Study of W+Jets background to top quark pair production cross section in ATLAS at the LHC

The measurement of top quark pair production cross section in p-p collisions at center-of-mass energy of 7 TeV is one of the first measurements that will be made by ATLAS at the LHC. The most promising channel is the semileptonic final state, where one of the top decays into a W decaying into hadrons and the other top into a W decaying leptonically. The dominant background to this channel of top quark pairs is given by direct p-p production of W+jets. Monte Carlo predictions for the rate of this background have large uncertain- ties; however it will be important to know it with precision in order to make an accurate measurement of top quark pair production cross section. In the following, a data-driven technique developed in order to reduce this uncertainty is presented and first results on real data are discussed.