Observation of a resonance X(1835) in J/ψ--> γπ^+ π^- η^{\prime}

Abstract

The decay channel J/ψ--> γπ^+ π^- η^{\prime} is analyzed using a sample of 5.8\times 10^7 Jψevents collected with the BESII detector. A resonance, the X(1835), is observed in the π^+ π^- η^{\prime} invariant mass spectrum with a statistical significance of 7.7 σ. A fit with a Breit-Wigner function yields a mass M = 1833.7 \pm 6.1(stat) \pm 2.7(syst) MeV/c^2, a width Γ= 67.7 \pm 20.3(stat) \pm 7.7(syst) MeV/c^2 and a product branching fraction B(J/ψ--> γX) B(X --> π^+ π^- η^{\prime}) = (2.2 \pm 0.4(stat) \pm 0.4(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\bar{p} mass threshold enhancement observed in the Jψ-->γp \bar{p} process at BESII.