We present a search for [special characters omitted] oscillations using semileptonic B_s → D_sμX (D_s → [special characters omitted]). The data were collected using the DØ detector from events produced in [special characters omitted] = 1.96 TeV proton-antiproton collisions at the Fermilab Tevatron. The Tevatron is currently the only place in the world that produces [special characters omitted] mesons and will be until early 2008 when the Large Hadron Collider begins operating at CERN.

One of the vital ingredients for the search for [special characters omitted] oscillations is the determination of the flavor of the [special characters omitted] candidate ([special characters omitted] or [special characters omitted]) at the time of its production, called initial state flavor tagging. We develop an likelihood based initial state flavor tagger that uses objects on the side of the event opposite to the reconstructed B meson candidate. To improve the performance of this flavor tagger, we have made it multidimensional so that it takes correlations between discriminants into account. This tagging is then certified by applying it to sample of semimuonic B^(0,+) decays and measuring the well-known oscillation frequency Δm_d. We obtain Δ m_d = 0.486 ± 0.021 ps^-1, consistent with the world average. The tagging performance is characterized by the effective efficiency, [special characters omitted] = (1.90 ± 0.41)%. We then turn to the search for [special characters omitted] oscillations in the above-named channel. A special two-dimensional mass fitting procedure is developed to separate kinematic reflections from signal events. Using this mass fitting procedure in an unbinned likelihood framework, we obtain a 95% C.L. of Δm_s > 1.10 ps^-1 and a sensitivity of 1.92 ps^-1. This result is combined with other analyzed [special characters omitted] decay channels at DØ to obtain a combined 95% C.L. of Δ m_s > 14.9 ps.1 and a sensitivity of 16.5 ps^-1. The corresponding log likelihood scan has a preferred value of Δ m_s = 19 ps^-1 with a 90% confidence level interval of 17 < Δm_s < 21 ps ^-1, assuming Gaussian uncertainties. A comparison of the change in the likelihood between Δm_s = 19 ps ^-1 and Δm_s = ∞ yields an 8% expectation for a background fluctuation.