Search for production of vector-like quark pairs and of four top quarks in the lepton plus jets final state in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

A search is presented for pair production of vector-like quarks, both up-type ($T$) and down-type ($B$), as well as for four-top-quark production. The search is based on $pp$ collisions at $\sqrt{s}=8$ TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider and corresponding to an integrated luminosity of 20.3 fb$^{-1}$. 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. Dedicated analyses are performed targeting three cases: a $T$ quark with significant branching to a $W$ boson and a $b$ quark ($T\bar{T} \to Wb$+X), and both a $T$ quark and a $B$ quark with significant branching ratio to a Higgs boson and a third generation quark ($T\bar{T} \to Ht$+X and $B\bar{B} \to Hb$+X respectively). The analyses exploit characteristic features of the signals to discriminate against the dominant background from top quark pair production, such as the high total transverse momenta of all final state objects, the presence of boosted hadronically-decaying $W$ bosons, or the presence of Higgs bosons decaying into $b\bar{b}$, resulting in high $b$-jet multiplicity. No significant excess of events above the Standard Model expectation is observed, and 95% CL lower limits are derived on the masses of the vector-like $T$ and $B$ quarks under several branching ratio hypotheses assuming contributions from $T \to Wb$, $Zt$, $Ht$ and $B \to Wt$, $Zb$, $Hb$ decays. The 95% CL observed lower limits on the $T$ quark mass range between 715 GeV and 950 GeV for all possible values of the branching ratios into the three possible decay modes, representing the most stringent constraints to date. Additionally, the most restrictive existing upper bounds on four-top-quark production are set in a number of new physics scenarios.