Production of strange particles in antineutrino interactions at the CERN PS

We have studied neutral strange particle production in the bubble chamber Gargamelle filled with propane and exposed to antineutrinos from the CERN PS. Cross sections are presented for Λ, Σ0 and K0 production. Associated production reactions (ΔS = 0) have been observed in the charged and the neutral current channels. From 45 candidates for the quasi-elastic reaction νp → μ+Λ, on bound and free protons, estimates of the axial transition form factor MA have been made using the total cross section and the t distribution. The weighted average from the two methods is MA = 862 ± 190 MeV. From a subsample of 15 candidates for ν interactions on free protons in propane, with (28±6)% background from ν interactions on bound protons, we obtain MA = 883 ± 243 MeV. The polarization of the Λ hyperons has been studied for the quasi-elastic reaction. We observe a significant Λ polarization in the production plane, normal to the Λ line of flight. The observed polarizations agree well with the predicted theoretical polarizations without any contribution from second-class axial coupling.