High-statistics study ofϒ(2S)→π+π−ϒ(1S)

A detailed investigation of the decay UPSILON(2S)..--> pi../sup +/..pi../sup -/UPSILON(1S) has been made from 128 000 UPSILON(2S) 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(2S)..--> pi../sup +/..pi../sup -/UPSILON(1S), UPSILON(1S)..-->..e/sup +/e/sup -/ or ..mu../sup +/..mu../sup -/. Assuming e-..mu.. universality we find B(UPSILON(1S)..--> mu../sup +/..mu../sup -/) = (2.84 +- 0.18 +- 0.20)%. A search for UPSILON(2S)..-->..etaUPSILON(1S) yields an upper limit of 1% at the 90% confidence level for this branching ratio. We have also searched for (UPSILON(2S) or UPSILON(3S))..--> pi../sup +/..pi../sup -/UPSILON(1S), UPSILON(1S)..-->..noninteracting particles.