Study of np → θπ+ and np → ωπ+ annihilation reactions in flight

The results of a study of the annihilation reactions np → θπ+ and np → ωπ+ are reported; the data were collected by the OBELIX apparatus, with antineutrons annihilating in flight (momenta from ∼ 50 MeV/c to 405 MeV/c). Annihilation frequencies and annihilation cross sections have been deduced, for both channels, as a function of antineutron momentum. From the cross section ratio, a substantial deviation from OZI rule expectations is observed. An ss quark content in the nucleon offers a fairly plausible explanation for such an effect.