Speaker
Remi Louvot
Description
The general-purpose Belle dectector is located at the KEK $B$ factory (Tsukuba, Japan). This asymmetric $e^+e^-$ collider was designed to produce a large number of $B0$ and $B^+$ meson pairs by running at a center-of-mass energy near the $\Upsilon(4S)$ resonance ($\sqrt s\approx10.58$ GeV). By increasing the energy above the $B_s0\bar B_s0$ threshold, the large potential of the $B$ factories for exploring the poorly-known $B_s0$ meson was quickly demonstrated. So far, the Belle detector have accumulated more than $\approx100$ fb$^{-1}$ at the $\Upsilon(5S)$ energy ($\sqrt s\approx10.87$ GeV) representing $\sim3$ millions of $B_s0$ pairs. This represents 99\% of the world's sample at this energy. The study of the $B_s0$ is of high interest and the Belle sample, where the number of produced $B_s0$ is accessible, is a unique chance to measure precisely an absolute branching fraction. The measurement of five $B_s0$ branching fractions using the first 23.6 fb$^{-1}$ of data will be described, and prospects for improving the precision through a better normalization will be discussed. The full reconstruction of a clean $B_s0\to D_s^-\pi^+$ sample ($161\pm15$ events) provided the current most precise measured $B_s0$ branching fraction. Additionally, the $B_s^{\ast}$ and $B_s0$ masses and the $B_s0$ production-mode fractions were also measured. A $3.5\sigma$ evidence for its Cabibbo-suppress counterpart $B_s0\to D_s^{\mp}K^{\pm}$ was obtained. This analysis was recently extended to three other dominant modes including photons in their final states and led to the first observations of the $B_s0\to D_s^{\ast-}\pi^+$, $B_s0\to D_s^-\rho^+$ and $B_s0\to D_s^{\ast-}\rho^+$ modes.
