A detailed investigation of the decay $\Upsilon \mathrm{}\left(2S\right)\mathrm{}\to {\pi }^{+}{\pi }^{-}\Upsilon \mathrm{}\left(1S\right)\mathrm{}$ has been made from 128 000 $\Upsilon \mathrm{}\left(2S\right)\mathrm{}$ decays observed in the CLEO detector at the Cornell Electron Storage Ring (CESR). We find this branching ratio to be (19.1±1.2±0.6)%. The properties of the $\pi \pi$ system have been studied using 491 exclusive events of the type $\Upsilon \mathrm{}\left(2S\right)\mathrm{}\to {\pi }^{+}{\pi }^{-}\Upsilon \mathrm{}\left(1S\right)\mathrm{}$, $\Upsilon \mathrm{}\left(1S\right)\mathrm{}\to e^{+}{e}^{-} or {\mu }^{+}{\mu }^{-}$. Assuming $e-\mu$ universality we find $B\left(\Upsilon \mathrm{}\right(1S)\mathrm{}\to {\mu }^{+}{\mu }^{-})=(2.84\mathrm{}\pm 0.18\pm 0.20)$%. A search for $\Upsilon \mathrm{}\left(2S\right)\mathrm{}\to \eta \Upsilon \mathrm{}\left(1S\right)\mathrm{}$ yields an upper limit of 1% at the 90% confidence level for this branching ratio. We have also searched for $\left[\Upsilon \mathrm{}\right(2S)\mathrm{} or \Upsilon \mathrm{}(3S\left)\right]\to {\pi }^{+}{\pi }^{-}\Upsilon \mathrm{}\left(1S\right)\mathrm{}$, $\Upsilon \mathrm{}\left(1S\right)\mathrm{}\to \mathrm{noninteracting}\mathrm{particles}$.

• Received 5 March 1984

DOI:https://doi.org/10.1103/PhysRevD.30.1433