Search for top squarks in events with a $Z$ boson using 139 fb$^{-1}$ of $pp$ collision data at $\sqrt{s}=13$ TeV with the ATLAS detector

A search for direct top squark pair production in events with a same-flavour opposite-sign dilepton pair with invariant mass consistent with a $Z$ boson plus missing transverse momentum is presented. The analysis is performed using the proton-proton collision data at $\sqrt{s}$=13 TeV collected by the ATLAS experiment during the LHC Run-2, corresponding to an integrated luminosity of 139 fb$^{-1}$. No excess is observed in the data with respect to the SM predictions. The results are interpreted in simplified models featuring direct production of pairs of either the lighter top squark ($\tilde{t}_1$) or the heavier top squark ($\tilde{t}_2$), excluding at 95% confidence level $\tilde{t}_1$ and $\tilde{t}_2$ masses up to about 1140 and 875 GeV, respectively.