Search for pair production of heavy top-like quarks decaying to a high-pT W boson and a b quark in the lepton plus jets final state at sqrt(s)=7 TeV with the ATLAS detector

TL;DR

This paper searches for heavy top-like quarks decaying to W bosons and b quarks in proton-proton collisions at 7 TeV, setting mass limits and exploring decay branching ratios with no significant excess found.

Contribution

It provides the first mass exclusion limit for a fourth-generation t' quark at 656 GeV and explores decay branching ratios for vector-like quarks using ATLAS data.

Findings

Excluded t' quark masses below 656 GeV at 95% CL.

No significant excess above Standard Model expectations.

Derived limits on decay branching ratios of vector-like quarks.

Abstract

A search is presented for production of a heavy up-type quark (t') together with its antiparticle, assuming a significant branching ratio for subsequent decay into a W boson and a b quark. The search is based on 4.7 fb^-1 of pp collisions at sqrt(s)=7 TeV recorded in 2011 with the ATLAS detector at the CERN Large Hadron Collider. Data are analyzed in the lepton+jets final state, characterized by a high-transverse-momentum isolated electron or muon, large missing transverse momentum and at least three jets. The analysis strategy relies on the substantial boost of the W bosons in the t't'bar signal when mt'>=400 GeV. No significant excess of events above the Standard Model expectation is observed and the result of the search is interpreted in the context of fourth-generation and vector-like quark models. Under the assumption of a branching ratio BR(t'->Wb)=1, a fourth-generation t' quark with mass lower than 656 GeV is excluded at 95% confidence level. In addition, in light of the recent discovery of a new boson of mass ~126 GeV at the LHC, upper limits are derived in the two-dimensional plane of BR(t'->Wb) versus BR(t'->Ht), where H is the Standard Model Higgs boson, for vector-like quarks of various masses.