Observation of a Resonance $X(1835)$ in $J/\psi \to \gamma {\pi }^{+}{\pi }^{-}{\eta }^{\prime }$

The decay channel $J/\psi \to \gamma {\pi }^{+}{\pi }^{-}{\eta }^{\prime }$ is analyzed using a sample of $5.8\times {10}^7$ $J/\psi$ events collected with the BESII detector. A resonance, the $X(1835)$, is observed in the ${\pi }^{+}{\pi }^{-}{\eta }^{\prime }$ invariant-mass spectrum with a statistical significance of $7.7\sigma$. A fit with a Breit-Wigner function yields a mass $M=1833.7\pm 6.1(\mathrm{stat})\pm 2.7(\mathrm{syst})\mathrm{MeV}/c^2$, a width $\Gamma =67.7\pm 20.3(\mathrm{stat})\pm 7.7(\mathrm{syst})\mathrm{MeV}/c^2$, and a product branching fraction $B(J/\psi \to \gamma X)·B(X\to {\pi }^{+}{\pi }^{-}{\eta }^{\prime })=[2.2\pm 0.4(\mathrm{stat})\pm 0.4(\mathrm{syst})]\times {10}^{-4}$. The mass and width of the $X(1835)$ are not compatible with any known meson resonance. Its properties are consistent with expectations for the state that produces the strong $p\overline{p}$ mass threshold enhancement observed in the $J/\psi \to \gamma p\overline{p}$ process at BESII.

Figure 1

Invariant-mass distributions for selected $J/\psi \to \gamma {\pi }^{+}{\pi }^{-}{\eta }^{\prime }({\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}\eta ,\eta \to \gamma \gamma )$ candidate events. (a) The invariant-mass distribution of $\gamma \gamma$ pairs. (b) The ${\pi }^{+}{\pi }^{-}\eta$ invariant-mass distribution. (c) The ${\pi }^{+}{\pi }^{-}{\eta }^{\prime }$ invariant-mass distributions; the open histogram is data and the shaded histogram is $J/\psi \to \gamma {\pi }^{+}{\pi }^{-}{\eta }^{\prime }$ phase-space MC events (with arbitrary normalization).

Figure 2

Invariant-mass distributions for the selected $J/\psi \to \gamma {\pi }^{+}{\pi }^{-}{\eta }^{\prime }({\eta }^{\prime }\to \gamma \rho )$ candidate events. (a) The invariant-mass distribution for $\gamma \gamma$ pairs. (b) The $\gamma {\pi }^{+}{\pi }^{-}$ invariant-mass distribution. (c) The ${\pi }^{+}{\pi }^{-}{\eta }^{\prime }$ invariant-mass distributions; the open histogram is data and the shaded histogram is from $J/\psi \to \gamma {\pi }^{+}{\pi }^{-}{\eta }^{\prime }$ phase-space MC events (with arbitrary normalization).

Figure 3

The ${\pi }^{+}{\pi }^{-}{\eta }^{\prime }$ invariant-mass distribution for selected events from both the $J/\psi \to \gamma {\pi }^{+}{\pi }^{-}{\eta }^{\prime }({\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}\eta ,\eta \to \gamma \gamma )$ and $J/\psi \to \gamma {\pi }^{+}{\pi }^{-}{\eta }^{\prime }({\eta }^{\prime }\to \gamma \rho )$ analyses. The bottom panel shows the fit (solid curve) to the data (points with error bars); the dashed curve indicates the background function.