Combination of searches for singly produced vector-like top quarks in pp collisions at $\sqrt{s} = 13$ TeV with the ATLAS detector

TL;DR

This paper combines multiple searches for singly produced vector-like top quarks at 13 TeV with the ATLAS detector, setting new exclusion limits up to 2.1 TeV for certain models.

Contribution

It provides the first combined analysis of various single production channels of vector-like top quarks using the full Run 2 ATLAS data set.

Findings

No significant excess over the Standard Model background.

Excluded vector-like top quark masses below 2.1 TeV for specific coupling assumptions.

Set the most restrictive limits to date on vector-like top quark production.

Abstract

A combination of searches for the single production of vector-like top quarks ($T$) is presented. These analyses are based on proton$-$proton collisions at $\sqrt{s}=13$ TeV recorded in 2015$-$2018 with the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb$^{-1}$. The $T$-quark decay modes considered in this combination are into a top quark and either a Standard Model Higgs boson or a $Z$ boson ($T \to Ht$ and $T \to Zt$). The individual searches used in the combination are differentiated by the number of leptons ($e$, $\mu$) in the final state. The observed data are found to be in good agreement with the Standard Model background prediction. Interpretations are provided for a range of masses and couplings of the vector-like top quark for benchmark models and generalized representations in terms of 95% confidence level limits. For a benchmark signal prediction of a vector-like top quark SU2 singlet with electroweak coupling, $\kappa$, of 0.5, masses below 2.1 TeV are excluded, resulting in the most restrictive limits to date.