Observation of the decay Υ(4S)→Υ(1S)π+ π-

We study transitions between Y states with the emission of charged pions using 477 fb(-1) of data collected with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider. The measured product branching fraction B(Y(4S)-> Y(1S)pi(+)pi(-))xB(Y(1S)->mu(+)mu(-))=(4.42 +/- 0.81(stat)+/- 0.56(sys))x10(-6). When the PDG value for B(Y(1S)->mu(+)mu(-)) is used, this corresponds to B(Y(4S)-> Y(1S)pi(+)pi(-))=(1.78 +/- 0.33(stat)+/- 0.23(sys))x10(-4) and a partial decay width Gamma(Y(4S)-> Y(1S)pi(+)pi(-))=(3.65 +/- 0.67(stat)+/- 0.65(sys)) keV.