Exclusive analysis of the Lambda(1405) resonance in the charged Sigmapion decay channels in proton proton reactions with HADES

Abstract

The HADES experiment is located at the heavy ion research facility GSI. It was originally designed for the measurement of the rare decays of the vector mesons _s13_s17961_s19, _s13_s17969_s19 and _s13_s17966_s19 into e_s16e_s13_s178722_s19-pairs at a beam energy of 1-2 AGeV. For this purpose different experiments with e.g. C _s16 C, Ar _s16 KCl and p _s16 Nb colliding systems are already accomplished, where the predicted in-medium modifications of the mass and width of the vector mesons can be investigated [Aga07]. As a reference measurement for the p _s16 Nb reaction system (taken in September 2008) a p _s16 p beam time with a beam energy of 3.5 GeV was processed in April 2007. Besides the feasibility of e_s16e_s13_s178722_s19-pair identification, it was shown that it is also possible to detect hadrons with HADES [Sch08]. Thus the data of the p_s16p run, with 1.2·109 detected events, can for instance be used for the analysis of particles with strangeness content. This thesis concentrates mainly on the investigation of the _s13_s17923_s19(1405) by its decay into the (_s13_s17931_s19±_s13_s17960_s19_s13_s178723_s19) states. The _s13_s17923_s19(1405) is currently of high interest, since it is predicted to be not only a three-quark baryon but a molecular state of a meson and a baryon. As the K¯N bound state should have a certain contribution to this resonance, the investigation of the _s13_s17923_s19(1405) is interesting in terms of a deeper understanding of the kaon-nucleon interaction. In order to improve the mass resolution of particles, reconstructed via invariant- or missingmass technique, a kinematic refit was developed for the exclusive analysis of the _s13_s17923_s19(1405). The mathematical details about the kinematic refit and the effects of its application are described in this thesis. Besides the improvement of the _s13_s17923_s19(1405) analysis, the refit can also be important for the investigation for other reactions, like the production of _s13_s17951_s19 and _s13_s17969_s19 mesons. An essential step in the presented analysis is the exact modeling and understanding of the background below the _s13_s17923_s19(1405) signal. Especially for that purpose a sophisticated sideband analysis was developed, which can describe the background due to misidentification quite well. Finally all background contributions could be identified and subtracted and thus a pure _s13_s17923_s19(1405) was extracted with a total yield of about 330 entries in each decay channel. In this way the thesis presents the first measurement of the _s13_s17923_s19(1405), produced in p _s16 p reactions and reconstructed in the two charged decay channels