Polarisation observables in π0 photoproduction at MAMI

[摘要] This thesis presents measurements of polarisation observables in neutral pion photoproduction conducted at the Mainzer Microtron (MAMI) in Mainz using the Crystal Ball detector. The double polarisation observable G and the polarisation observable Σ have been measured on a frozen spin butanol target in the γp → pπ 0 reaction between the ∆(1232) and second resonance regions. The measurements have been made using a linearly polarised photon beam in the energy range 370 < Eγ < 770 MeV.This work is part of a world wide program aimed at obtaining information on the nucleon excitation spectrum to better understand the internal dynamics and structure by measuring polarisation observables. Measurement of the G observable in the energy region of this work will help determine the M1− partial wave. The M1− partial wave is highly sensitive to the N(1440) 1 2 + Roper resonance which is the first radial excitation of the proton. The properties of the Roper resonance are poorly understood and the mass, which is predicted to be higher than the negative parity resonance N(1535) 1 2 − , cannot currently be reconciled by quark models based on SU(6) symmetry.The sPlot technique was used to extract the polarisation observables in this work. The results are compared with the MAID, SAID and Bonn-Gatchina partial wave analysis curves.These measurements will contribute to the world dataset of polarisation observables and provide information on the excitation spectrum of the nucleon towards an unambiguous, model independent, solution to partial wave analyses.

[发布机构] University:University of Glasgow;Department:School of Physics and Astronomy

[关键词] MAMI, linear polarisation, hadrons, proton, pion, resonance, observable, polarisation observable.