Search for flavour-changing neutral current top-quark decays to $qZ$ in $pp$ collision data collected with the ATLAS detector at $\sqrt{s}=8$ TeV

TL;DR

This paper reports a search for rare top-quark decays to a quark and a Z boson using ATLAS data, setting upper limits on the decay rate due to no observed signal.

Contribution

It provides the first stringent upper limit on the branching ratio of the rare decay $t o qZ$ at 8 TeV collision energy.

Findings

No evidence for $t\to qZ$ decay was observed.

Upper limit on the branching ratio is $7\times 10^{-4}$ at 95% CL.

Analysis used leptonic decays of Z and W bosons.

Abstract

A search for the flavour-changing neutral-current decay $t\to qZ$ is presented. Data collected by the ATLAS detector during 2012 from proton-proton collisions at the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=8$ TeV, corresponding to an integrated luminosity of 20.3 fb$^{-1}$, are analysed. Top-quark pair-production events with one top quark decaying through the $t\to qZ$ ($q=u,c$) channel and the other through the dominant Standard Model mode $t\to bW$ are considered as signal. Only the decays of the $Z$ boson to charged leptons and leptonic $W$ boson decays are used. No evidence for a signal is found and an observed (expected) upper limit on the $t\to qZ$ branching ratio of $7\times 10^{-4}$ ($8\times 10^{-4}$) is set at the 95% confidence level.