|dc.description.abstract||The resonance spectrum of the nucleon gives direct information on the
dynamics and interactions of its constituents. This offers an important
challenge to the theoretical models of nucleon structure, including the
emerging Lattice QCD predictions, conformal field theories and more phenomenological,
QCD-based approaches. Although the various models
predict different features for the excitation spectra, the experimental information
is currently of too poor quality to differentiate between these
Pion photoproduction from the nucleon is a powerful probe of the spectrum
as most resonances are expected to couple to the pion decay channel.
However, cross-sections alone are not sensitive enough to allow identification
of the underlying excitation spectrum, as the resonances have energy
widths larger than their separations. A major world effort is underway to
additionally measure polarisation observables in the production process.
For a model-independent analysis a “complete” set of single- and doublepolarisation
observables needs to be measured in experiments involving
polarised beams, targets and a means of determining recoil nucleon polarisation.
In particular, the beam asymmetry is a critical observable for
the constraint of partial wave analyses (PWA) used to extract the nucleon
excitation spectrum from the data.
Almost all of the available world data on the beam asymmetry has been
taken on the proton, with the neutron dataset sparse, containing only three
experiments at fixed angles and in a limited photon energy range. The
lack of extensive data on the neutron is a major deficiency, as different
resonances have very different electromagnetic couplings to the proton and
neutron. As a result, the data from the two targets will have very different
relative contributions from, and sensitivities to, the spectrum of nucleon
resonances. Moreover, neutron data is essential for the separation of the
isoscalar and isovector components of the reaction amplitudes.
This thesis presents a very high statistics measurement of the photon beam
asymmetry on the neutron with close-to-complete angular coverage and
a wide range of invariant mass (1610 – 2320 MeV) extending over the
third resonance region, where the excitation spectrum is particularly ill
defined. The experiment was conducted at the Thomas Jefferson National
Accelerator Facility (JLab) using a tagged, linearly polarised photon beam,
a liquid deuterium target and the CEBAF Large Acceptance Spectrometer
(CLAS). The quality and quantity of the data has allowed an invariantmass
resolution of 10 MeV and an angular resolution of 0.1 in the cosine of the
centre-of-mass pion production angle, θ.
Good agreement is evident in the regions where there is kinematic overlap
with sparse previous data. Comparison of the new data is made with the
two main partial wave analyses, SAID andMAID. Significant discrepancy
is observed at backward θ with SAID (across most of the energy range) and
MAID (up to ∼ 1750 MeV) and also below ∼ 35◦ in θ with both analyses.
This extensive new dataset will help significantly to constrain partial wave
analyses and will be a crucial part of the current world effort to use meson
photoproduction to tackle long-standing uncertainties in the fundamental
excitation spectrum of the nucleon. As a first step towards this the refitting
of the SAID partial wave analysis incorporating the new data was carried
out and shows very significant changes in the properties of the magnetic
P11, P13, D13, D35, F15, G17 and G19 partial waves.||en