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\ifmmode^\circ\else\textdegree\fi{} and 153\ifmmode^\circ\else\textdegree\fi{}. The excitation functions show a resonance type shape with maxima at about 320 Mev. Angular distributions are analyzed in the form $A+Bcos\ensuremath{\theta}+C{cos}^{2}\ensuremath{\theta}$ in the center-of-momentum system. The coefficient $B$, which gives the front-back asymmetry, is small at all energies; and the ratio $\ensuremath{-}\frac{A}{C}$ is 1.22\ifmmode\pm\else\textpm\fi{}0.10 at all energies between 295 and 450 Mev. The maximum cross section at 90\ifmmode^\circ\else\textdegree\fi{} in the c.m. system is 26\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}30}$ ${\mathrm{cm}}^{2}$/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 $\frac{3}{2}$.