Observation of $\Upsilon(4S) decays to $\pi^+pi^-\Upsilon(1S)$ and $\pi^+pi^-\Upsilon(2S)

TL;DR

This paper reports the first measurement of specific decay modes of the $Upsilon(4S)$ meson into lower states with pion pairs, providing new data on bottomonium decay processes.

Contribution

It presents the first measurement of $Upsilon(4S)$ decays to $pi^+pi^-Upsilon(1S)$ and $pi^+pi^-Upsilon(2S)$, including branching fractions and partial widths.

Findings

Measured product branching fractions for the decays.

Derived partial widths for the decay modes.

First experimental evidence of these decay channels.

Abstract

We present the first measurement of $\Upsilon(4S)$ decays to $\pi^+pi^-\Upsilon(1S)$ and $\pi^+pi^-\Upsilon(2S)$ based on a sample of 230$\times10^6$ $\Upsilon(4S)$ mesons collected with the BABAR detector. We measure the product branching fractions ${\cal B}(\Upsilon(4S)\to \pi^+pi^-\Upsilon(1S))\times{\cal B}(\Upsilon(1S)\to\mu^+\mu^-)=(2.23\pm0.25_{stat} \pm0.27_{sys})\times 10^{-6}$ and ${\cal B}(\Upsilon(4S)\to \pi^+pi^-\Upsilon(2S))\times{\cal B}(\Upsilon(2S)\to\mu^+\mu^-)=(1.69\pm0.26_{stat}\pm0.20_{sys})\times 10^{-6}$, from which we derive the partial widths $\Gamma(\Upsilon(4S)\to \pi^+pi^-\Upsilon(1S))=(1.8\pm0.4)$~keV and $\Gamma(\Upsilon(4S)\to \pi^+pi^-\Upsilon(2S))=(2.7\pm0.8)$~keV.