Abstract
The efficiency to identify jets containing b-hadrons (b-jets) is measured using a high purity
sample of dileptonic top quark–antiquark pairs (tt¯) selected from the 36.1 fb−1 of data collected
by the ATLAS detector in 2015 and 2016 from proton–proton collisions produced by the Large
Hadron Collider at a centre-of-mass energy √s = 13 TeV. Two methods are used to extract the
efficiency from tt¯ events, a combinatorial likelihood approach and a tag-and-probe method. A
boosted decision tree, not using b-tagging information, is used to select events in which two b-
jets are present, which reduces the dominant uncertainty in the modelling of the flavour of the
jets. The efficiency is extracted for jets in a transverse momentum range from 20 to 300 GeV, with
data-to-simulation scale factors calculated by comparing the efficiency measured using collision
data to that predicted by the simulation. The two methods give compatible results, and achieve a
similar level of precision, measuring data-to-simulation scale factors close to
unity with uncertainties ranging from 2% to 12% depending on the jet transverse momentum.