A pion at rest decays into a muon and neutrino (+). Rest masses of particles are m. m, and m = 0. If t is the proper life time of u and c is velocity of light, on the average the distance travelled by muon in vacuum before disintegrating is m + m - m_mu m (m-m 2m-m m CT 2m- mu - m ) et (m + m) c 2mm CT