Comparison of Monte Carlo generator predictions for gap fraction and jet multiplicity observables in top-antitop events

Predictions from several Monte Carlo generators are compared with each other and the data for observables in events with top-antitop quark pair at the LHC. Data taken by ATLAS in 2011 at $\sqrt{s}=7$ TeV is used. The focus is on observables sensitive to additional parton radiation: jet multiplicities and gap fraction observables. Generators that have been used for ATLAS analyses of the data collected in the first LHC proton physics run as well as new generators that will be used in the upcoming LHC run are included. The goal of the note is to collect information and studies for discussions between the communities of the ATLAS and CMS experiments and theoretical colleagues.