Observation of a charged charmoniumlike structure in e+e- to pi+pi-J/psi at \sqrt{s}=4.26 GeV

Abstract

We study the process e+e- to pi+pi-J/psi at a center-of-mass energy of 4.260 GeV using a 525 pb^{-1} data sample collected with the BESIII detector operating at the Beijing Electron Positron Collider. The Born cross section is measured to be (62.9\pm 1.9\pm 3.7) pb, consistent with the production of the Y(4260). We observe a structure at around 3.9 GeV/c^2 in the π^\pm J/psi mass spectrum, which we refer to as the Z_c(3900). If interpreted as a new particle, it is unusual in that it carries an electric charge and couples to charmonium. A fit to the π^\pm J/psi invariant mass spectrum, neglecting interference, results in a mass of (3899.0\pm 3.6\pm 4.9) MeV/c^2 and a width of (46\pm 10\pm 20) MeV. Its production ratio is measured to be R=\frac{σ(e+e- to π^\pm Z_c(3900)^\mp to pi+pi-J/psi))} {σ(e+e- to pi+pi-J/psi)}=(21.5\pm 3.3\pm 7.5)%. In all measurements the first errors are statistical and the second are systematic.

Figures (4)

• Figure 1: The distributions of $M(\mu^+\mu^-)$ (left panel) and $M(e^+e^-)$ (right panel) after performing a 4C kinematic fit and imposing all selection criteria. Dots with error bars are data and the curves are the best fit described in the text.
• Figure 2: Dalitz distributions of $M^2(\pi^+\pi^-)$ vs. $M^2(\pi^+J/\psi)$ for selected $e^+e^-\to \pi^+\pi^-J/\psi$ events in the $J/\psi$ signal region. The inset shows background events from the $J/\psi$ mass sidebands (not normalized).
• Figure 3: One dimensional projections of the $M(\pi^+J/\psi)$, $M(\pi^-J/\psi)$, and $M(\pi^+\pi^-)$ invariant mass distributions in $e^+e^-\to \pi^+\pi^-J/\psi$ for data in the $J/\psi$ signal region (dots with error bars), data in the $J/\psi$ sideband region (shaded histograms), and MC simulation results from $\sigma(500)$, $f_0(980)$ and non-resonant $\pi^+\pi^-$ amplitudes (red dot-dashed histograms). The pink blank histograms show a MC simulation of the $Z_c(3900)$ signal with arbitrary normalization.
• Figure 4: Fit to the $M_{\rm max}(\pi^\pm J/\psi)$ distribution as described in the text. Dots with error bars are data; the red solid curve shows the total fit, and the blue dotted curve the background from the fit; the red dot-dashed histogram shows the result of a phase space MC simulation; and the green shaded histogram shows the normalized $J/\psi$ sideband events.