Evidence for the Production of a Neutral Pion in an Antiproton Annihilation Event

An antiproton of kinetic energy 110 Mev annihilates in flight, producing a star in nuclear emulsion with 3 charged mesons and 4 heavier particles. An electron-positron pair is found, 76 μ from the vertex and closely aligned with it. The pair is best explained as resulting from a $\gamma$ ray from the decay of a ${\pi }^0$ meson which was formed in the annihilation. The most likely interpretation of the event is that the antiproton annihilated with a proton in ${\mathrm{N}}^{14}$ or ${\mathrm{O}}^{16}$ with the production of two positive pions, a neutral pion and a negative pion all of which escaped the nucleus; and a negative pion which was absorbed. The pion energies, including $m_{\pi }{c}^2$, are as follows (in Mev): ${\pi }^{+}$, 214±2; ${\pi }^{\pm }$, 635±138; ${\pi }^{\mp }$, 628±131; ${\pi }^0$, $190<\mathrm{}{E_{\pi }}^0<400\mathrm{}$; ${\pi }^{-}$, $290<\mathrm{}{E_{\pi }}^{-}<480\mathrm{}$.