}$ of proton-proton collisions at $\sqrt{s}=14\;\mathrm{T\kern -0.1em eV}$ with an average pile-up of 200 at the ATLAS detector are presented. The study uses stable particles from generated Monte Carlo events. The energy and momentum of leptons and jets are smeared to simulate the response of the upgraded ATLAS detector and pile-up collisions. Events are selected from the semi-leptonic final state of the $t\bar t$ system with one electron or muon, two $b$-quark jets, two light jets and missing momentum from the neutrino, along with an additional four $b$-jets from the Higgs boson decay. After applying event selections requiring at least 5 jets to be $b$-tagged, and assuming Standard Model rates of signal and background processes, 25 $t \bar t H H$ events are selected, with a background of 7,100 events. By considering events in the $5~b$-tag and $\ge 6~b$-tag regions separately, the statistical significance of $t \bar t H H$ production is estimated to be $0.35\,\sigma$. It is concluded that, once systematic uncertainties are included, the $t \bar t H H$ production mechanism at the level predicted in the Standard Model will make, at best, a small contribution to evidence of Higgs boson pair production and to a measurement of the Higgs self-coupling parameter $\lambda_{HHH}$ at the High Luminosity-LHC.