A Search for Supersymmetry in Multi-$b$ Jet Events with the ATLAS Detector

A search for supersymmetry in pair-produced gluinos decaying via top squarks to the lightest neutralino is presented. Events with multiple hadronic jets, of which at least three must be identified as originating from $b$-quarks, and large amounts of missing transverse energy in the final state, are selected for study. The dataset utilized encompasses proton-proton collisions with a center-of-mass energy of $\sqrt{s}$ = 13 TeV and integrated luminosity of 79.9 fb$^{-1}$ collected by the ATLAS experiment at the LHC from 2015 to 2017. The search employs a parameterized boosted decision tree (BDT) to separate supersymmetric signal events from standard model backgrounds. New methods for optimal BDT parameter point selection and signal region creation, as well as new soft kinematic variables, are exploited to increase the search's expected exclusion limit beyond prior analyses of the same dataset by 100-200 GeV in the gluino and neutralino mass plane. No excess is observed in data above the predicted background, extending the previous exclusion limit at the 95% confidence level by 250 GeV to approximately 1.4 TeV in neutralino mass. The analytical and machine learning techniques developed here will benefit future analysis of additional Run 2 data from 2018.