Search for heavy particles in the $b$-tagged di-jet mass distribution with additional $b$-tagged jets in proton-proton collisions at $\sqrt{s} = 13$ TeV with the ATLAS experiment

A search for new heavy particles decaying to two $b$-quarks is reported. This analysis is optimized for a new heavy vector boson produced in association with $b$-quarks. The sensitivity is improved by $b$-tagging jets in addition to the two highest $p_{\text{T}}$ jets. The data used in this search correspond to an integrated luminosity of 103~$\text{fb}^{-1}$ collected with a dedicated tri-jet trigger during the 2017 and 2018 $\sqrt{s} = 13$ TeV proton-proton collision runs with the ATLAS detector at the LHC. The search looks for resonant peaks in the $b$-tagged di-jet invariant mass spectrum over a smoothly falling background. An innovative data-driven background estimation method not relying on empirical functional forms is applied. The observed $b$-tagged di-jet invariant mass spectrum is compatible with the background-only hypothesis. Upper limits at 95% confidence level on the heavy vector boson production cross section times its branching ratio to a pair of $b$-quarks are derived.