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

TL;DR

In a study of Y(4260) → π+ π- J/φ decays, a structure is observed in the M(π(±)J/ψ) mass spectrum with 5.2σ significance that can be interpreted as a new charged charmoniumlike state.

Abstract

The cross section for $e^+ e^- \to π^+ π^- J/ψ$ 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 $Υ(nS)$ ($n = 1,\ 2,\ ...,\ 5$) resonances. The Y(4260) state is observed, and its resonance parameters are determined. In addition, an excess of $π^+ π^- J/ψ$ 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 $Y(4260) \to π^+ π^- J/ψ$ decays, a structure is observed in the $M(π^\pm\jpsi)$ mass spectrum with $5.2σ$ significance, with mass $M=(3894.5\pm 6.6\pm 4.5) {\rm MeV}/c^2$ and width $Γ=(63\pm 24\pm 26)$ MeV/$c^{2}$, where the errors are statistical and systematic, respectively. This structure can be interpreted as a new charged charmonium-like state.

Figures (4)

• Figure 1: (a) Invariant mass distributions of $\pi^+\pi^-\ell^+\ell^-$. Points with error bars are data, and the shaded histograms are the normalized $J/\psi$ mass sidebands. The solid curves show the total best fit with two coherent resonances and contribution from background. The dashed curves are for solution I, while the dot-dashed curves are for solution II. The inset shows the distributions on a logarithmic vertical scale. The large peak around 3.686 GeV/$c^2$ is the $\psi(2S)\to \pi^+\pi^-J/\psi$ signal. (b) Cross section of $e^+e^-\to \pi^+\pi^-J/\psi$ after background subtraction. The errors are statistical only.
• Figure 2: Dalitz plot for $Y(4260)\to \pi^+\pi^-J/\psi$ decays for $4.15~\hbox{GeV}/c^2 < M(\pi^+\pi^-J/\psi) < 4.45$ GeV/$c^2$. The inset shows background events from the $J/\psi$-mass sidebands (not normalized).
• Figure 3: Invariant mass distributions of (a) $\pi^+\pi^-$, (b) $\pi^+J/\psi$ and (c) $\pi^-J/\psi$ for events in the $Y(4260)$ signal region. Points with error bars represent data, shaded histograms are normalized background estimates from the $J/\psi$-mass sidebands, solid histograms represent MC simulations of $\pi^+\pi^-$ amplitudes pwa (normalized $J/\psi$-mass sideband events added) and dashed histograms are MC simulation results for a $Z(3900)^\pm$ signal
• Figure 4: Unbinned maximum likelihood fit to the distribution of the $M_{\mathrm{max}}(\pi J/\psi)$. Points with error bars are data, the curves are the best fit, the dashed histogram is the phase space distribution and the shaded histogram is the non-$\pi^+\pi^-J/\psi$ background estimated from the normalized $J/\psi$ sidebands.