Search for Scalar Top Quark Pair Production in Natural Gauge Mediated Supersymmetry Models with the ATLAS Detector in $pp$ Collisions at $\sqrt{s}=$~7~TeV

The results of a search for pair production of the lighter scalar partners of top quarks ($\tone$) in 2.05 fb-1 of $pp$ collisions at $\sqrt{s}=7$~TeV using the ATLAS experiment are reported. Scalar top quarks are searched for in events with two same flavour opposite-sign leptons ($e,\mu$) with invariant mass consistent with the $Z$ boson mass, large missing transverse momentum and jets in the final state. At least one of the jets is identified as originating from a $b$-quark. No excess over Standard Model expectations is found. The results are interpreted in the framework of $R$-parity conserving, gauge-mediated Supersymmetry breaking `natural' scenarios where the neutralino ($\tilde{\chi}_{1}^{0}$) is the next-to-lightest supersymmetric particle. Scalar top quark masses up to 310~GeV are excluded for 115~GeV~$< m_{\tilde{\chi}_{1}^{0}}<$~230~GeV at 95\% confidence-level, reaching an exclusion of $m_{\tone}<$~330~GeV for $m_{\tilde{\chi}_{1}^{0}}=190$~GeV.