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Title: Radiative π⁰ photoproduction in the region of the Δ(1232) resonance
Author: Downie, Evangeline Joy
ISNI:       0000 0001 3431 6642
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2006
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The Delta+(1232) is the first excited state of the proton. The Delta-isobar states (the Delta-, Delta0, Delta+ and Delta++) are the lowest lying of the baryon resonances, and very well isolated. As such they form an excellent testing ground for the variety of Quantum Chromodynamics inspired models of hadronic structure. The resonance electromagnetic multipoles would be of particular theoretical interest. Due to the strongly decaying nature of the Delta-isobar, physicists have only been able to ascertain the masses and widths of these resonances and some of their transition multipoles, but not their static properties. However, due to its short lifetime, the Delta+ has a comparatively large width of ~120 MeV. This allows us to access the dipole magnetic moment of the Delta+ (muDelta+) through radiative self-decay whereby a Delta+ is formed at the upper end of its mass range and radiatively decays to a Delta+ of lower mass. This process has an amplitude that depends on muDelta+. The secondary Delta+ decays to a nucleon and a pion. This work is concerned with the Delta+ →pi0p decay channel and the pi0 subsequently decays to a photon pair. We observed this three-photon, single proton, final state to reconstruct the gammap → gamma'pi0p reaction. We studied this reaction in the A2 collaboration's Tagged Photon Experimental Hall at the Institut fur Kernphysik in Mainz, Germany. The Glasgow Photon Tagging Spectrometer converted part of the MAMI B 883 MeV electron beam to an energy-tagged linearly polarised photon beam. Reaction products were detected in the new CB MAMI 4pi spectrometer setup composed of the Crystal Ball and TAPS detectors. The Crystal Ball was augmented with a pair of Multi-Wire Proportional Chambers for charged particle tracking. A plastic scintillator barrel, the Particle Identification Detector, was also specifically designed for the CB MAMI experimental series to provide charged particle identification by DeltaE/E methods. The experiment took place throughout several beam periods between July 2004 and January 2005. We have analysed the new CB MAMI data and have found ~20,000 radiative pi0 photoproduction events. From this event set we have produced preliminary total cross sections which agree well with the previous result and have a much improved statistical error. The series of photon asymmetry measurements for gammap → gamma'pi0 we present herein represent the first measurement of this observable and indicate a downward trend in photon asymmetry both with increasing photon beam energy and with increasing production photon energy. The photon asymmetry results are well established. Although not yet at the stage of muDelta+ extraction, due to the factor of ~40 higher statistics of this measurement, the additional physical observables that have been measured and the recent intensive development of the theoretical models, we believe that the muDelta+ result extracted from this data will be far more accurate than that obtained from the only previous experiment carried out in 2000/01.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available