Measurement of the cross section in $t\bar t$ + jets using a multivariate kinematics fit method with the ATLAS detector

A measurement of the production cross-section for $t\bar t$ events with an additional jet ($t\bar t$ + jet) is presented, using 5.0 fb$^{-1}$ of data collected by the ATLAS detector at $\sqrt{s} =7$ TeV at the LHC in 2011. The measurement is performed in the semileptonic decay channel ( $t\bar t \rightarrow \ell\nu b\,bjj$) with electrons or muons in the final state. A multivariate kinematic approach is used in order to extract the background events, a 2D likelihood is formed to simultaneously measure the $t\bar t$ + jet and inclusive $t\bar t$ cross-sections.