Measurement of jet multiplicity in top pair events

The normalized differential $t\bar t$ cross section in jet multiplicity bins is presented, measured in proton-proton collisions using 5.0~fb$^{-1}$ of data collected at $\sqrt{s}$ = 7~\TeV. The analysis is performed in the electron + jets and the muon + jets channels. The jet multiplicity distribution is sensitive to initial state radiation. A comparison of the data with different Monte Carlo generators is shown. After background subtraction, the data are in agreement with $t\bar t$ signal Monte Carlo distributions. Additionally, the measurement of the top quark pair differential cross-section in the number of radiated additional hard partons in the muon + jets channel is presented. The measured fractions of events with $t\bar t$ + 0, 1, and $\geq$ 2 additional partons are in good agreement with different Monte Carlo predictions.