We construct a threshold $K$ matrix for the dominant $\mathrm{PS}11\left(\epsilon N\right)$ contribution to ${\pi }^{-}p\to {\pi }^{+}{\pi }^{-}n$ that agrees with (1) the result of partial-wave isobar-model analyses above 1300 MeV, (2) the result of a single-arm-spectrometer experiment below 1300 MeV, and (3) the predictions of the effective Lagrangian at threshold. We find that the chiral-symmetry-breaking parameter $\xi$ is -0.2±0.3, and that by only 10 MeV above threshold, isobar production accounts for roughly one-third of the total cross section.

• Received 3 October 1979

DOI:https://doi.org/10.1103/PhysRevLett.44.66