A Study of the Upper Limit of v_τ-Mass at BEPC

By using the new result of τ-mass from Beijing spectrometer and the data of τ→5πv_τ from ARGUS, the upper limit of v_τ-mass of 30 MeV is obtained at 95% confidence level. The upper limit of the v_τ-mass could be largely reduced if a high resolution detector is used in the improved Beijing electron positron collider, which is under discussion.