Search for pair production of heavy vector-like quarks decaying into high-$p_T$ $W$ bosons and top quarks in the lepton-plus-jets final state in $pp$ collisions at $\sqrt{s}$=13 TeV with the ATLAS detector

TL;DR

This paper reports a search for heavy vector-like B quarks decaying into W bosons and top quarks in proton-proton collisions at 13 TeV, setting mass limits based on ATLAS data without observing significant deviations from the Standard Model.

Contribution

It provides the first mass limits for vector-like B quarks decaying into Wt, Zb, and Hb final states using 13 TeV data, expanding previous searches.

Findings

Mass limit of 1350 GeV for B quarks with 100% decay to Wt.

Lower mass limit of 1170 GeV in the SU(2) singlet scenario.

Constraints on B quark production as a function of decay branching ratios.

Abstract

A search is presented for the pair production of heavy vector-like $B$ quarks, primarily targeting $B$ quark decays into a $W$ boson and a top quark. The search is based on $36.1$ $fb^{-1}$ of $pp$ collisions at $\sqrt{s}$ = 13 TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. Data are analysed in the lepton-plus-jets final state, characterised by a high-transverse-momentum isolated electron or muon, large missing transverse momentum, and multiple jets, of which at least one is $b$-tagged. No significant deviation from the Standard Model expectation is observed. The 95% confidence level lower limit on the $B$ mass is 1350 GeV assuming a 100% branching ratio to $Wt$. In the SU(2) singlet scenario, the lower mass limit is 1170 GeV. This search is also sensitive to a heavy vector-like $B$ quark decaying into other final states ($Zb$ and $Hb$) and thus mass limits on $B$ production are set as a function of the decay branching ratios. The 100% branching ratio limits are found to be also applicable to heavy vector-like $X$ production, with charge $+$5/3, that decay into $Wt$.