Mean Life of the NeutralπMeson

We report an experiment to measure the ${\pi }^0$ mean life, using a new method that utilizes the relativistic flight-path dilation produced by a high pion velocity. A ${\pi }^{-}$ beam of 3.5 BeV/c made interactions in Ilford K.5 emulsion. Some 103 secondary neutral pions were observed to decay by the Dalitz mode (${\pi }^0\to e^{+}+e^{-}+\gamma$) in a study of 3600 such stars. The ${\pi }^0$ path lengths could be accurately measured and were automatically calculated by specially developed equipment. We assumed the ${\pi }^0$ momentum spectrum to be the same as that of the secondary charged pions found by multiple-scattering measurements. The mean transverse momentum of secondary charged pions at all angles is 274±10 MeV/c. The opening-angle distribution of the electron pairs was measured and compared with extensive calculations of the expected laboratory-system distribution in order to evaluate the number of unobserved events. We used a statistical approach to evaluate the mean life (${{\tau }_{\pi }}^0$) of the ${\pi }^0$. To evaluate the complex effects of electron scattering, the finite grain size, and the random grain spacing, pairs of electron tracks were generated artificially by a Monte Carlo program and the results compared with the observations. Our estimate of the neutral-pion mean life is (1.7±0.5)×${10}^{-16\mathrm{}}$ sec.