20-27 July 2011
Alpes Congrès - Alpexpo
Europe/Paris timezone

Bs decays at Belle

21 Jul 2011, 15:00
Oisans (Alpes Congrès - Alpexpo)


Alpes Congrès - Alpexpo

Parallel session talk Flavour Physics and Fundamental Symmetries Flavour Physics and Fundamental Symmetries


Dr Remi Louvot (EPFL, Lausanne)


Using fully reconstructed $B_{s}$ mesons, we measure the branching fractions for the decays of $B_s \to D_s^{(*)+}D_s^{(*)-}$ exclusively. Assuming these decay modes saturate decays to CP-even final states, the branching fraction determines the relative width difference between the $CP$-odd and $CP$-even $B_s$ states. We study the decay $B^0_s\to J/\psi \phi$, $\phi \to K^+ K^-$. In this sample $B^0_s$ mesons are produced in three $\Upsilon(5S)$ decays: $\Upsilon(5S) \to B^*_s\overline{B}^*_s$, $B^*_s\overline{B}_s$ and $B_s\overline{B}_s$, where the $B^*_s$ mesons decay to $B_s \gamma$. We determine the absolute branching fraction of the decay $B^0_s\rightarrow J/\psi \phi$, which is an important mode for measuring the CP violating phase $\beta_s$ in the $B_s \overline{B}_s$ mixing. The latter is of particular interest as it is sensitive to physics beyond the Standard Model. We also report the first observation of $B_s^0\to J/\psi f_0(980)$ and first evidence for $B_s^0\to J/\psi f_0(1370)$, which are CP-eigenstate decay modes. We measure the branching fractions $\mathcal{B}(B_s^0\to J/\psi f_0(980);f_0(980)\to\pi^+\pi^-)=(1.16^{+0. 31}_{-0.19}(\mathrm{stat.})^{+0.15}_{-0.17}(\mathrm{syst.})^{+0.26}_{-0. 18}(N_{B_s^{(*)}\bar B_s^{(*)}})) \times 10^{-4}$ with a significance of $8.4\sigma$, and $\mathcal{B}(B_s^0\to J/\psi f_0(1370);f_0(1370)\to\pi^+\pi^-)=(0.34 ^{+0.11}_{-0.14}(\mathrm{stat.})^{+0.03}_{-0.02}(\mathrm{syst.})^{+0.08} _{-0.05}(N_{B_s^{(*)}\bar B_s^{(*)}})) \times 10^{-4}$ with a significance of $4.2\sigma$. The last error listed is due to uncertainty in the number of produced $B_s^{(*)}\bar B_s^{(*)}$ pairs. We have also performed the first search for charmed baryonic $B_s^0$ decay, $\bar{B}_s^0\rightarrow\Lambda_c^+\pi^-\bar{\Lambda}$. The measurement precision of the $B_s^0$ branching fraction at Belle is limited by the poor knowledge of the $B_s^0$ production fraction, $f_s=\sigma(B_s^{(\ast)}\bar B_s^{(\ast)})/\sigma(b\bar b)$. We present the status of the current measurements and discuss their main limitations. Using dilepton events, a new independent measurement of $f_s$ is presented, together with its impact on the precision of Belle's $B_s^0$ measurements. We present a measurement of the CP violation parameter sin2$\phi_1$ at the $\Upsilon$(5S) resonance using a new tagging method called the B-$\pi$ tagging. This method in general uses a $\Upsilon$(5S) decay to a charged B meson, a neutral B meson, and a charged pion. The neutral B meson decays to a CP eigenstate, and the charged B meson is reconstructed as missing mass of the neutral B meson and the charged pion. The initial flavor of the neutral B meson is determined by the charge of the pion at the moment of the $\Upsilon$(5S) decay. CP violating parameters can be derived from asymmetry of the number of BB$\pi^+$ and BB$\pi^-$ events without measuring decay time. Since the B-$\pi$ tagging method have different systematic errors from the analyses on the $\Upsilon$(4S) resonance and thus complementary. In this analysis, the neutral B meson is reconstructed as J/$\psi$K$_{\rm S}$. All these results are based on a data sample collected with the Belle detector at the $\Upsilon(5S)$ resonance with an integrated luminosity of 121.4 fb$^{-1}$ at the KEKB asymmetric-energy $e^+ e^-$ collider.

Primary author

Dr Remi Louvot (EPFL, Lausanne)

Presentation Materials