The elastic scattering of photons by protons has been measured for 100 MeV to 290 MeV photons at 90° c.m.s. and 139° c.m.s. scattering angles. The expected large increase in cross-section is observed at energies approaching that of (3/2, 3/2) pion-nucleon resonance. The scattering can be qualitatively explained by the ordinary Thomson amplitude combined with that of the (3/2, 3/2) resonance. A more detailed examination of the cross-section in the region just above the photo-meson threshold has shown that it is sensitive to the π0 photon coupling. From the experimental data, one may conclude that the π0 mean life should be between 10−16 and 10−18 s.
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The scattering of photons by protons has been measured with a spark-chamber technique using 335-MeV bremsstrahlung. The experimental values obtained at 90° and 135° are compared with those calculated by Contogouris using dispersion relations. The agreement is reasonable except for a persistently low point for 310 MeV at 90°.
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