Σ+P scattering experiments with a scintillating fiber active target

Hyperon-proton scattering experiments, KEK-PS E251 and E289 had been carried out at KEK-PS K2 beamline and the analyses for deriving scattering cross sections are under way. With use of a scintillating fiber active target, we had performed the direct observation of hyperon tracks and their scattering vertex which had never been done after bubble chamber experiments in 1960's and 1970's. Pion beam of 1.64 GeV/c is incidented on the active target and Σ+ is produced via (π+, Κ+) reaction. The Σ+ travels through the target and scatters on a proton inside the target. E251 resulted 11 Σ+ p elastic scattering events from 3 x 105 events of (π+, Κ+) reactions. E289 collected a few times more scattering events than E251. These statistics are larger than the result from past bubble chamber experiments for the Σ+ momentum region from 400 MeV/c to 600 MeV/c. (author)