Study of $\chi_{bJ}(2P)\to\omega\Upsilon(1S)$ at Belle

TL;DR

This paper presents the first evidence for the near-threshold transition $oldsymbol{ ext{χ}_{b0}(2P) ooldsymbol{ extomega}oldsymbol{ extUpsilon}(1S)}$ and measures branching fractions for $ ext{χ}_{b1,2}(2P)$ transitions, revealing deviations from QCD predictions.

Contribution

It provides the first evidence for a near-threshold transition in bottomonium and measures branching fractions, challenging existing QCD multipole expansion expectations.

Findings

First evidence for $ ext{χ}_{b0}(2P) o extomega extUpsilon(1S)$ transition.

Measured branching fractions for $ ext{χ}_{b1,2}(2P)$ transitions.

Observed deviation in the transition ratio from QCD predictions.

Abstract

We report a study of the hadronic transitions $\chi_{bJ}(2P)\to\omega\Upsilon(1S)$, with $\omega\to\pi^{+}\pi^{-}\pi^{0}$, using $28.2\times10^6~\Upsilon(3S)$ mesons recorded by the Belle detector. We present the first evidence for the near--threshold transition $\chi_{b0}(2P)\to\omega\Upsilon(1S)$, the analog of the near-threshold charm sector decay $\chi_{c1}(3872)\to\omega J/\psi$, with a branching fraction of $\cal{B}\big(\chi_{b0}(2P)\to\omega\Upsilon(1S)\big) = \big(0.55\pm0.19\pm0.07\big)\%$. We also obtain branching fractions of $\cal{B}\big(\chi_{b1}(2P)\to\omega\Upsilon(1S)\big) = \big(2.39{}^{+0.20}_{-0.19}\pm0.24\big)\%$ and $\cal{B}\big(\chi_{b2}(2P)\to\omega\Upsilon(1S)\big) = \big(0.47{}^{+0.13}_{-0.12}\pm0.06\big)\%$, confirming the measurement of the $\omega$ transitions of the $J=1,2~P$--wave states. The ratio for the $J=2$ to $J=1$ transitions is also measured and found to differ by 3.3 standard deviations from the expected value in the QCD multipole expansion.