  A measurement of the production cross-section for top quark pairs (t tbar) in
pp collisions at sqrt{s} = 7 TeV is presented using data recorded with the
ATLAS detector at the Large Hadron Collider. Events are selected in the single
lepton topology by requiring an electron or muon, large missing transverse
momentum and at least three jets. With a data sample of 35/pb, two different
multivariate methods, one of which uses b-quark jet identification while the
other does not, use kinematic variables to obtain cross-section measurements of
sigma (t tbar) = 187 +/- 11 (stat.) +18-17 (syst.) +/- 6 (lumi.) pb and sigma
(t tbar) = 173 +/- 17 (stat.) +18-16 (syst.) +/- 6 (lumi.) pb} respectively.
The two measurements are in agreement with each other and with QCD
calculations. The first measurement has a better a priori sensitivity and
constitutes the main result of this Letter.
