Measurement Of The Cross Section For Pi P Pi Pi N Near Threshold PDF Download

The doubly differential cross sections for the .pi. from the reaction .pi.−p .-->. .pi.pi.−n were measured at about twenty points for each of five energies between 245 MeV and 356 MeV. The experiment was carried out at the Clinton P. Anderson Meson Physics Facility, where a double focusing magnetic spectrometer detected the .pi. mesons produced in a liquid hydrogen target. The measurements were normalized by comparison with .pi.−p elastic scattering measured with the same apparatus. These are the first such measurements in this energy range, and have an accuracy between 4.7% and 39%. The integrated reaction cross section was determined at each energy with an accuracy of about 5%. These agree with but are an improvement over previous measurements in this energy range. Comparison of the extrapolated threshold value of the mean square modulus of the matrix element with the soft pion calculations favors the symmetry breaking mechanism of Weinberg (xi = 0). It also demonstrates the futility of attempting to determine the symmetry breaking parameter xi from a single measurement of the integrated reaction cross section.