We report the first observation of $B^{\pm }\to \psi (2S){\pi }^{\pm }$, a Cabibbo- and color-suppressed decay. This analysis is based on $657\times {10}^6$ $B\overline{B}$ events collected at the ${\rm Y}(4S)$ resonance with the Belle detector at the KEKB energy-asymmetric $e^{+}{e}^{-}$ collider. The measured branching fraction is $(2.44\pm 0.22\pm 0.20)\times {10}^{-5}$ and the charge asymmetry is $\mathcal{A}=0.022\pm 0.085\pm 0.016$. The ratio of the branching fractions $\mathcal{B}(B^{\pm }\to \psi (2S){\pi }^{\pm })/\mathcal{B}(B^{\pm }\to \psi (2S)K^{\pm })=(3.99\pm 0.36\pm 0.17)\%$ is also determined.

• Received 15 July 2008

DOI:https://doi.org/10.1103/PhysRevD.78.051104