Measurement of the decay Υ(2S)→ππΥ(1S)

Abstract

We have investigated the hadronic transitions Υ(2S)→${\pi }^0$ ${\pi }^0$Υ(1S)→γγγγl +$l^{\mathrm{-}}$ (l=μ or e) and Υ(2S)→${\pi }^{+}$ ${\pi }^{\mathrm{-}}$Υ(1S)→${\pi }^{+}$π ${}_{}{}^{\mathrm{-}}{}_{}{}^{}e_{}^{+}{}_{}{}^{}$ $e^{\mathrm{-}}$ using the Crystal Ball detector at the DESY $e^{+}$ $e^{\mathrm{-}}$ storage ring DORIS II. Using the present world-average value of $B_{\mathrm{ll}}$(Υ(1S))=2.9±0.3)% we derive branching ratios B(Υ(2S)→${\pi }^0$ ${\pi }^0$Υ(1S))= (8.0±1.5)% and B(Υ(2S)→${\pi }^{+}$ ${\pi }^{\mathrm{-}}$Υ(1S))= (16.9±4.0)%. We also present results on the invariant-mass spectra and the angular distributions of the dipion system.