At the Nuclear Physics energy scale our understanding of the structure of the constituents of nuclei, nucleons and mesons, is still rather limited. We are confronted with a type of strongly interacting many-body systems (hadrons) whose mass is largely determined by their relativistic quark-gluon dynamics. There is an enormously large mass gap between the nucleon and its constituents: the current quark masses account for only 2% of the mass of a nucleon. It is a unique case in physics that we cannot separate out the constituents of hadrons - there are no free quarks in nature. Rather they are confined in hadrons and we do not yet have a theory that explains this properly. Finally, we know very little about correlations of quarks or gluons which should determine the long range structure of the nucleon, and the origin of the nucleon's spin is still a puzzle.
There are essentially two methods to unravel the structure of composite systems such as hadrons:
- by scattering high energy pointlike particles or photons off hadrons, or
- by investigating the excitation spectrum of hadrons through resonant production mechanisms - hadron spectroscopy.
The measurement of the nucleon's magnetic moment gave the first indication that it is not an elementary particle but composed of partons, quarks and gluons. We expect to learn more about the complex structure of excited nucleons by measuring their magnetic moments. The first ever experiments to determine the magnetic moment of a highly excited nucleon, the D-resonance, which lives for just 10-24 seconds, were recently performed at the Glasgow Photon Tagger in the A2-Collaboration at MAMI. In Hall B at Jefferson Lab the Glasgow-Edinburgh team pursues a rigorous programme to investigate higher lying nucleon resonances using linearly polarised high energy photon beams.
The long-range structure of nucleons is best investigated by measuring form factors in lepton (electron, positron, muon) scattering. The figure shows a recent example of the charge distribution within a neutron as inferred from our double polarisation measurements of the neutron's electric form factor at the Triple Spectrometer Facility of the A1 Collaboration at MAMI. These experiments will be continued at higher electron beam energies in Hall A at Jefferson Lab.
Sum rules are very powerful tools in physics. They relate dynamic properties of composite systems to their static properties making use of fundamental symmetry principles. We will continue the pioneering measurements of the Gerasimov-Drell-Hearn sum rule, which we performed at the A2 Glasgow Tagger and at HERMES, at the upgraded MAMI C electron accelerator.
The nucleon spin puzzle
The nucleon's short-range structure is accessed by measuring parton distribution functions, where we concentrate in HERMES on the spin structure functions. It came as a big surprise, when the EMC Collaboration at CERN found evidence that the contribution of the spin of the quarks to the nucleon's spin is very small: only about 25% according to the most recent HERMES measurements. The other 75% must then come from orbital angular momenta or the spin of the gluons, in a very delicate balance to arrive at the spin 1/2 of the nucleon. The recently developed theoretical framework of Generalised Parton Distributions (GPD) promises access to quark angular momenta via the measurement of hard exclusive lepton scattering processes such as Deeply Virtual Compton Scattering. Under Scottish project leadership the HERMES experiment is being upgraded by setting up a Recoil Detector around the target region (cf. figure), with which we will perform the first GPD measurement world-wide.