High-statistics measurements of the (p)over-bar-p ->(Lambda)over-bar-Lambda and (p)over-bar-p ->(Lambda)over-bar-Sigma(0)+cc reactions at threshold -art. no. 055203

We report on precision measurements of the cross sections for the reactions (p) over barp--><(<Lambda>)over bar>Lambda (p) over barp--><(<Lambda>)over bar>Sigma (0), and (p) over barp-->Sigma (0)Lambda in the region below 5 M eV excess kinetic energy above reaction threshold. For the (p) over barp--> <(<Lambda>)over bar>Lambda measurement, the data represent a third and defi nitive scan through the threshold region by this experiment. Improvements i n the detector system and analysis techniques, coupled with greatly increas ed statistics, have negated the question of the existence of a near-thresho ld resonance as had been indicated in our earlier, lower-statistics studies . The high statistics permit the first extraction of the polarization infor mation at these low energies. We also report the first measurement of the c ross section for the complementary reactions (p) over barp--><(<Lambda>)ove r bar>Sigma (0)+c.c. at excess energies up to 2.5 MeV. This allows a compar ison of the onset of <(<Lambda>)over bar>Lambda production to that of <(<La mbda>)over bar>Sigma (0) and <(<Sigma>)over bar>(0)Lambda. The data show no evidence for near-threshold resonance structures in either reaction channe l. Further, the (p) over barp--><(<Lambda>)over bar>Sigma (0)+ c.c. reactio ns exhibit a larger P-wave fraction close to threshold compared to (p) over barp--><(<Lambda>)over bar>Lambda.