Search for stop decay into stable gluino with the ALEPH detector

A search for a scalar top quark decaying into a stable gluino has been performed using 207 pb$^{-1}$ data collected by the ALEPH detector at center-of-mass energies between 204 and 209 GeV. No evidence of any signal is observed in the channel $e^+e^-\rightarrow \tilde{t_1}\bar{\tilde{t_1}} \rightarrow c\tilde{g}\bar{c} \tilde{g}$. Preliminary excluded regions in the (gluino mass-stop mass) plane are derived in the framework of R-parity conserving supersymmetric models with unconstrained gaugino masses, under the assumption that the above channel is the dominating gluino production channel. For a gluino mass larger than 20 GeV/$c^2$ and mass differences between stop and gluino larger than 3 GeV/$c^2$, the 95$\%$ C.L. lower mass limit on the stop, decaying into a stable gluino, is 75 GeV/$c^2$.