Illuminating strongly interacting matter

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The work will give new information addressing fundamental gaps in our understanding of the structure of the nucleon (proton and neutron) and strongly interacting matter using the world's most intense energy tagged photon beams of appropriate wavelength (delivering ~1 billion polarised photons per second). 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. 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.This situation is particularly disappointing given the recent advances in theoretical descriptions of the spectra, such as first predictions from Lattice QCD. We will obtain unique new data to constrain the structure of the nucleon utilising polarised photon beams with (i) polarised neutron targets in which the spin of the target nucleons an be oriented in a chosen direction with respect to the beam (ii) measurement of the transfer of polarisation from the photon to the recoiling nucleon(s) in photoreactions. The first aspect utilises new HD target technology while the latter uses the Edinburgh nucleon polarimeter which employs a novel and highly cost-effective design to measure the polarisation of nucleons with a large angular acceptance. The polarimeter 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 also carry out world's first measurements of polarisation transfer in deuterium photodisintegration using the Edinburgh nucleon polarimeter. Polarisation transfer provides the most sensitive information on whether the appropriate degrees of freedom in the reaction process are the nucleon or the underlying quark structure of the deuteron. This will be the benchmark measurement to address intriguing indications of this transition at unexpectedly low photon energies. Finally, we will obtain important new information on how nucleons combine to make nuclei by accurately measuring the size of neutron skins. Neutron skins with thicknesses of up to ~1/3 of a neutron diameter are predicted to exist for heavy nuclei, and accurate measurement would give long sought constraints on the poorly established density dependence of the asymmetry parameter for neutron rich matter. This asymmetry parameter fixes the scale of the binding energy penalty for moving away from symmetric matter (equal numbers of protons and neutrons) and has fundamental consequences for neutron star physics including establishing the feasibility of cooling mechanisms and constraining mass-radius relationships. The size of the skin is also a crucial constraint on modern theoretical descriptions of the nucleus, which we will challenge by determining the change in skin thickness when the neutron number increases for a fixed proton number.