A search for neutral B meson oscillations at the Tevatron collider experiment DO

We present a search for B0s oscillations using semileptonic Bs → DsmuX (Ds → K0SK ). The data were collected using the DO detector from events produced in s = 1.96 TeV proton-antiproton collisions at the Fermilab Tevatron. The Tevatron is currently the only place in the world that produces B0s 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 B0s oscillations is the determination of the flavor of the B0s candidate ( B0s or B&d1;0s ) 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 Deltamd. We obtain Delta md = 0.486 +/- 0.021 ps-1, consistent with the world average. The tagging performance is characterized by the effective efficiency, eD2 = (1.90 +/- 0.41)%. We then turn to the search for B0s 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 Deltams > 1.10 ps-1 and a sensitivity of 1.92 ps-1. This result is combined with other analyzed B0s decay channels at DO to obtain a combined 95% C.L. of Delta ms > 14.9 ps.1 and a sensitivity of 16.5 ps-1. The corresponding log likelihood scan has a preferred value of Delta ms = 19 ps-1 with a 90% confidence level interval of 17 < Deltams < 21 ps -1, assuming Gaussian uncertainties. A comparison of the change in the likelihood between Deltams = 19 ps -1 and Deltams = infinity yields an 8% expectation for a background fluctuation.