Search for Higgs boson pair production in the single-lepton $WWb\bar{b}$ channel with the ATLAS detector

A search for the Standard Model (SM) Higgs boson pair production in the $WWb\bar{b}$ single-lepton channel is presented. This search uses 139 $\mathrm{fb}^{-1}$ of proton-proton collisions data at a centre-of-mass energy of 13 TeV provided by the Large Hadron Collider and recorded by the ATLAS experiment from 2015 to 2018. A new analysis strategy, based on the application of an optimised multi-output Deep Neural Network classifier, has been used to improve the background rejection and the sensitivity of the search. No evidence of events beyond the background expectation is found. An observed (expected) $95\%$ confidence level upper limit on the SM Higgs pair production cross section of 40.2 (34.8) times the predicted SM cross section is obtained, improving the previous ATLAS result on the same final state.