2015 Theses Doctoral

Search for Heavy Down-Type Vector-Like Quarks in the Lepton-Plus-Jets Final State in 8 TeV proton-proton Collisions Using the ATLAS Detector at the LHC

Hu, Diedi

This dissertation presents a search for pair production of heavy down-type vector-like quarks (VLQ B) using the full 2012 data set of proton-proton collisions at the center of mass 8 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 20.3 1/fb. Events with a single charged lepton, which can be either an electron or muon, missing transverse energy, at least six jets, at least one tagged as a b-jet, and at least one reconstructed hadronically-decaying W/Z boson candidate are analyzed. No significant deviation from the Standard Model background prediction is observed.

Under the assumption that only the B->Wt/Zb/Hb decay modes are allowed, 95% CL upper limits are derived in the two dimensional plane of the B->Wt branching ratio versus B->Hb branching ratio. In the specific case where the branching ratios are consistent with a SU(2) singlet scenario, the observed (expected) 95% CL lower limit on the VLQ B mass is 640 GeV (505 GeV). In the chiral B scenario with a 100% branching ratio of the decay B->Wt, the observed (expected) 95% CL lower limit on the B mass is 814 GeV (756 GeV).