Novel approach to investigate ? decays via ?' ? pp?

To avoid the impact from the background events directly from e+e- annihilations or J=& psi; decays, we propose a novel approach to investigate & eta; decays, in particular for its rare or forbidden decays, by using & eta;' & RARR; & pi;& pi;& eta; produced in J=& psi; decays at the & tau;-charm factories. Based on the Monte Carlo studies of a few typical decays, & eta; & RARR; & pi;& pi;, & gamma;l+l-(l = e; & mu;), l+l-, as well as l+l-& pi;0, the sensitivities could be obviously improved by taking advantage of the extra constraint of & eta;'. Using 1 x 1012 J=& psi; events accumulated at the super & tau;-charm facility, the precision on the investigation of & eta; decays could be improved significantly and the observation of the rare decay & eta; & RARR; e+e- is even accessible.