Search for pair production of heavy top-like quarks decaying to a high-$p_{\rm T}$ $W$ boson and a $b$ quark in the lepton plus jets final state in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

A search is presented for production of a heavy top-like 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 14.3 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}=8$ TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider. Data are analysed in the lepton+jets final state, characterised by a high-transverse-momentum isolated electron or muon, large missing transverse momentum and at least four jets, with at least one of them $b$ tagged. The analysis strategy relies on the substantial boost of the $W$ bosons in the $T \bar{T}$ signal when $m_{T}\gtrsim 400$ GeV. No significant excess of events above the Standard Model expectation is observed. For a chiral fourth generation quark, and under the assumption of a branching ratio $BR(T \to Wb) = 1$, a mass lower than 740 GeV is excluded at 95% CL. For vector-like $T$ quarks, and under the assumption that only the $T \to Wb$, $T \to Zt$ and $T \to Ht$ decay modes contribute, 95% CL upper limits are derived for various masses in the two-dimensional plane of $BR(T \to Wb)$ versus $BR(T \to Ht)$, where $H$ is the Standard Model Higgs boson. These limits are significantly improved by combining this search with a previous search for $T\bar{T} \to Ht+X$, with $H\to b\bar{b}$.