Search for non-resonant Higgs boson pair production in the $bb\ell\nu\ell\nu$ final state with the ATLAS detector in $pp$ collisions at $\sqrt{s} = 13$ TeV

A search for non-resonant Higgs boson pair production, as predicted by the Standard Model, is presented, where one of the Higgs bosons decays via the H→bb channel and the other via one of the H→WW⁎/ZZ⁎/ττ channels. The analysis selection requires events to have at least two b -tagged jets and exactly two leptons (electrons or muons) with opposite electric charge in the final state. Candidate events consistent with Higgs boson pair production are selected using a multi-class neural network discriminant. The analysis uses 139 fb −1 of pp collision data recorded at a centre-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. An observed (expected) upper limit of 1.2 ( 0.9−0.3+0.4 ) pb is set on the non-resonant Higgs boson pair production cross-section at 95% confidence level, which is equivalent to 40 ( 29−9+14 ) times the value predicted in the Standard Model.