Search for Higgs boson decays into two new low-mass spin-0 particles in the 4b channel with the ATLAS detector using pp collisions at √s= 13 TeV

This paper describes a search for beyond the Standard Model decays of the Higgs boson into a pair of new spin-0 particles subsequently decaying into b-quark pairs, H -> aa -> (bb̅)(bb̅), using proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider at center-of-mass energy √s = 13 TeV. This search focuses on the range 15 GeV ≤ m_a ≤ 30 GeV, where the decay products are collimated; it is complementary to a previous search in the same final state targeting the range 20 GeV ≤ m_a ≤ 60 GeV, where the decay products are well separated. A novel strategy for the identification of the a -> bb̅ decays is deployed to enhance the efficiency for topologies with small separation angles. The search is performed with 36 fb^-1 of integrated luminosity collected in 2015 and 2016 and sets upper limits on the production cross section of H -> aa -> (bb̅)(bb̅), where the Higgs boson is produced in association with a Z boson.