An Experimental Test of Weinberg's Off-Mass-Shell Dependence of theππScattering Amplitude

The off-mass-shell behavior of the $S$-wave $\pi \pi$ amplitude predicted by Weinberg implies that the $S-P$-wave interference term in pion production vanishes for certain values of momentum transfer and $\pi \pi$ effective mass. Such behavior is shown to be consistent with experiment. The analysis determines the ratio of scattering lengths and the isospin content of the sigma commutator of current algebra.