Improved measurement of branching fractions for ππ transitions among Υ(nS) states

Using samples of (5.93 ± 0.10) × 10 6 Y(3S) decays and (9.11 ± 0.14) X 10 6 Y(2S) decays collected with the CLEOdetector, we report improved measurements of the following branching fractions: B(Y(3S) → Y(1S)π + π - ) = (4.46 ± 0.01 ± 0.13)%, B(Y(2S) → Y(1S)π + π - ) = (18.02 ± 0.02 ± 0.61)%, B(Y(3S) → Y(1S)π 0 π 0 ) = (2.24 ± 0.09 ± 0.11)%, B(Y(2S)→ Y(1S)π 0 π 0 )=(8.43±0.16±0.42)%, and B(Y(3S)→Y(2S)π 0 π 0 ) = (1.82 ±0.09 ±0.12)%. In each case, the first uncertainty reported is statistical, while the second is systematic.