Λ(1405) photoproduction with the BGO-OD experiment

Abstract

The Λ(1405) has been considered for many years a candidate for an unconventional hadronic state. Being difficult to understand within the framework of a three valence quark, constituent quark model, models explicitly including meson-baryon interactions have had improved success in describing the Λ(1405). Current understandings assume a molecule of N- K̄, which consequently has influence on the mass distribution, called the line shape. Measurements of this and the differential production cross section over a broad momentum range are crucial constraints to determine the nature of the Λ(1405). In this thesis the Λ(1405) is studied with the BGO-OD experiment at the Electron Stretcher Accelerator (ELSA) in Bonn via meson photoproduction. BGO-OD is a unique setup combining a central calorimeter with a forward spectrometer. The production mechanism of a molecule like structure could favor extreme forward angles, where the transferred momentum is minimal. This kinematic regime is particularly well covered by BGO-OD.<br /> Cross section measurements require an understanding of the detector geometry and response, and an absolute flux normalization, both of which were improved as part of this thesis work. Simulations of the data acquisition triggering system were accurately determined, and consistency checks measuring the γp → π0 p reaction showed good agreement in the cross section compared to other experimental data, demonstrating that the current hardware trigger system is well understood. Furthermore, the reactions γp to π0p, ηp, ωp, η′p and K +Σ0 were identified, to check the performance of the kinematic fitting algorithm implemented in this thesis. The results agree well with previous data. The achieved results on Λ(1405) → π0 Σ0 line shape and differential cross section are statistically comparable to the previous experimental data. In addition, first preliminary results were achieved with the forward spectrometer for production angles not accessible by other experiments.