The radiative capture of pions on protons

One of the major goals of nuclear physics is to understand the strong interaction. Quantum Chromodynamics explains the strong interaction in terms of underlying quark and gluon degrees of freedom. The study of the structure of baryons and their excitations in terms of the elementary quark and gluon constituents is thus pivotal to our understanding of the nuclear force. The reaction π−p → nγ gives us a particularly good understanding of the dynamics within the nucleus because of the well known nature of the electromagnetic interaction. The electromagnetic interaction is one of the fundamental interactions in physics; in the π −p → nγ reaction it allows us to gain insight into the spectroscopy of baryon resonances. Data for the radiative capture reaction π−p → nγ were taken at the Brookhaven National Laboratory during 1997 and 1998. The CRYSTAL BALL detector was used to measure the energy and position of all neutral particles. Radiative capture data were taken with eighteen beam momenta ranging from 238 MeV/c to 748 MeV/c. Absolute differential cross sections have been extracted with about 10% statistical errors and 4% systematic errors. Total cross sections and excitation functions for this reaction are also shown.