Mean Lifetime of the Neutral Pion

An estimate of the mean lifetime of the ${\pi }^0$ meson has been obtained from an experiment employing a direct time-of-flight technique first attempted by Harris et al. in 1957. This method is based upon the observation in nuclear emulsion of the decay of the ${K_{\pi 2}}^{+}$ meson ($K^{+}\to {\pi }^{+}+{\pi }^0$) and the subsequent decay of the ${\pi }^0$ via the Dalitz mode, ${\pi }^0\to e^{+}+e^{-}+\gamma$. In the present experiment we were able to utilize a new fine-grained emulsion (Ilford L.4) that yielded markedly improved resolution. The availability of the separated $K^{+}$ beam from the Bevatron at Berkeley permitted detection and measurement of 76 Dalitz decays. We obtain, for the mean lifetime of the ${\pi }^0$, $\tau =(1.9\mathrm{}\pm 0.5)\times {10}^{-16\mathrm{}}$ sec.