Measurement of the production cross-section of a single top-quark in association with a $Z$ boson at 13 TeV with the ATLAS detector

TL;DR

This paper reports the measurement of the production cross-section of a single top-quark in association with a Z boson at 13 TeV using ATLAS data, employing neural networks for background suppression, and achieving a significance of 4.2 sigma.

Contribution

It presents the first measurement of the single top-quark plus Z boson production cross-section at 13 TeV with neural network analysis for background rejection.

Findings

Measured cross-section: 600 ± 170 (stat) ± 140 (syst) fb

Signal significance: 4.2 sigma in data

Expected significance: 5.4 sigma

Abstract

The production of a top quark in association with a $Z$ boson is studied in the trilepton channel. The data collected by the ATLAS experiment at the LHC in 2015 and 2016 at a centre-of-mass energy of $\sqrt{s} = 13\,\text{TeV}$ are used, corresponding to an integrated luminosity of $36.1\,\text{fb}^{-1}$. Events containing three identified leptons (electron and/or muon) and two jets, one of which is identified as a $b$-quark jet are selected. The major backgrounds come from diboson, $t\bar{t}$ and $Z+$jets production. A neural network is used to improve the background rejection and extract the signal. The resulting significance of the signal is $4.2\sigma$ in the data and the expected significance is $5.4\sigma$. The measured cross-section is $600 \pm 170\text{(stat)} \pm 140\text{(syst)}\,\text{fb}$.