Search for vector-like $B$ quarks in events with one isolated lepton, missing transverse momentum and jets at $\sqrt{s}=$ 8 TeV with the ATLAS detector

TL;DR

This paper reports a search for vector-like B quarks in proton-proton collisions at 8 TeV using the ATLAS detector, setting mass limits based on the absence of excess events in the lepton-plus-jets final state.

Contribution

It provides the first limits on vector-like B quark masses at 8 TeV, considering various decay branching ratios, and also sets limits on exotic T_{5/3} fermions.

Findings

No significant excess observed above background.

Lower mass limits for B quarks range from 640 GeV to 810 GeV.

Limits on T_{5/3} mass are set at 840 GeV.

Abstract

A search has been performed for pair production of heavy vector-like down-type ($B$) quarks. The analysis explores the lepton-plus-jets final state, characterized by events with one isolated charged lepton (electron or muon), significant missing transverse momentum and multiple jets. One or more jets are required to be tagged as arising from $b$-quarks, and at least one pair of jets must be tagged as arising from the hadronic decay of an electroweak boson. The analysis uses the full data sample of $pp$ collisions recorded in 2012 by the ATLAS detector at the LHC, operating at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb$^{-1}$. No significant excess of events is observed above the expected background. Limits are set on vector-like $B$ production, as a function of the $B$ branching ratios, assuming the allowable decay modes are $B \rightarrow Wt/Zb/Hb$. In the chiral limit with a branching ratio of 100% for the decay $B \rightarrow Wt$, the observed (expected) 95% CL lower limit on the vector-like $B$ mass is 810 GeV (760 GeV). In the case where the vector-like $B$ quark has branching ratio values corresponding to those of an $SU(2)$ singlet state, the observed (expected) 95% CL lower limit on the vector-like $B$ mass is 640 GeV (505 GeV). The same analysis, when used to investigate pair production of a colored, charge 5/3 exotic fermion $T_{5/3}$, with subsequent decay $T_{5/3} \rightarrow Wt$, sets an observed (expected) 95% CL lower limit on the $T_{5/3}$ mass of 840 GeV (780 GeV).