Expected sensitivity of ATLAS to FCNC top quark decays $t \rightarrow Zu$ and $t \rightarrow Hq$ at the High Luminosity LHC

The expected sensitivity of the upgraded ATLAS experiment at HL-LHC to the flavour changing neutral current (FCNC) top quark decays $t \rightarrow Zu$ and $t \rightarrow Hq$ is estimated for an integrated luminosity of $3000\;\mathrm{fb}^{-1}$ at a centre-of-mass energy of 14 TeV. The analysis is based on tt̄ events produced in proton-proton collisions in which one top quark decays to $W(\to \ell \nu)b$ final states, while the other top quark decays to $Z(\to \ell\ell)u/c$ or $H(→ b\bar{b})u/c$ final states via FCNC interactions. Three different ATLAS detector upgrade scenarios are considered in this study. For the most optimistic upgrade scenario the expected limits on the FCNC top quark branching ratios are found to be $(2.4-5.8) \times 10^{-5}$ for $t \rightarrow Zq$ and $(0.6-1.2) \times 10^{-4}$ for $t \rightarrow Hq$ channels respectively at the 95% confidence level, depending on the FCNC modelling. When systematic uncertainties are also considered, the sensitivity decreases to $(8.3 - 41) \times 10^{-5}$ and $(1.1 - 2.4) \times 10^{-4}$, respectively, depending on the detailed assumptions.