Dynamics of Internally Converted Electron-Positron Pairs

Abstract

The reactions studied were ${\pi }^{-}+p\to n+{\pi }^0$ (${\pi }^0\to \gamma +e^{+}+e^{-}$) and ${\pi }^{-}+p\to n+e^{+}+e^{-}$. From a sample of ∼15 000 internally converted electron-positron pairs, 7000 were measured, of which 4200 were used in the detailed analysis. The differential distributions in $y$ (the energy partition) and $x^2$ (the virtual mass of the photon) agree with the theoretical quantum electrodynamic calculations. A measure of the form factor for the ${\pi }^0\to 2\gamma$ reaction gave a value $\Gamma \left(\frac{x}{\mu }\right)=1-\frac{(0.24\mathrm{}\pm 0.16)x^2}{{\mu }^2}$, where $\mu$ is the mass of the pion. It was further demonstrated that the number of events necessary to determine the contribution of the longitudinally polarized virtual $\gamma$ rays in the second reaction is of the order of 50 times that in the present experiment.