The composite structure of the nucleon leads to a spectrum of excited states, or resonances, which contain important constraints on the dynamics and interactions of its constituents. A major world programme measuring meson photoproduction from the nucleon is currently under way to improve our knowledge of this fundamental spectrum. These new generation of measurements exploit polarisation degrees of freedom of the incident photon along with spin degrees of freedom in the target and recoiling nucleons. The meson photoproduction process can be described by 16 independent experimental observables. To fully constrain the four underlying complex photoproduction amplitudes requires the measurement of a least 8 of these. Measurements of double polarisation observables, where the photon beam and nucleon target are polarised are crucial to achieve this requirement. This thesis will present the analysis and extraction of the first preliminary measurement of the double polarisation observable. F in the single pion photoproduction reaction γ→ + p →n + π+ was undertaken to measure both F and T. The resulting measurement of polarisation observables from this reaction in conjunction with with those from the Pπ0 channel will provide stringent constraints on partial wave analyses aiming to extract precision information on the properties of the low lying nucleon resonances.

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The coherent nuclear ¼0 photoproduction reaction is sensitive to the nuclear matter form factor i.e. the distribution of matter within the nucleus. Accurate measurements of the nuclear matter distribution as charaterised by the r.m.s. radius are important for applications to nuclear theories, neutron stars, atomic parity non-conservation and heavy ion collisions. This thesis presents the results from a new experiment to measure coherent ¼0 photoproduction on 208Pb, 40Ca, 16O and 12C. The ultimate goal of the research programme is to make a high precision measurement of the nature of the neutron skin of 208Pb. The first major step in this direction is to achieve an accurate data set of coherent pion photoproduction which is presented in this thesis. The experiment was performed in the A2 hall of the MAMI electron accelerator facility at the InstitÄut fur Kernphysik, Mainz, Germany. An 883 MeV beam of electrons was directed on to a 10¹m nickel radiator producing a Bremsstrahlung photon beam which was then 'tagged' with a resolution of 2 MeV using the Glasgow Photon Tagging Spectrometer. The photon beam was incident on one of the 4 experimental targets inducing the reaction A(°,¼0)A. The neutral ¼0s were then detected via their two photon decay in the newly installed 4¼ Crystal Ball and TAPS detector systems. The results from all four targets are presented as differential and total cross sections covering the energy range E°=(135-300) MeV and covering the full 180± of the pion polar angle. Comparisons have been made with previous data and with the latest theoretical calculations of Dreschel et. al. which employ detailed pion optical potentials in describing the pion-nucleus final state interaction. It is concluded that the 208Pb cross sections show good agreement with the calculations indicating that the pion-nucleus FSI distortions are well accounted for by the model. A pleasing reduction in statistical and systematic uncertainties from previous measurements is also observed. While the 208Pb cross sections are finalised it is suggested that the 12C and 16O data would benefit from a further analysis utilising the coincident detection of nuclear decay photons to isolate incoherent events and allow a more detailed comparison with theory. A first comparison of the theoretical model with the new high quality data gives first indications of a neutron skin on 208Pb. A future scheme for the full detailed extraction of the matter distribution from the new data set is also suggested.

Antibaryon production has been investigated since the advent of nuclear physics, largely motivated by the potential for baryon-antibaryon bound states. Due to the recent availability of high statistics experiments there has been a renewed interest in how antibaryons are created in photoproduction. The g12 (E04-005) experiment conducted at Thomas Jefferson National Accelerator Fa- cility used the CEBAF Large Acceptance Spectrometer to provide the world’s largest dataset for baryon-antibaryon photoproduction on a liquid hydrogen tar- get. The focus of this work is to investigate the photoproduction mechanism of γp → ppp ̄ through detailed differential cross section measurements from 3.95 to 5.45 GeV. These first time results as well as the resulting total cross section measurement are reported. In particular the energy dependence (σ(Eγ)) and the angular dependence (dσ/dcos(θCM)) have interesting features that have never p ̄ been seen before. On the other hand, the mass distributions (dσ/dM(pp) and dσ/dM(pp ̄) do not show evidence for previously reported narrow resonances. In addition, the first time total cross section mesurement of an antineutron in photoproduction using the reaction γp → ppn ̄π− is reported. The significance and implications of these results will be discussed as well.

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Establishing the resonance spectrum of the nucleon with accuracy would provide important new information about the dynamics and degrees of freedom of its constituents. The spectrum and properties of nucleon resonances are a fundamental test of the emerging predictions from Lattice QCD calculations and will guide re finements to QCD-based phenomenological models. Pion photoproduction is an excellent tool to study the nucleon resonance spectrum, as this channel is expected to couple strongly to most resonances. The new generation of measurements for this reaction, of which the measurement presented in this thesis forms a crucial part, will provide a great improvement in the quality of available experimental data. For the photoproduction process in particular, the use of photon beams and targets with high degrees of polarisation, coupled with large acceptance particle detectors is essential for disentangling the spectrum of excited states. There are many nucleon resonances predicted by recent Lattice QCD calculations and by phenomenological nucleon models which are only observed inconsistently by different analyses of the same experimental data or which are not observed at all. It is of upmost importance to establish if this means that the resonances do not exist in nature, reflecting inappropriate degrees of freedom in the theoretical description of the nucleon or if the current experimental measurements have not been sensitive enough. As such, there is a current world effort at modern tagged photon facilities to measure the \complete set" of photoproduction observables necessary to fully constrain the partial wave analyses used to extract the experimental excitation spectrum from the data. This thesis will present the first detailed measurement to date of positive pion photoproduction in the 730-2300 MeV photon energy (1400-2280 MeV centre-of-mass energy) region with a linearly polarised photon beam and a longitudinally polarised proton target with a close-to-complete angular coverage in detection of the reaction products. This unique set up allows for the extraction of the double-polarisation observable, G. The data were taken as part of the g9 experiment at the Thomas Jefferson National Accelerator Facility in Virginia, using a tagged, polarised photon beam and the Frozen Proton Spin Target, FROST, in conjunction with the CEBAF Large Acceptance Spectrometer, CLAS. The results of the study presented here are compared to the sparse existing data set for the G double-polarisation observable along with the current solutions of the the three main partial wave analyses: MAID, SAID and Bonn-Gatchina. Some agreement is obtained with the expectations of these PWA at lower energies, while disagreement at higher energies is clearly evident. This is the energy region where many of the missing resonances are expected to lie. Once incorporated into the MAID, SAID and Bonn-Gatchina models, these new data will provide an important contribution to constraining the amplitudes and therefore the resonance spectrum and properties of the nucleon. The new data will form a central part of the world effort to accurately establish the nucleon excitation spectrum for the first time by achieving the first complete measurement of experimental observables in meson photoproduction.