Extract signals by fitting chi(2) distribution of the kinematic fit

In measuring the psi(2S) radiative decays at BES II, contribution of the background is serious inmost of the final states. To extract the number of signal events, a fit to the chi(2) distribution of kinematic fit using signal and background components is proposed. Extensive Monte-Carlo simulations (MCS) are performed, and the results show that the shape of X 2 distribution of the signal channel looks different from those of the background channels, thus it provides us by fitting the X 2 distribution of the data to extract the number of signal events. An input-output test is performed using MCS, and the uncertainty of the fit method is found to be less than 2%.