The Mass Difference of Neutral and NegativeπMesons

The angular correlation of the decay $\gamma$ rays from neutral $\pi$ mesons produced in the reaction ${\pi }^{-}+p\to n+{\pi }^0$ has been measured, using counter techniques. Analysis of the observed correlation function yields a value of the ${\pi }^{-}-{\pi }^0$ mass difference $m_{{\pi }^{-}}-m_{{\pi }^0}=8.8\mathrm{}\pm 0.6m_e$.