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

A search for the flavour-changing neutral-current decay $t\rightarrow qZ$ 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 $t\rightarrow qZ$ ( $q=u,c$ ) channel and the other through the dominant Standard Model mode $t\rightarrow bW$ 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 $t\rightarrow qZ$ branching ratio of $7\times 10^{-4}$ ( $8\times 10^{-4}$ ) is set at the 95 % confidence level