Abstract
This Letter presents a search for pair production of heavy down-type quarks decaying via b' _ Wt in the lepton + jets channel, as b'b' _ W- tW+ t _ bbW+ W- W+ W- _ l± _bbqqqqqq. In addition to requiring exactly one lepton, large missing transverse momentum, and at least six jets, the invariant mass of nearby jet pairs is used to identify high transverse momentum W bosons. In data corresponding to an integrated luminosity of 1.04__fb(-1) from pp collisions at sqrt[s] = 7__TeV recorded with the ATLAS detector, a heavy down-type quark with mass less than 480 GeV can be excluded at the 95% confidence level.