| Summary: | 博士 === 國立中山大學 === 物理學系研究所 === 97 === GeV photon beams are provided at SPring-8/LEPS facility by backward-Compton scattering (BCS) from Argon laser photon against the 8-GeV electrons inside the SPring-8 storage ring. The energy range of tagged photon by measuring the recoil electron is from 1.5 to 2.4 GeV, and enables us to study the strange meson- and baryon-photoproduction, such as φ(1020), Λ(1405) and Λ(1520) . In the past 30 years, other than the high-energy Λ(1520) photoproduction at NINA/LAMP2 experiment, there is no further study of the Λ(1520) photoproduction near the threshold. Based on this exclusive measurement at NINA/LAMP2 experiment, several theoretical calculations predict that the Λ(1520) photoproduction near the threshold can be explained by t-channel K* exchange, t-channel K exchange or the contact term together with the t-channel K exchange under the gauge invariance.
This experiment was performed at SPring-8/LEPS facility with liquid hydrogen and deuterium targets by a forward spectrometer from 2002 to 2003. The electroproduction experiment with similar c.m. energy region performed by JLab/CLAS is included for comparison. We study three two-track detection modes to reconstruct the Λ(1520) resonance and other resonant/non-resonant backgrounds by the invariant mass and missing mass spectra. Utilizing the one-/two-step side-band subtraction methods and Monte Carlo simulation method, we measure the differential cross sections, K^- decay asymmetries in t-channel helicity frame and photon beam asymmetries to understand the production mechanism of Λ(1520) . This is the first time, the differential cross sections were measured at low energies and with a deuterium target. A large asymmetry of the production cross sections from protons and neutrons was observed at backward K^{+/0} angles. It can be explained by the dominance of the contact term along with the t-channel K exchange under the satisfaction of gauge invariance. This interpretation was also compatible with the differential cross sections, K^- decay asymmetry, and the photon beam asymmetry measured in the production from protons in the forward K^+ angles.
A detector upgrade for the acceptance complement is done by introducing a time projection chamber, the configuration and operation principles except for the physics results will also be discussed in this thesis.
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