Illuminating strongly interacting matter

The work will give new information on the structure of the atomic nucleus and the nucleon using the world's most intense energy tagged photon beam of appropriate wavelength (delivering ~1 billion polarised photons per second) and the famous Crystal Ball detector. The photon probe is an ideal way to investigate the structure of matter as its interaction is well understood and it probes the whole volume of the object under study. The central part of our programme will utilise polarised photon beams and measure the transfer of polarisation in interactions of the photons with nucleons (protons and neutrons). To achieve this we have developed the Edinburgh nucleon polarimeter which employs a novel and highly cost-effective design to determine with a large angular acceptance the degree of spin polarisation of the nucleons produced after interactions of the photon beam with the target. Nucleons are composite structures made of 3 much lighter particles called quarks, which exist in a sea of virtual gluons and quark-antiquark pairs. This system can be excited into a number of very short lived excited states which can be detected from their subsequent decay back to a nucleon via the emission of mesons. The spectrum of these excited states and their properties are fundamental observables reflecting the dynamics of the nucleon's internal constituents. Despite being studied for decades in various experiments information on the mass, lifetime and photon coupling of the known excited states have been obtained only with large uncertainties for many parts of the spectrum. For example the lifetime of the second excited state of the proton is only known to 50% accuracy. Some excited states only show up inconsistently in different theoretical analyses of the same experimental data and many 'missing' excited states predicted by quark models are not yet observed. Our programme will enable the long sought 'complete measurement' of experimental observables in meson photoproduction from the nucleon. This will be a real milestone in the field, providing sufficient experimental constraints to allow a model independent analysis for the first time in the extraction of the fundamental excitation spectrum. We will supplement the nucleon recoil polarisation measurements with similar observables in the photoproduction of mesons containing strange quarks, made possible by a new strange meson tagging technique. By utilising light nuclear targets the recoil polarimeter will also make possible new measurements to establish the transition between where nucleons are a valid basis to describe matter, to where quarks become the valid basis. The point at which this transition occurs in reaction processes is a fundamental observable for our understanding of how nucleons interact. There has been a great deal of excitement in recent years because the cross section for the photodisintegration of deuterium shows characteristics consistent with quark descriptions of matter at unexpectedly low photon energies of ~1 GeV. This observation compels the need for a more sensitive test of the onset of this quark regime. We will achieve this using the Edinburgh polarimeter to measure the transfer of polarisation from the photon beam to the final state nucleons. This benchmark measurement will allow a more sensitive test as to whether the transition has occurred and will also challenge the new generation of 'exact' nucleon and quark based theoretical models of the Deuteron and its disintegration process. We will also obtain important new information on how nucleons combine to make nuclei by accurately measuring the size of the neutron skins. These are predicted to form on the surface of heavy nuclei and establishing their nature is a crucial constraint on modern theoretical descriptions of the nucleus, has important consequences in understanding neutron star physics and will reduce systematic errors in low energy tests of the standard model by atomic experiments.