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

Abstract: Measurements of the jet activity in $t\bar{t}$ 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 $e\mu$ 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 pseudorapidity range $|\eta|<4.5$. 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 regions of the invariant mass of the $e\mu b\bar{b}$ 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 $t\bar{t}$ event generators with appropriate parameter choices. The results can be used to further optimise the parameters used in such generators.