Search for Down-Type Fourth Generation Quarks with the ATLAS Detector in Events with One Lepton and Hadronically DecayingWBosons

This Letter presents a search for pair production of heavy down-type quarks decaying via b^'→Wt in the lepton+jets channel, as ${b}^{{'}}{b̄}^{{'}}{→}{W}^{{-}}t{W}⁺t̄{→}bb̄{W}⁺{W}^{{-}}{W}⁺{W}^{{-}}{→}{l}^{±{}}{ν}bb̄qq̄qq̄qq̄$. 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 $√s=7 TeV$ recorded with the ATLAS detector, a heavy down-type quark with mass less than 480 GeV can be excluded at the 95% confidence level.