Evidence for the decay $\Upsilon(4S) \to \Upsilon(1S) \pi^{+} \pi^{-} $

TL;DR

This paper presents evidence for a rare decay of the $5S$ resonance into $1S$ with two pions, marking the first non-$Bar{B}$ decay observed for this state, based on analysis of Belle detector data.

Contribution

It provides the first experimental evidence of the decay $5S o 1S \pi^+\pi^-$, expanding understanding of bottomonium decay modes.

Findings

Evidence of $5S o 1S \pi^+\pi^-$ decay with statistical significance.

Estimated branching fraction of this decay is approximately $1.1 imes 10^{-4}$.

First observation of a non-$Bar{B}$ decay mode for the $5S$ resonance.

Abstract

A study of transitions between $\Upsilon$ states with the emission of charged pions using 398 fb$^{-1}$ of data collected with the Belle detector at the KEKB asymmetric energy $e^+e^-$ collider is presented. A clear peak from the decay $\Upsilon(1S)\to\mu^+\mu^-$ is observed in the invariant mass distribution of $(\mu^+ \mu^-)$ pairs from the $(\mu^+ \mu^- \pi^+ \pi^- X)$ event sample. The mass difference distribution ($M_{\mu^+\mu^-\pi^+\pi^-}-M_{\mu^+\mu^-}$) for $M_{\mu^+\mu^-}$ from the $\Upsilon(1S)$ mass region has two peaks from $\Upsilon(2S,3S)\to \Upsilon(1S)\pi^+\pi^-$ decays, with no background. A third peak at $\Delta M = (1119.0\pm 1.4)$~MeV/$c^2$ can be interpreted as evidence of a signal from the decay $\Upsilon(4S)\to \Upsilon(1S)\pi^+\pi^-$ with a subsequent $ \Upsilon(1S) \to \mu^+ \mu^-$ transition. This is the first example of a non-$B \bar{B}$ decay of the $\Upsilon(4S)$ resonance. The preliminary estimated branching fraction is equal to $\mathcal{B}(\Upsilon(4S)\to \Upsilon(1S)\pi^+\pi^-) = (1.1 \pm 0.2(\mathrm{stat.}) \pm 0.4(\mathrm{sys.}))\times 10^{-4}.$