Selection and study of the $B_{s}0 \to D_{s}-\pi^{+}$ decay at LHCb

The decay channel B_s0 -> D_s- pi+ will be used in LHCb to extract the physics parameters Delta M_s and Delta Gamma_s and the wrong tag fraction omega. This flavour specific decay may also serve as a control channel for B_s0 -> D_s+- K+- which is used to extract the CP-violating phases gamma + phi_s. This study of fully Monte Carlo simulated events shows that an annual signal yield of (111.4 +- 0.5)k reconstructed, triggered and selected events can be obtained. The background-to-signal ratio for this channel is expected to be 0.83 +- 0.09 where the dominant source of background is assumed to be bb~. The effective combined tagging efficiency is expected to be (6.10 +-0.19)%. The proper time resolution is also investigated and a value of 41.8 fs is found. This resolution is dominated by the spatial resolution on the B_s-decay vertex. This is still a very good resolution which will allow to resolve the fast B_s0<->B_s~0 oscillations. The efficiency of selection is analysed as a function of the true proper time before and after trigger. The results show that this efficiency is smaller for low proper times (due to the cuts requiring a displaced secondary vertex) and is also decreasing for large proper time. This effect is found to be enhanced by the Level-0 trigger and even more by the Level-1 trigger. Finally, the sensitivity to Delta M_s is calculated and the analysis shows that this parameter will be measurable (with a statistical significance of at least 5 sigma) up to 46 ps-1.