Photoproduction of Neutral Pions from Hydrogen at Forward Angles from 240 to 480 Mev

Recoil protons from the process γ+p→p+π⁰ have been detected by nuclear emulsions placed within a hydrogen-gas target and used to measure the differential cross section for production of neutral pions. In this manner protons of energies as low as 5 Mev can be detected at laboratory angles corresponding to emission of a pion at center-of-momentum (c.m.) angles as low as 26^∘{}. This experiment thus supplements that of Oakley and Walker which is in the same range of photon energies (240-480 Mev), but is restricted to pion c.m. angles greater than about 70^∘{} owing to higher minimum detectable proton energy. Common experimental points provide intercomparison of absolute values. Angular distributions are analyzed in the form dσdΩ=A+Bcosθ+Ccos²θ in the c.m. system. The combined Oakley-Walker and present data give the average value of the ratio A/C as -1.60±{}0.10 in the energy range from 260 to 450 Mev. The coefficient B, which gives the front-back asymmetry, passes through zero below the resonance energy of 320 Mev and is positive at higher energies. These results are consistent with magnetic dipole absorption leading to a state of the pion-nucleon system of angular momentum 3/2, together with a finite amount of S-wave interference.