Energy dependence of the pp→K+nΣ+ reaction near threshold

Hyperon production in proton-proton collisions in the close-to-threshold region has been extensively studied at different experimental facilities. The energy dependence of the total cross sections for pp→K+pΛ and pp→K+pΣ0 shows a phase-space behavior, though modified in the former case by the Λp Final-state interaction (FSI). However, new data on the pp→K+nΣ+ channel, published recently by the COSY-11 collaboration at excess energies of ε = 13 MeV and 60 MeV, show surprisingly high production rates. Using experimental data collected at the magnetic spectrometer ANKE (COoler-Synchrotron Juelich), new estimates for the total cross section for Σ+ production at 129 MeV excess energy have been done. Our new results, taken in conjunction with other experimental data, would indicate a close-to-threshold anomaly in the energy dependence of the total cross section for the pp→K+nΣ+ reaction channel. The status of analysis as well as plans for the future measurements will be presented. (orig.)