Search for pair production of vector-like top quarks in events with one lepton and an invisibly decaying Z boson in $\sqrt{s} = 13$ TeV pp collisions at the ATLAS detector

The results of a search for vector-like top quarks in the $Zt + X$ channel using final states with exactly one lepton, at least four jets, and large missing transverse momentum are reported. 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 the pair production of a vector-like $T$ quark as a function of its 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.