Study of $\eta$ and dipion transitions in $\Upsilon(4S)$ decays to lower bottomonia

TL;DR

This study measures branching fractions of hadronic transitions in bottomonium states at the $$ resonance, revealing ratios, searching for rare decays, and observing possible resonance contributions, using a large data sample from the Belle detector.

Contribution

It provides new measurements of transition branching fractions, ratios, and searches for rare decays in bottomonium, with evidence for resonance effects in dipion transitions.

Findings

Measured branching fractions for $(4S)$ transitions to lower states.

Found a ratio of branching fractions $((4S) o o o o o$ ratio.

Indications of a resonant contribution from the $f_0(980)$ meson in dipion transitions.

Abstract

We study hadronic transitions between bottomonium states using 496 fb$^{-1}$ data collected at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric energy $e^{+}e^{-}$ collider. We measure: ${\cal B}(\Upsilon(4S)\to\pi^+\pi^-\Upsilon(1S))=(8.2\pm 0.5 {\rm(stat.)} \pm 0.4 {\rm(syst.)})\times10^{-5}$, ${\cal B}(\Upsilon(4S)\to\pi^+\pi^-\Upsilon(2S))=(7.9\pm 1.0 {\rm(stat.)} \pm 0.4 {\rm(syst.)})\times10^{-5}$, and ${\cal B}(\Upsilon(4S)\to\eta\Upsilon(1S))=(1.70\pm 0.23 {\rm(stat.)} \pm 0.08 {\rm(syst.)})\times10^{-4}$. We measure the ratio of branching fractions ${\cal R} = {\cal B}(\Upsilon(4S)\to\eta\Upsilon(1S))/{\cal B}(\Upsilon(4S)\to\pi^+\pi^-\Upsilon(1S)) = 2.07\pm 0.30 {\rm(stat.)} \pm 0.11 {\rm(syst.)}$. We search for the decay $\Upsilon(1^3D_{1,2})\to\eta\Upsilon(1S)$, but do not find significant evidence for such a transition. We also measure the initial state radiation production cross sections of the $\Upsilon(2S,3S)$ resonances and we find values compatible with the expected ones. Finally, the analysis of the $\Upsilon(4S)\to\pi^+\pi^-\Upsilon(1S)$ events shows indications for a resonant contribution due to the $f_0(980)$ meson.