Observation of a Resonance X(1835) in J/{psi}{yields}{gamma}{pi}{sup +}{pi}{sup -}{eta}{sup '}

The decay channel J/{psi}{yields}{gamma}{pi}{sup +}{pi}{sup -}{eta}{sup '} is analyzed using a sample of 5.8x10{sup 7} J/{psi} events collected with the BESII detector. A resonance, the X(1835), is observed in the {pi}{sup +}{pi}{sup -}{eta}{sup '} 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{sup 2}, a width {gamma}=67.7{+-}20.3(stat){+-}7.7(syst) MeV/c{sup 2}, and a product branching fraction B(J/{psi}{yields}{gamma}X){center_dot}B(X{yields}{pi}{sup +}{pi}{sup -}{eta}{sup '})=[2.2{+-}0.4(stat){+-}0.4(syst)]x10{sup -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 pp mass threshold enhancement observed in the J/{psi}{yields}{gamma}pp process at BESII.