Ablikim, M (reprint author), Inst High Energy Phys, Beijing 100049, Peoples R China.
The decay channel J/psi ->gamma pi(+)pi(-)eta(') is analyzed using a sample of 5.8x10(7) J/psi events collected with the BESII detector. A resonance, the X(1835), is observed in the pi(+)pi(-)eta(') invariant-mass spectrum with a statistical significance of 7.7 sigma. A fit with a Breit-Wigner function yields a mass M = 1833.7 +/- 6.1(stat)+/- 2.7(syst) MeV/c(2), a width Gamma=67.7 +/- 20.3(stat)+/- 7.7(syst) MeV/c(2), and a product branching fraction B(J/psi ->gamma X)center dot B(X ->pi(+)pi(-)eta('))=[2.2 +/- 0.4(stat)+/- 0.4(syst)]x10(-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 (p) over bar mass threshold enhancement observed in the J/psi ->gamma p (p) over bar process at BESII.