Measurement of the reactions ? and ? close to threshold

The reactions (p) over bar p --> <(Sigma(+))over bar>Sigma(+) and (p) over bar p --> <(Sigma(-))over bar>Sigma(-) have been investigated at 1.922 GeV/c beam momentum by the PS185 Collaboration at LEAR, CERN. Of both reaction types together 170 events could be identified. The cross section of the reaction (p) over bar p --> <(Sigma(+))over bar>Sigma(+) was determined to be sigma(tot) = 3.68 +/- 0.43 mu b at an excess energy of 23 MeV. Its angular distribution exhibits a pronounced forward peaking. The ratio of the cross sections sigma((p) over bar p --> <(Sigma(+))over bar>Sigma(+))/sigma((p) over bar p --> <(Sigma(-))over bar>Sigma(-)) = 2.40(-1.3)(+3.0) was extracted with a confidence limit of 90%. It is the first time that experimental data of these reaction channels are available close to threshold. We compare our results with those of earlier experiments and with theoretical predictions. (C) 1997 Elsevier Science B.V.