Measurement of jet activity in top quark events using the $e\mu$ final state with two $b$-tagged jets in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

Measurements of the jet activity in $ t\overline{t} $ events produced in proton-proton collisions at $ \sqrt{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\mu b\overline{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\overline{t} $ event generators with appropriate parameter choices. The results can be used to further optimise the parameters used in such generators.