Measurement ofπ−p→π−π+nnear Threshold and Chiral-Symmetry Breaking

Abstract

$\frac{d^2\sigma }{d\Omega \mathrm{dT}}$ for ${\pi }^{+}$ mesons produced in ${\pi }^{-}p\to {\pi }^{-}{\pi }^{+}n$ was measured at seven incident energies between 203 and 357 MeV and the integrated reaction cross section was calculated. The matrix element, when extrapolated to threshold and compared with soft-pion calculations, determined the chiral-symmetry-breaking parameter $\xi =0.05\mathrm{}\pm 0.26$, which is consistent with the Weinberg Lagrangian. The large hard-pion contributions at 203 MeV demonstrated the absolute necessity for comparing at threshold.