Observation of Υ(4S) Decays to π+π−Υ(1S) and π+π−Υ(2S)

Abstract

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