Improved search for B-S(0)-(B)over-bar(S)(0) oscillations

An improved search for B-s(0) oscillations is performed in the ALEPHdata sample collected during the first phase of LEP, and reprocessed in 1998. Three analyses based on complementary eventselections are presented. First, decays of B-s(0) mesons into hadronic flavour eigenstates are fully reconstructed. This selection yields a small sample of candidates with excellent decay length and momentum resolution and high average B-s(0) purity. Semileptonic decayswith a reconstructed D-s(-) meson provide a second sample with larger statistics, high average B-s(0) purity, but a poorer momentum and decay length resolution due to the partial decay reconstruction. Finally, semileptonic b-hadron decays are inclusively selected and yield the data sample with the highest sensitivity to B-s(0) oscillations, as the much higher statistics compensate for the low average B-s(0) purity and poorer time resolution. A lower limit is set at Deltam(s) > 10.9 ps(-1) at 95%C.L., significantly lower than the expected limit of 15.2 ps(-1).