Associated strangeness production was studied in the pp ! ! p + p reaction at the CERN antiproton facility LEAR using the experimental setup of PS185. Results from two high{statistics measurements at incident a n tiproton momenta of 1:642 and 1.918 GeV/c are reported. Approximately 40 000 reconstructed events at each momentum have allowed us to measure the total and dierential cross-sections, the spin polarizations, the spin correlations, and the singlet fractions of the pair. Since the decays of both the and the w ere simultaneously observed in the same detector, we are able to provide upper limits on CP and CPT violation phenomena in the weak interaction.
We report on precision measurements of the cross sections for the reactions p p→⌳ ⌳, p p→⌳ ⌺ 0 , and p p→⌺ 0 ⌳ in the region below 5 MeV excess kinetic energy above reaction threshold. For the p p→⌳ ⌳ measurement, the data represent a third and definitive scan through the threshold region by this experiment. Improvements in the detector system and analysis techniques, coupled with greatly increased statistics, have negated the question of the existence of a near-threshold resonance as had been indicated in our earlier, lower-statistics studies. The high statistics permit the first extraction of the polarization information at these low energies. We also report the first measurement of the cross section for the complementary reactions p p →⌳ ⌺ 0 ϩc.c. at excess energies up to 2.5 MeV. This allows a comparison of the onset of ⌳ ⌳ production to that of ⌳ ⌺ 0 and ⌺ 0 ⌳. The data show no evidence for near-threshold resonance structures in either reaction channel. Further, the p p→⌳ ⌺ 0 ϩc.c. reactions exhibit a larger P-wave fraction close to threshold compared to p p→⌳ ⌳.
Interest in the production of hyperon-antihyperon pairs following antiproton-proton annihilation stems largely from attempts to understand the nature of flavor production. To date the major focus of both the experimental and the theoretical effort has been on the pp→⌳⌳ reaction. In this paper, we present data on the complementary channels pp→⌺ 0 ⌳ and pp→⌳⌺ 0. Events from the kinematically similar pp→⌳⌳ reaction were obtained in parallel. The procedure to distinguish these three separate reactions is described and results for all channels are presented. These include the total and differential cross sections, hyperon polarizations, and spin correlation coefficients. Data were obtained at incident antiproton lab momenta of 1.726 and 1.771 GeV/c which correspond to excess kinetic energies in the pp→⌳⌺ 0 ϩc.c. reaction of 26 and 40 MeV, respectively, above threshold. Comparisons are made to earlier work at similar excess energies in the pp→⌳⌳ channel. The low-energy regime has been highlighted in this experiment to reduce the complexity in the theoretical analysis. ͓S0556-2813͑96͒01612-3͔
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