Production of High-Energy Electron Pairs by Negative Pions in Nuclear Collisions

Abstract

Nuclear plates were exposed to negative pion beams of the 450-Mev Chicago cyclotron to search for high-energy pairs produced in nuclear collisions. The seven pairs found exhibit the following characteristics: (a) the mass of the particles is less than 10 electron masses, so the pairs probably consist of electrons; (b) the energies of the pairs range from 22 to 205 Mev; (c) the angles between the particles of the pair range from 0.3° to 25.9°; (d) the distance from the star to the point of origin of the pair—"gap"—was definitely less than 2 microns (determined by reprojection); (e) the angle between the center of mass of the pair and the direction of the incoming pion was 125° wth a spread of ±15°. The most reasonable assumption seems that they are due to the process: ${\pi }^0\to e^{+}+e^{-}+\gamma$ first discussed by Dalitz. If this explanation should hold, then from the measured gaps the lifetime must be definitely less than 4.8×${10}^{-15\mathrm{}}$ sec. However, if all the seven pairs were due to the proposed decay scheme of the ${\pi }^0$ meson, the probability of the observed angular distribution (e) is certainly less than 2×${10}^{-5\mathrm{}}$. A possible explanation for the observed effect is that the pair may tend to line up in direction with the neutral pion direction.