Search for heavy particles in the b -tagged dijet mass distribution with additional b -tagged jets in proton-proton collisions at √{s }=13 TeV with the ATLAS experiment

A search optimized for new heavy particles decaying to two b -quarks and produced in association with additional b -quarks is reported. The sensitivity is improved by b -tagging at least one lower-p_T jet in addition to the two highest-p_T jets. The data used in this search correspond to an integrated luminosity of 103 fb^-1 collected with a dedicated trijet trigger during the 2017 and 2018 √{s }=13 TeV proton-proton collision runs with the ATLAS detector at the LHC. The search looks for resonant peaks in the b -tagged dijet invariant mass spectrum over a smoothly falling background. The background is estimated with an innovative data-driven method based on orthonormal functions. The observed b -tagged dijet invariant mass spectrum is compatible with the background-only hypothesis. Upper limits at 95% confidence level on a heavy vector-boson production cross section times branching ratio to a pair of b -quarks are derived.