A search for Supersymmetry in events containing a leptonically decaying $Z$ boson, jets and missing transverse momentum in $\sqrt{s}=13~$TeV $pp$ collisions with the ATLAS detector

A search for supersymmetric particles in final states containing a same-flavour opposite-sign lepton (electron or muon) pair with an invariant mass consistent with that of the $Z$ boson, jets and large missing transverse momentum is presented. This search makes use of proton--proton collision data, collected at a centre-of-mass energy $\sqrt{s}=13$ TeV by the ATLAS detector at the Large Hadron Collider, which correspond to an integrated luminosity of $3.2$ fb$^{-1}$. A total of 21 events are observed on $10.3 \pm 2.3$ expected, resulting in an excess of events above the Standard Model expectation, with a significance of $2.2$ standard deviations. This result is interpreted in the context of a simplified model involving the production of gluinos, which decay via the second lightest neutralino ($\tilde{\chi}_2^0$) to $Z$ bosons and stable lightest-supersymmetric-particles. Within this model, gluinos are excluded up to a mass of $1.1$ TeV for a $\tilde{\chi}_2^0$ mass of $700$ GeV.