Observation of Upsilon(4S) decays to pi+ pi- Upsilon(1S) and pi+ pi- Upsilon(2S)

We present the first measurement of Υ(4S)Υ(4S) decays to π+pi−Υ(1S)π+pi−Υ(1S) and π+pi−Υ(2S)π+pi−Υ(2S) based on a sample of 230×106×106 Υ(4S)Υ(4S) mesons collected with the BABAR detector. We measure the product branching fractions B(Υ(4S)→π+pi−Υ(1S))×B(Υ(1S)→μ+μ−)=(2.23±0.25stat±0.27sys)×10−6B(Υ(4S)→π+pi−Υ(1S))×B(Υ(1S)→μ+μ−)=(2.23±0.25stat±0.27sys)×10−6 and B(Υ(4S)→π+pi−Υ(2S))×B(Υ(2S)→μ+μ−)=(1.69±0.26stat±0.20sys)×10−6B(Υ(4S)→π+pi−Υ(2S))×B(Υ(2S)→μ+μ−)=(1.69±0.26stat±0.20sys)×10−6, from which we derive the partial widths Γ(Υ(4S)→π+pi−Υ(1S))=(1.8±0.4)Γ(Υ(4S)→π+pi−Υ(1S))=(1.8±0.4)~keV and Γ(Υ(4S)→π+pi−Υ(2S))=(2.7±0.8)Γ(Υ(4S)→π+pi−Υ(2S))=(2.7±0.8)~keV.