Radiative Decay of the Delta Resonance

All meson theories predicting charged pion photoproduction have in common the term which arises from the interaction of the meson current with the incident photon (t-channel Born term in the Feynman diagrams). The t-channel Born term has a large effect at small angles. However, few data are available at small angles. Also the previous experiments for the negative pion photoproduction around Delta-resonance region either used a deuteron target, which means that corrections for deuteron structure or for Coulomb force are needed; or measured both the gamma-ray and the neutron in coincidence for the time reversal reaction pi^{- }p to gamma n. Both methods could cause large uncertainty of the absolute cross section measurement (10% or even larger). We studied the pi ^{-}p to gamma n reaction using high resolution gamma-ray detectors without detecting the neutron in coincidence for distinguishing the radiative gamma-ray from the more prolific gamma-ray of the pi^0 decay of CEX. The pi^{-}p to gamma n reaction was studied for the laboratory pion kinetic energies T_{pi^ {-}} = 45.6, 62.2, 74.7, 91.0, 107.8 and 121.9 MeV at the laboratory gamma-ray angles 5 ^circ and 20^circ , which completed the differential cross section measurements with the existing data at those energies; and for T_{pi^{- }} = 148, 173, 198 and 231 MeV at 6 or 7 angles between 5^circ and 145^circ. The small angle measurements confirm the t-channel Born term contribution with no ambiguity. The differential and total cross sections are more accurate than previous data due to the use of the high resolution gamma-ray detectors.