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

TL;DR

Fits to the mass spectrum indicate that a broad resonance with a mass of about 4.26 GeV/c2 is required to describe the observed structure, and the presence of additional narrow resonances cannot be excluded.

Abstract

We study initial-state radiation events, $e^+e^- \to γ_{ISR}π^+π^-J/ψ$, 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 $π^+π^-J/ψ$. 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.

Figures (3)

• Figure 1: The $\pi^+\pi^-{J / \psi }\xspace$ invariant mass spectrum in the range 3.8$-$5.0${\mathrm{\,Ge V\!/}c^2}$ and (inset) over a wider range that includes the $\psi{(2S)}\xspace$. The points with error bars represent the selected data and the shaded histogram represents the scaled data from neighboring $e^+e^-$ and $\mu^+\mu^-$ mass regions (see text). The solid curve shows the result of the single-resonance fit described in the text; the dashed curve represents the background component.
• Figure 2: The distribution of $m_{Rec}^2$. The points represent the data events passing all selection criteria except that on $m_{Rec}^2$ and having a $\pi^+\pi^-{J / \psi }\xspace$ mass near 4260${\mathrm{\,Me V\!/}c^2}$, minus the scaled distribution from neighboring $\pi^+\pi^-{J / \psi }\xspace$ mass regions (see text). The solid histogram represents ISR $Y$ Monte Carlo events, and the dotted histogram represents the ISR $\psi{(2S)}\xspace$ data events.
• Figure 3: The dipion mass distribution for $Y(4260)\rightarrow\xspace\pi^+\pi^-{J / \psi }\xspace$ data is shown as points with error bars. The histogram shows the distribution for Monte Carlo events where $Y(4260)\rightarrow\xspace\pi^+\pi^-{J / \psi }\xspace$ is generated according to an $S$-wave phase space model.