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Search for pair production of a new heavy quark that decays into a $W$ boson and a light quark in $pp$ collisions at $\sqrt{s} = 8$ TeV with the ATLAS detector

The ATLAS collaboration
Phys.Rev.D 92 (2015) 112007, 2015.

Abstract (data abstract)
CERN-LHC. Results from a search is presented for pair production of a new heavy quark ($Q$) that decays into a $W$ boson and a light quark ($q$) in the final state where one $W$ boson decays leptonically (to an electron or muon plus a neutrino) and the other $W$ boson decays hadronically. The analysis is performed using an integrated luminosity of 20.3 fb$^{-1}$ of $pp$ collisions at $\sqrt{s} = 8$ TeV collected by the ATLAS detector at the LHC. No evidence of $Q\bar{Q}$ production is observed. New chiral quarks with masses below 690 GeV are excluded at 95% confidence level, assuming BR$(Q\to Wq)=1$. Results are also interpreted in the context of vectorlike quark models, resulting in the limits on the mass of a vectorlike quark in the two-dimensional plane of BR$(Q\to Wq)$ versus BR$(Q\to Hq)$ The main selection requirements for this analysis were: - exactly one isolated lepton with pT(lep) > 25 GeV - missing transverse momentum ETmiss > 30 GeV - "transverse" W mass + ETmiss > 60 GeV - at least four jets with pT(jet) > 25 GeV - zero b-jets - a leptonic W candidate consisting of the lepton+ETmiss system with pT > 125 GeV - a hadronic W candidate consisting of a dijet system with pT > 200 GeV, 65 GeV < mass 100 GeV, and deltaR < 1.0 between the two jets. - two reconstructed heavy quarks (W+1jet systems) with a mass difference less than 120 GeV - At least two jets (not used for the hadronic W) with pT > 120 GeV. - At least one jet (not used for the hadronic W) with pT > 160 GeV. - Scalar sum of |pT| from hadronic W, leptonic W, and two highest pT jets not used for hadronic W must be > 1.1 TeV - Angular cuts between reconstructed objects (see details in paper).

  • Table 1

    Data from Figure 10


    The expected and observed 95% CL upper limits on the cross section as a function of $m_Q$ when setting BR$(Q\to...

  • Table 2

    Data from Figures 11 and 12


    The upper and lower bounds on the range of heavy quark masses expected and observed to be excluded at 95%...

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