Investigating Hadron Structure at Low and Medium Energies

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The objective of this research proposal is to conduct a program of precision hadron-structure experiments with electromagnetic probes at low and intermediate energies. In this energy region the description of structure and reaction dynamics encompasses nucleon/meson-based models and transitions to QCD-inspired models (or effective theories) based on quark/gluon degrees of freedom. Understanding hadronic structure in both the low-energy and transition regions of non-perturbative QCD remains at the forefront of worldwide efforts in nuclear physics. This program will focus on complementary experiments at three different laboratories: the Mainz Microtron (MAMI) in Germany, Jefferson Laboratory (JLab) in Virginia, and the High-Intensity Gamma Source (HIGS) based at Duke University in North Carolina. Our main focus over the past decade has been the study of polarizabilities, which are fundamental structure quantities like mass and charge except that they characterize a particle's internal response to an external electromagnetic field. They are of interest not only in the study of hadron structure where they can provide input to the QCD puzzle, but also in other fields such as precision atomic physics and astrophysics; for example, they yield an appreciable correction to the proton charge radius via Lamb shift and hyperfine structure in addition to defining neutron star properties. In Mainz, we will continue our leadership role in the polarizability and hadron-structure program in the A2 Collaboration; at JLab we will participate in measurements of the charged- and neutral-pion polarizabilities via the Primakoff effect using the GlueX detector in Hall D; and at HIGS we will make use of the high-flux free-electron laser to conduct lower-energy Compton scattering measurements on protons and light nuclei. At MAMI and HIGS we will concentrate on 1) light-nuclei experiments to improve the accuracy of the neutron scalar polarizabilities and 2) measurements of single- and double-polarization observables to access higher-order nucleon spin polarizabilities. In addition, we are involved in a measurement of elastic electron-proton scattering in Mainz using a new time-projection chamber (TPC), in order to better understand the low-Q^2 behaviour of the proton electric form factor and to help clarify the proton radius puzzle. Our group (HQP and PI) will contribute predominantly to data acquisition and analysis while also contributing to detector development, testing, and commissioning.