Study of $e^+ e^- \to \pi^+ \pi^- J/\psi$ and Observation of a Charged Charmonium-like State at Belle

TL;DR

This paper measures the cross section of $e^+ e^- o \pi^+ \pi^- \\ J/\\\psi$ over a wide energy range, observes the Y(4260) resonance, and reports a new charged charmonium-like state around 3.9 GeV with significant evidence.

Contribution

It reports the observation of a new charged charmonium-like state and provides detailed measurements of the Y(4260) resonance parameters using Belle data.

Findings

Observation of Y(4260) resonance with determined parameters.

Detection of an excess around 4 GeV consistent with Y(4008).

Evidence for a new charged charmonium-like state at 3894.5 MeV/c^2.

Abstract

The cross section for $e^+ e^- \to \pi^+ \pi^- J/\psi$ between 3.8 GeV and 5.5 GeV is measured with a 967 fb$^{-1}$ data sample collected by the Belle detector at or near the $\Upsilon(nS)$ ($n = 1,\ 2,\ ...,\ 5$) resonances. The Y(4260) state is observed, and its resonance parameters are determined. In addition, an excess of $\pi^+ \pi^- J/\psi$ production around 4 GeV is observed. This feature can be described by a Breit-Wigner parameterization with properties that are consistent with the Y(4008) state that was previously reported by Belle. In a study of $Y(4260) \to \pi^+ \pi^- J/\psi$ decays, a structure is observed in the $M(\pi^\pm\jpsi)$ mass spectrum with $5.2\sigma$ significance, with mass $M=(3894.5\pm 6.6\pm 4.5) {\rm MeV}/c^2$ and width $\Gamma=(63\pm 24\pm 26)$ MeV/$c^{2}$, where the errors are statistical and systematic, respectively. This structure can be interpreted as a new charged charmonium-like state.