Abstract
The results of a search for vector-like top quarks using events with exactly one lepton, at least four jets, and large missing transverse momentum are reported. The search is optimised for pair production of vector-like top quarks in the Z(→νν) t + X decay channel. LHC pp collision data at a centre-of-mass energy of sqrt{s}=13 TeV recorded by the ATLAS detector in 2015 and 2016 are used, corresponding to an integrated luminosity of 36.1 fb−1. No significant excess over the Standard Model expectation is seen and upper limits on the production cross-section of a vector-like T quark pair as a function of the T quark mass are derived. The observed (expected) 95% CL lower limits on the T mass are 870 GeV (890 GeV) for the weak-isospin singlet model, 1.05 TeV (1.06 TeV) for the weak-isospin doublet model and 1.16 TeV (1.17 TeV) for the pure Zt decay mode. Limits are also set on the mass as a function of the decay branching ratios, excluding large parts of the parameter space for masses below 1 TeV.
Highlights
Background estimationThe dominant background in the signal region (SR) is due to ttproduction, followed by about equal contributions from with associated jets (W +jets) and single-top production
Limits are set on the mass as a function of the decay branching ratios, excluding large parts of the parameter space for masses below 1 TeV
The number of events observed in the SR, Control regions (CRs) and the validation regions (VRs) is shown in table 4, together with the backgrounds estimated in the simultaneous fit
Summary
The ATLAS detector [24] is a multi-purpose particle detector at the LHC with nearly 4π coverage around the collision point. Closest to the beam is the inner detector (ID), 1ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the centre of the detector and the z-axis along the beam pipe. The ATLAS detector [24] is a multi-purpose particle detector at the LHC with nearly 4π coverage around the collision point.. The ID is surrounded by a superconducting solenoid providing a 2 T axial magnetic field, followed by an electromagnetic lead/liquid-argon (LAr) sampling calorimeter and a hadronic (steel/scintillator-tile) calorimeter. The outer part of the detector consists of a muon spectrometer (MS) with high-precision tracking chambers for coverage up to |η| = 2.7, fast detectors for triggering over |η| < 2.4, and three large superconducting toroidal magnets with eight coils each. The data were collected using triggers that select events with high missing transverse momentum, with online thresholds of 70 GeV in 2015 and 90 GeV to 110 GeV in 2016. The triggers have an efficiency greater than 99% for an offline ETmiss requirement of at least 300 GeV
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