Observation of $\Upsilon(4S)\to \eta' \Upsilon(1S)$

TL;DR

This paper reports the first observation of a rare hadronic transition from the $S$ to $
S$ state involving an $$ meson, using data from the Belle detector, with a measured branching fraction and high statistical significance.

Contribution

First observation of the $S o '
S$ transition, providing new insights into bottomonium decay processes.

Findings

Measured branching fraction: (3.43 ± 0.88(stat.) ± 0.21(syst.))×10⁻⁵.

Observation significance: 5.7σ.

First experimental evidence of this transition.

Abstract

We report the first observation of the hadronic transition $\Upsilon(4S)\to\eta'\Upsilon(1S)$, using 496 fb$^{-1}$ data collected at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider. We reconstruct the $\eta'$ meson through its decays to $\rho^0\gamma$ and to $\pi^+\pi^-\eta$, with $\eta\to\gamma\gamma$. We measure: ${\cal B}(\Upsilon(4S)\to\eta'\Upsilon(1S))=(3.43\pm 0.88 {\rm(stat.)} \pm 0.21 {\rm(syst.)})\times10^{-5}$, with a significance of 5.7$\sigma$.