Measurement of jet activity in top quark events using the eμ final state with two b -tagged jets in pp collisions at s = 8 TeV with the ATLAS detector

Measurements of the jet activity in t t ¯ events produced in proton-proton collisions at s = 8 TeV are presented, using 20.3 fb −1 of data collected by the ATLAS experiment at the Large Hadron Collider. The events were selected in the dilepton eμ decay channel with two identified b -jets. The numbers of additional jets for various jet transverse momentum ( p T ) thresholds, and the normalised differential cross-sections as a function of p T for the five highest- p T additional jets, were measured in the jet pseudo-rapidity range | η | < 4 . 5. The gap fraction, the fraction of events which do not contain an additional jet in a central rapidity region, was measured for several rapidity intervals as a function of the minimum p T of a single jet or the scalar sum of p T of all additional jets. These fractions were also measured in different intervals of the invariant mass of the e μ b b ¯ system. All measurements were corrected for detector effects, and found to be mostly well-described by predictions from next-to-leading-order and leading-order t t ¯ event generators with appropriate parameter choices. The results can be used to further optimise the parameters used in such generators.