  The ATLAS Collaboration has reported the first results of the search for SUSY
particles in 1-lepton + 3 jets + MET final states. An essential ingredient for
these results is a reliable background estimation in the signal region, in
particular of the ttbar, W+jets and QCD backgrounds. The estimation of these
three backgrounds is explained in this paper. The ttbar and W+jets backgrounds
are obtained from a background dominated control region and extrapolated to the
signal region, whereas for the estimation of the QCD background a matrix method
is used.
