Measurement of branching fractions for B \to J/ψηK decays and search for a narrow resonance in the J/ψηfinal state

Abstract

We report an observation of the $B^{\pm }\to J/\ddot{I}ˆ\hat{I}\cdot K^{\pm }$ and $B^0\to J/\ddot{I}ˆ\hat{I}\cdot K_S^0$ decays using 772$\times 10^6$ $B\bar{B}$ pairs collected at the $\hat{I}\yen (4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We obtain the branching fractions $\mathcal{B}(B^{\pm }\to J/\ddot{I}ˆ\hat{I}\cdot K^{\pm })=(1.27\pm 0.11(stat.)\pm 0.11(syst.))\times 10^{-4}$ and $\mathcal{B}(B^0\to J/\ddot{I}ˆ\hat{I}\cdot K_S^0)=(5.22\pm 0.78(stat.)\pm 0.49(syst.))\times 10^{-5}$. We search for a new narrow charmonium(-like) state $X$ in the $J/\ddot{I}ˆ\hat{I}\cdot$ mass spectrum and find no significant excess. We set upper limits on the product of branching fractions, $\mathcal{B}(B^{\pm }\to X{K}^{\pm })\mathcal{B}(X\to J/\ddot{I}ˆ\hat{I}\cdot )$, at 3872 MeV$/c^2$ where a $C$-odd partner of X(3872) may exist, at $\ddot{I}ˆ(4040)$ and $\ddot{I}ˆ(4160)$ assuming their known mass and width, and over a range from 3.8 to 4.8 GeV$/c^2$. % at a 5 MeV$/c^2$ step. The obtained upper limits at 90% confidence level for $X^{C-\mathrm{odd}}(3872)$, $\ddot{I}ˆ(4040)$ and $\ddot{I}ˆ(4160)$ are 3.8$\times 10^{-6}$, 15.5$\times 10^{-6}$ and 7.4$\times 10^{-6}$, respectively.

Submitted to PTEP