Scattering and production amplitudes involving scalar resonances are known,
according to Watson's theorem, to share the same phase δ(s). We show
that, at low energies, the production amplitude is fully determined by the
combination of δ(s) with another phase ω(s), which describes
intermediate two-meson propagation and is theoretically unambiguous. Our main
result is a simple and almost model independent expression, which generalizes
the usual K-matrix unitarization procedure and is suited to be used in
analyses of production data involving scalar resonances.Comment: 10 pages, 4 figures. Minor changes, references added, version to
appear in Phys. Rev.
