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 top squark pair production using events with at least two leptons including a same-flavour opposite-sign pair with invariant mass consistent with the Z boson mass, jets tagged as originating from b -quarks and missing transverse momentum. The analysis is performed with proton–proton collision data at sqrt(s)=8 TeV collected with the ATLAS detector at the LHC in 2012 corresponding to an integrated luminosity of 20.3 fb -1 . No excess beyond the Standard Model expectation is observed. Interpretations of the results are provided in models based on the direct pair production of the heavier top squark state ( t~2 ) followed by the decay to the lighter top squark state ( t~1 ) via t~2?Zt~1 , and for t~1 pair production in natural gauge-mediated supersymmetry breaking scenarios where the neutralino ( chi~01 ) is the next-to-lightest supersymmetric particle and decays producing a Z boson and a gravitino ( G~ ) via the chi~01 -> ZG~ process.