First Bs mixing results at CDF II

I report the results of the first search for Bs flavor oscillations performed at CDF II. We analyze a dataset of approximately 355 pb¡1 from proton-antiproton collisions at p s = 1.96 TeV collected in 2002-2004 with the CDF II Detector at the Tevatron Collider. Samples of both fully recon- structed, Bs ! Dsπ, and partially reconstructed, Bs ! Dslν, decays are employed. A combination of tagging algorithms is used to determine the flavor of the Bs mesons at production time. We fit the data using an unbinned likelihood technique which combines mass, proper decay time, and flavor tagging information of the Bs candidates. Information about the oscillation frequency of the system Δms is obtained by performing an amplitude scan of the data, from which an exclusion limit Δms > 7.9 ps¡1 (95% C.L.) is achieved, with a measured sensitivity of 8.4 ps¡1. Improvements currently under development, together with the increasing data samples, are expected to enhance the sensitivity beyond the Standard Model favored region before the end of Run II.