Evidence for the decay ${\rm Y}(4S)\to {\rm Y}(1S){\pi }^{+}{\pi }^{-}$

Abstract

A study of transitions between ${\rm Y}$ states with the emission of charged pions using 398 fb${}^{-1}$ of data collected with the Belle detector at the KEKB asymmetric energy $e^{+}{e}^{-}$ collider is presented. A clear peak from the decay ${\rm Y}(1S)\to {\mu }^{+}{\mu }^{-}$ is observed in the invariant mass distribution of $({\mu }^{+}{\mu }^{-})$ pairs from the $({\mu }^{+}{\mu }^{-}{\pi }^{+}{\pi }^{-}X)$ event sample. The mass difference distribution ($M_{{\mu }^{+}{\mu }^{-}{\pi }^{+}{\pi }^{-}}-M_{{\mu }^{+}{\mu }^{-}}$) for $M_{{\mu }^{+}{\mu }^{-}}$ from the ${\rm Y}(1S)$ mass region has two peaks from ${\rm Y}(2S,3S)\to {\rm Y}(1S){\pi }^{+}{\pi }^{-}$ decays, with no background. A third peak at $\Delta M=(1119.0\pm 1.4)$~MeV/$c^2$ can be interpreted as evidence of a signal from the decay ${\rm Y}(4S)\to {\rm Y}(1S){\pi }^{+}{\pi }^{-}$ with a subsequent ${\rm Y}(1S)\to {\mu }^{+}{\mu }^{-}$ transition. This is the first example of a non-$B\bar{B}$ decay of the ${\rm Y}(4S)$ resonance. The preliminary estimated branching fraction is equal to $\mathcal{B}({\rm Y}(4S)\to {\rm Y}(1S){\pi }^{+}{\pi }^{-})=(1.1\pm 0.2(stat.)\pm 0.4(sys.))\times 10^{-4}.$

13 pages, 8 figures, contributed to Lepton-Photon 2005 and EPS 2005