Measurement of the $b$-jet identification efficiency with the $p_\text{T}^{\text{rel}}$ method in multi-jet events using pp collisions at $\sqrt{s} = 13$ TeV with the ATLAS Detector

The identification of jets containing \emph{b}-hadrons, or \emph{b}-tagging, is achieved in ATLAS using various approaches. A multivariate algorithm, denoted as MV2, has been widely used in many ATLAS Run 2 analyses. A technique is presented to measure the \emph{b}-tagging efficiency of MV2 using multi-jet events considering jets spatially matched to muons. Proton--proton collision data corresponding to 14-767 pb$^{-1}$ collected by the ATLAS detector between 2016 and 2018 at a centre-of-mass energy of $\sqrt{s}$ = 13 TeV are used. The magnitude of the momentum of the muon projected onto the plane that is transverse to the combined muon and jet axis, $p_{\text{T}}^{\text{rel}}$, is used in a profile-likelihood fit to determine the yields of jets containing \emph{b}-hadrons in data. Thus the \emph{b}-tagging efficiency in data can be extracted. The results are reported as ratios between the efficiency measured in data and that in simulation, referred to as scale factors. The scale factors for the \emph{b}-tagging algorithm MV2 are reported as a function of the transverse momentum for calorimeter jets.