Threshold hyperon production in proton-proton collisions at COSY-11


For the first time the pp -> nK+Sigma+ reaction has been measured in the threshold region and the cross section was determined. The measurement was performed at the COSY-11 detection system at two beam momenta p = 2.6 GeV/c and 2.74 GeV/c, corresponding to excess energies Q = 13 MeV and 60 MeV.

COSY-11 is an internal magnetic spectrometer experiment at the COoler SYnchroton and storage ring COSY in Jülich, Germany. It is equipped with scintillator hodoscopes and drift chambers for charged particle detection and a scintillator/lead sandwich detector for neutrons.

Experimentally, the Sigma+ hyperon was identified via the missing mass technique, by detecting the remaining reaction products - K+ meson and neutron. Extensive background studies in the missing mass spectra have been performed and the possible influence of the the higher partial waves on the detection efficiency discussed.

The investigation on the Sigma+ production is a part of the long ongoing studies of the hyperons production performed by the COSY-11 collaboration. In the previous analysis of the Lambda and Sigma0 hyperon production in the pp -> pK+Lambda(Sigma0) reactions, respectively, the unexpectedly high cross section ratio sigma(Lambda) / sigma(Sigma0) in the close to threshold region was observed. To explain this behavior, various theoretical scenarios were proposed, but although they differ even in the dominant basic reaction mechanism, all more or less reproduce the data. In order to get more information for disentangling the contributing reaction mechanisms, data from an other isospin channel were taken, namely pp -> nK+Sigma+.

Within this thesis the method of the measurement and the data analysis is given. The total cross section is presented and the results are discussed in view of available theoretical models.

PhD thesis as PDF (~1.8MB) file