Sensitivity of searches for the flavour-changing neutral current decay $t\rightarrow qZ$ using the upgraded ATLAS experiment at the High Luminosity LHC

The sensitivity of the ATLAS experiment in the search for flavour-changing neutral-current top quark decays is presented. The study is performed in the context of the high luminosity phase of the Large Hadron Collider with a centre-of-mass energy of 14 TeV and an integrated luminosity of 3000 fb$^{-1}$. The three charged lepton final state of top-quark pair events is considered, in which one of the top quarks decays through the $t\to qZ$ $(q=u,c)$ flavour-changing neutral-current channel and the other one decays to $bW$ ($t\bar t\to bWqZ\to b\ell\nu q\ell\ell$). An improvement by a factor of four is expected over the current Run-2 analysis results of $\mathcal{B}(t\to uZ) < 1.7\times 10^{-4}$ and $\mathcal{B}(t\to cZ) < 2.4\times 10^{-4}$ with 36.1 fb$^{-1}$ integrated luminosity. Obtained branching ratio limits are at the level of 4 to 5 $\times~10^{-5}$ depending on the considered scenarios for the systematic uncertainties.