The Hadronic Transitions $Upsilon(2S)\to Upsilon(1S)$

TL;DR

This study measures hadronic transition rates between Upsilon(2S) and Upsilon(1S) states, providing precise branching fractions and analyzing dipion system parameters, thus testing theoretical models of quarkonium transitions.

Contribution

First precise measurements of multiple Upsilon(2S) to Upsilon(1S) hadronic transition branching fractions using CLEO II data, including dipion and neutral pion transitions.

Findings

Branching fraction for $$ transition: 0.0192+-0.002+-0.010

Leptonic branching ratios of $$ state: B_{ee}=0.0229+-0.0008+-0.0011, B_{}=0.0249+-0.0008+-0.0013

Upper limits for $$ and $$ transitions: 0.0028 and 0.0011

Abstract

Using a data sample of $\Upsilon(2S)$ events collected with the CLEO II detector at the Cornell Electron Storage Ring, we have investigated the hadronic transitions between the $\Upsilon(2S)$ and the $\Upsilon(1S)$. The charged dipion transition $\Upsilon(2S)\to \Upsilon(1S)\pi^+\pi^-$ was studied using two different analysis techniques. Selecting events in which $\Upsilon(1S)$ decays into a $\mu\mu$ or $ee$ lepton pair(``exclusive'' analysis), we measured the branching fraction of B=0.189+-0.004+-0.009, while using a method allowing $\Upsilon(1S)$ to decay into anything (``inclusive'' analysis) we found B=0.0196+-0.002+-0.010. The appropriate average of the two measurements gives B=0.0192+-0.002+-0.010. Combining the exclusive and inclusive results we derive the $\Upsilon(1S)$ leptonic branching ratios B_{ee}=0.0229+-0.0008+-0.0011 and B_{\mu\mu}=0.0249+-0.0008+-0.0013 We also studied $\Upsilon(2S)\to \Upsilon(1S)\pi^0\pi^0$ and obtained the branching fraction of B=0.092+-0.006+-0.007. Parameters of the $\pi\pi$ system (dipion invariant mass spectra, angular distributions) were analyzed and found to be consistent with current theoretical models. Lastly, we searched for the $\eta$ and single $\pi^0$ transitions and obtained the upper limits of 0.0028 and 0.0011 respectively.