Search for down-type fourth generation quarks with the ATLAS detector in   events with one lepton and hadronically decaying W bosons
  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'->WtWt->bbWWWW->lvbbqqqqqq. 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 is excluded at the 95% confidence level. 
ATLAS Collaboration
hep-ex