Search for light scalar top quark pair production in final states with two leptons with the ATLAS detector in $\sqrt{s}=7\TeV$ proton--proton collisions

A search is presented for the pair production of light scalar top quarks in $\sqrt{s}=7$~TeV proton--proton collisions with the ATLAS detector at the Large Hadron Collider. This analysis uses the full data sample collected during 2011 running that corresponds to a total integrated luminosity of 4.7~fb$^{-1}$. Light scalar top quarks are searched for in events with two leptons ($e$, $\mu$), large missing transverse momentum and at least one jet in the final state. No excess over Standard Model expectations is found. By assuming that the light scalar top decays to a $b$ quark in addition to an (on-shell) chargino of mass 106~GeV whose decay occurs through a virtual $W$ boson, light scalar top quark masses below 130~GeV are excluded for neutralino masses below 65~GeV. This extends the previous limit set by the CDF experiment in the same scenario, which assumes that the neutralino mass is heavier than 45 GeV.