Search for direct top squark pair production in events with a $Z$ boson, $b$-jets and missing transverse momentum in $\sqrt{s}$=13 TeV $pp$ collisions with the ATLAS detector

A search for direct top squark pair production resulting in events with at least three leptons, including a same-flavour opposite-sign pair with invariant mass compatible with that of $Z$ boson, $b$-quark jets and missing transverse momentum is presented. The analysis is performed using proton-proton collision data at $\sqrt{s}=13$ TeV collected with the ATLAS detector at the LHC in 2015-2016 corresponding to an integrated luminosity of 13.3 fb$^{-1}$. The results are interpreted in models featuring direct pair production of the heavier top squark $\tilde{t}_{2}$ decaying to the lighter top squark $\tilde{t}_{1}$ via $\tilde{t}_{2} \rightarrow Z \tilde{t}_{1}$.