Identification of additional b-jets in the tt^- plus heavy flavor production with two leptons of the opposite sign in the final state using BDT for differential cross- section measurements

Top quark pair-production in association with one or two additional b-jets is one of the main backgrounds in the search for tt^-+H production, and in BSM physics searches involving top and bottom quarks in the final states. Precise measurement of tt^-+b and tt^-+bb differential cross sections will help to significantly reduce systematic and modeling uncertainties in future BSM searches and measurements of the top-Higgs coupling. A BDT that discriminates between b-jets which do or do not originate from top quarks decays has been developed in the opposite-sign dilepton channel. The method has higher purity compared to the existing method based on the angular distributions between b-jets and leptons. Preliminary results show that with the new method we can accurately measure the p_T spectra of b-jets from top quark decays and of the leading additional b-jet up to 400 GeV, and the sub-leading additional b-jets p_T up to 300 GeV. The analysis uses LHC proton-proton collision data at the center-of-mass energy √s=13 TeV with an integrated luminosity of 139 fb^-1 collected with the ATLAS detector in 2015-2018.