Measurement of exclusive ${\rm Y}(1S)$ and ${\rm Y}(2S)$ decays into Vector-Pseudoscalar final states

Abstract

Using samples of 102 million ${\rm Y}(1S)$ and 158 million ${\rm Y}(2S)$ events collected with the Belle detector, we study exclusive hadronic decays of these two bottomonium resonances to $\text{ks}{K}^{+}{\pi }^{-}$ and charge-conjugate (c.c.) states, ${\pi }^{+}{\pi }^{-}{\pi }^0{\pi }^0$, and ${\pi }^{+}{\pi }^{-}{\pi }^0$, and to the two-body Vector-Pseudoscalar ($K^{\ast }(892{)}^0{\bar{K}}^0+c.c.$, $K^{\ast }(892{)}^{-}{K}^{+}+c.c.$, $\omega {\pi }^0$, and $\rho \pi$) final states. For the first time, signals are observed in the modes ${\rm Y}(1S)\to \text{ks}{K}^{+}{\pi }^{-}+c.c.$, ${\pi }^{+}{\pi }^{-}{\pi }^0{\pi }^0$, and ${\rm Y}(2S)\to {\pi }^{+}{\pi }^{-}{\pi }^0{\pi }^0$, and evidence is found for the modes ${\rm Y}(1S)\to {\pi }^{+}{\pi }^{-}{\pi }^0$, $K^{\ast }(892{)}^0{\bar{K}}^0+c.c.$, and ${\rm Y}(2S)\to \text{ks}{K}^{+}{\pi }^{-}+c.c.$ Branching fractions are measured for all the processes, while 90% confidence level upper limits on the branching fractions are also set for the modes with a statistical significance of less than $3\sigma$. The ratios of the branching fractions of ${\rm Y}(2S)$ and ${\rm Y}(1S)$ decays into the same final state are used to test a perturbative QCD prediction for OZI-suppressed bottomonium decays.

Comments: 8 pages, 3 figures, 2 tables, accepted for publication as a Rapid Communication in Physical Review D