Photoproduction of Neutral Pions in Hydrogen: Magnetic Analysis of Recoil Protons

Abstract

The photoproduction of neutral pions from hydrogen has been studied by counting the recoil protons with a magnetic spectrometer and scintillation counters. The process has been studied between photon energies of 260 and 450 Mev and between center-of-momentum pion angles of 70° and 153°. The excitation functions show a resonance type shape with maxima at about 320 Mev. Angular distributions are analyzed in the form $A+Bcos\theta +C{cos}^2\theta$ in the center-of-momentum system. The coefficient $B$, which gives the front-back asymmetry, is small at all energies; and the ratio $-\frac{A}{C}$ is 1.22±0.10 at all energies between 295 and 450 Mev. The maximum cross section at 90° in the c.m. system is 26×${10}^{-30\mathrm{}}$ ${\mathrm{cm}}^2$/steradian for 320-Mev photons. The total cross section divided by the square of the c.m. photon wavelength has a maximum near 340 Mev, and drops by nearly a factor of two at 450 Mev. These results are consistent with magnetic dipole and electric quadrupole absorption leading to a resonant state of the pion-nucleon system of angular momentum $\frac{3}{2}$ and isotopic spin $\frac{3}{2}$.