Search for direct stop pair production in events with a Z boson, b-jets and missing transverse energy with the ATLAS detector using 21~fb$^{-1}$ from proton-proton collision at $\sqrt{s}=8$ TeV

A search is presented for direct scalar top pair production using events with $Z$ bosons, $b$-jets and large missing transverse momentum in final states with two or three leptons forming a same-flavor opposite-sign lepton pair with invariant mass consistent with the $Z$ boson mass. The analysis is performed with 21~fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=8$~TeV with the ATLAS detector. No excess beyond the Standard Model expectation is observed. Interpretations of this result are provided for natural gauge-mediated supersymmetry breaking scenarios where the neutralino ($\tilde{\chi}^0_1$) is the next-to-lightest supersymmetric particle producing $Z$ bosons and a gravitino ($\tilde{G}$) in the $\tilde{\chi}^0_1 \to Z\tilde{G}$ decay, and in models based on the direct production of heavy stop states ($\tilde{t}_2$) that decay to light stop states ($\tilde{t}_1$) via $\tilde{t}_2 \to Z\tilde{t}_1$.