Lifetime of the Neutral Pion

Abstract

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