Abstract
A search for pair production of vector-like quarks, both up-type ($T$) and down-type ($B$), as well as for four-top-quark production, is presented. 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 an isolated electron or muon with high transverse momentum, large missing transverse momentum and multiple jets. Dedicated analyses are performed targeting three cases: a $T$ quark with significant branching ratio 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). 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 decay modes, and are the most stringent constraints to date. Additionally, the most restrictive upper bounds on four-top-quark production are set in a number of new physics scenarios.
Highlights
The discovery of a new particle consistent with the Standard Model (SM) Higgs boson by the ATLAS [1] and CMS [2] collaborations is a major milestone in high-energy physics
The results presented in this paper complete the program of searches for pair production of vector-like quarks decaying into√third-generation quarks by the ATLAS Collaboration using the pp dataset collected at s = 8 TeV
Samples of single-top-quark backgrounds corresponding to the t-channel, s-channel and W t production mechanisms are generated with Powheg-Box 2.0 [102, 103] using the CT10 parton distribution functions (PDF) set and interfaced to Pythia 6.425 with the CTEQ6L1 PDF set and the Perugia2011C underlying event (UE) tune
Summary
The discovery of a new particle consistent with the Standard Model (SM) Higgs boson by the ATLAS [1] and CMS [2] collaborations is a major milestone in high-energy physics. Resulting f√rom the combination of several analyses in lepton-plus-jets and multilepton final states at s = 8 TeV This search set 95% confidence level (CL) lower limits on the T quark mass ranging between 690 GeV and 780 GeV for all possible values of the branching ratios into the three decay modes. To reduce the background from non-prompt electrons resulting from semileptonic decays of b- or c-hadrons, and from jets with a high fraction of their energy deposited in the EM calorimeter, electron candidates must satisfy calorimeterand track-based isolation requirements. This section describes the different signal scenarios considered in the interpretation of the results, together with details of how they are modelled in the analysis
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