Measurements on additional jet production in top pair events

The large centre-of-mass energy available at the proton-proton collider LHC allows for the copious production of top quark pairs in association with other final state particles at high transverse momentum. The ATLAS experiment has measured several final state observables which are sensitive to additional parton radiation in top anti-top quark final states. Examples are the multiplicity of jets for various transverse momentum thresholds or the probability to emit jets above a given threshold in a fixed rapidity region ("gap fraction"). These measurements are compared to modern Monte Carlo generators based on NLO QCD matrix element or LO multi-leg matrix elements and with systematic model parameter variations.