ar X iv : h ep - e x / 04 11 06 4 v 1 2 0 N ov 2 00 4 Open strangeness threshold production at COSY - 11

The open strangeness production near threshold is investigated at the internal experiment COSY-11 in different reaction channels. Recently, the main focus has been to extend measurements of the hyperon production pp → pK + Λ/Σ 0 in different isospin channels as well as the associated strangeness production in pp → ppK + K −. The experimental technique is based on the reconstruction of the four momentum for all positively charged ejectiles. Neutrons are detected in addition using a neutral particle detector. The unregistered hyperon or meson is identified by means of the missing mass technique. The present status of the analysis for both reaction channels is presented. In case of the reaction pp → ppK + K − , very preliminary cross sections are shown. The magnetic spectrometer COSY-11 [1] at the COoler SYnchrotron COSY [2] is shown in its principal layout in figure 1. The reaction takes place in a cluster target mounted FIGURE 1. Schematic view of the COSY-11 setup for an exemplary event in pp → ppK + K − , where the kaon decays before reaching the stop scintillator S3. The not shown cluster target is located in front of the left dipole magnet. in front of one of the ring dipoles and is operated with hydrogen or deuterium. The positively charged ejectiles are bent to the interior of the ring and their four momenta are reconstructed using the information of a set of drift chambers and a subsequent time of flight measurement. The K − for the pp → ppK + K − reaction is then identified via the missing mass method. In case of pp → nK + Σ + , additionally the neutron is detected in a lead-scintillator arrangement and the unregistered Σ + is again deduced from the missing