Observation of a Broad Structure in the $\pi^+\pi^-J/\psi$ Mass Spectrum around 4.26 GeV/$c^2$

TL;DR

This paper reports the observation of a broad resonance structure around 4.26 GeV/c^2 in the $^+^-J/$ mass spectrum from initial-state radiation events, suggesting a possible new charmonium-like state.

Contribution

It provides the first evidence of a broad resonance at 4.26 GeV/c^2 in the $^+^-J/$ spectrum using BaBar data, with detailed mass and width measurements.

Findings

A broad resonance at 4.26 GeV/c^2 observed

Resonance width estimated between 50 and 90 MeV/c^2

Additional narrow resonances not excluded

Abstract

We study initial-state radiation events, $e^+e^- \to \gamma_{ISR}\pi^+\pi^-J/\psi$, with data collected with the BaBar detector. We observe an accumulation of events near 4.26 GeV/$c^2$ in the invariant-mass spectrum of $\pi^+\pi^-J/\psi$. Fits of the mass spectrum indicate that a broad resonance with a mass of about 4.26 GeV/$c^2$ is required to describe the observed structure. The presence of additional narrow resonances cannot be excluded. The fitted width of the broad resonance is 50 to 90 MeV/$c^2$, depending on the fit hypothesis.