Lifetime of the Neutral Pion

Abstract

A new upper limit on the ${\pi }^0$ lifetime has been obtained by using 12 ${K_{\pi 2}}^{+}$ endings in nuclear emulsion where the ${\pi }^0$ decays by a direct pair, ${\pi }^0\to (e^{+}+e^{-})+\gamma$. The distance traveled by the ${\pi }^0$ before decay was measured from the intersection of the $K^{+}$ and ${\pi }^{+}$ tracks to the intersection of the pair and ${\pi }^0$ (colinear with ${\pi }^{+}$). The inherent accuracy of the measuring technique was found to be ∼0.5 micron per event by using ${\tau }^{+}$ secondaries as mock events. With the statistics of 12 events it should be possible to detect a ${\pi }^0$ decay length of ∼0.2 micron or greater. Our data give no indication of a displacement of pair origins from the $K$ endings. Thus the ${\pi }^0$ decay length is probably less than ∼0.2 microns. This corresponds to a lifetime of less than 5×${10}^{-16\mathrm{}}$ sec. Information on the upper limit of the ${\pi }^0$ lifetime is obtained from a plot of the likelihood function vs ${\pi }^0$ lifetime. The likelihood function is down by a factor of 30 for a lifetime of 1×${10}^{-15\mathrm{}}$ sec and is rapidly decreasing for longer lifetimes.