We study transitions between ${\rm Y}$ states with the emission of charged pions using $477{\mathrm{fb}}^{-1}$ of data collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. The measured product branching fraction $\mathcal{B}({\rm Y}(4S)\to {\rm Y}(1S){\pi }^{+}{\pi }^{-})\times \mathcal{B}({\rm Y}(1S)\to {\mu }^{+}{\mu }^{-})=(4.42\pm 0.81(\mathrm{stat})\pm 0.56(\mathrm{sys}))\times {10}^{-6}$. When the PDG value for $\mathcal{B}({\rm Y}(1S)\to {\mu }^{+}{\mu }^{-})$ is used, this corresponds to $\mathcal{B}({\rm Y}(4S)\to {\rm Y}(1S){\pi }^{+}{\pi }^{-})=(1.78\pm 0.33(\mathrm{stat})\pm 0.23(\mathrm{sys}))\times {10}^{-4}$ and a partial decay width $\Gamma ({\rm Y}(4S)\to {\rm Y}(1S){\pi }^{+}{\pi }^{-})=(3.65\pm 0.67(\mathrm{stat})\pm 0.65(\mathrm{sys}))\mathrm{keV}$.

• Received 15 November 2006

DOI:https://doi.org/10.1103/PhysRevD.75.071103