Search for $tZ$ Flavour Changing Neutral Currents in top-quark decays with the ATLAS detector

TL;DR

This paper reports a search for flavor-changing neutral current decays of the top quark into a $Z$ boson and an up or charm quark using ATLAS data, setting new upper limits on these rare processes.

Contribution

It provides the most stringent upper limits to date on the branching ratios of $t\to uZ$ and $t\to cZ$ decays using 13 TeV LHC data.

Findings

No evidence of FCNC top decays was observed.

Upper limits on $t\to uZ$ and $t\to cZ$ branching ratios are about three times better than previous results.

The analysis improves constraints on new physics models predicting FCNC in top decays.

Abstract

A search for flavour-changing neutral current (FCNC) processes in top-quark decays is presented. Data collected from proton-proton collisions at the LHC at a centre-of-mass energy of $\sqrt{s}=13$ TeV corresponding to an integrated luminosity of 36 fb$^{-1}$, are analysed. A search is performed for top-quark pair-production events, with one top-quark decaying through the $t\to qZ$ ($q=u,c$) FCNC channel, and the other through the dominant Standard Model mode $t\to bW$. Only the decays of the $Z$ boson to charged leptons and leptonic $W$ boson decays are considered as signal. No evidence for a signal is found. The observed and expected upper limits on the branching ratio of $t\to uZ$ and $t\to cZ$ are set at 95\% confidence level and are about a factor 3 better than the ones obtained with the Run 1 data of the ATLAS detector.