Study of Bs0-meson Production and Measurement of Bs0 Decays into a Ds(*)- and a Light Meson in e+e- Collisions at √s=10.87 GeV

This work is dedicated to the experimental study of Bs0 production in e+e– collisions at the Υ(5S) resonance (√s = 10.87 GeV), as well as measurement with fully reconstructed Bs0 decays involving one Ds()– meson. The analysed data sample was recorded between June 2005 and December 2009 with the Belle detector at the KEKB storage ring in Japan, and represents the largest statistics ever collected at the Υ(5S) resonance. After having performed a fit of the current existing measurements of the fraction of Υ(5S) events producing a Bs0 Bs0 meson pair, we perform a new measurement of the ratio between the Υ(5S) events producing a Bs0 Bs0 meson pair and those involving a non-strange BB meson pair, by implementing an alternative method based on sign correlations of dilepton events in 121 fb-1 of Υ(5S) data. The result is where the first quoted error includes the statistical uncertainty and the errors due to external physics parameters, and the second quoted error represents uncertainties due to the selection and to the fitting procedure. The total relative error is smaller than that of the average obtained from other existing measurements with the above-mentioned fit, We also fully reconstruct Bs0 → Ds–π+ decays with a sample of 23.4 fb-1. From this high-purity signal, we obtain the most precise measurement of a Bs0 exclusive decay (the dominant systematic error, due to the fs fraction, is quoted separately): together with the world most precise measurement of the Bs mass, and the second most precise measurement of the Bs0 mass, With the same signal, the production fractions of excited Bs* mesons at the Υ(5S) are obtained: These fruitful analyses are extended to four other Bs0 modes which branching fractions are measured: The three decays, Bs0 → Ds*– π+, Bs0 → Ds– ρ+ and Bs0 → Ds*– ρ+ decays are observed for the first time with significances in excess of 8σ. The Bs0 → Ds*– ρ+ decay involves two polarisations and we report a measurement of its longitudinal polarisation fraction, which is the first polarisation measurement of a Bs0 decay. All these results are in agreement with expectations from heavy-flavour theory and B0 counterparts.