  A search for direct pair production of the supersymmetric partner of the top
quark, decaying via a scalar tau to a nearly massless gravitino, has been
performed using 20 fb$^{-1}$ of proton-proton collision data at $\sqrt{s}=8$
TeV. The data were collected by the ATLAS experiment at the LHC in 2012. Top
squark candidates are searched for in events with either two hadronically
decaying tau leptons, one hadronically decaying tau and one light lepton, or
two light leptons. No significant excess over the Standard Model expectation is
found. Exclusion limits at 95% confidence level are set as a function of the
top squark and scalar tau masses. Depending on the scalar tau mass, ranging
from the 87 GeV LEP limit to the top squark mass, lower limits between 490 GeV
and 650 GeV are placed on the top squark mass within the model considered.
