# Search for pair production of vector-like top quarks in events with one lepton, jets, and missing transverse momentum in $\sqrt{s} = 13$ TeV pp collisions with the ATLAS detector

## TL;DR

This paper reports a search for vector-like top quarks in proton-proton collisions at 13 TeV using the ATLAS detector, setting mass limits and excluding certain parameter spaces based on the absence of observed excess events.

## Contribution

First search for vector-like top quarks in the specific decay channel with detailed limits set on mass and branching ratios using 13 TeV data from ATLAS.

## Key findings

- No significant excess observed over Standard Model predictions.
- Lower mass limits for vector-like T quarks range from 870 GeV to 1.16 TeV.
- Excludes large parameter space for T quark masses below 1 TeV.

## 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 $Z(\rightarrow \! \! \nu \nu) \, t + X$ 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 $\mathrm{fb}^{-1}$. 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.