Grant: Subatomic Physics Envelope - Individual

The objective underlying my research program is understanding the fundamental properties of nucleons and nuclei through electromagnetic and electroweak interactions. My research gives students excellent training in analytical and computational aspects of theoretical physics, as well as opportunities to work with international researchers. In the electromagnetic sector, calculations of two-photon exchange (TPE) radiative corrections have been instrumental in resolving the discrepancy between measurements of electron-nucleon scattering form factors using Rosenbluth and polarization -transfer techniques. Pioneering calculations by my group have focussed on a hadronic approach to evaluating the model--dependent TPE amplitudes. Experimental interest in TPE has grown considerably in the last decade, in part because it has implications for a number of precision measurements, such as the proton radius puzzle and parity--violating electron scattering. It is also important for extracting proton and neutron form factors, electro-production of resonances and pions, and nuclear form factors. Experiments seeking to directly identify TPE effects, such as through the angular -dependence of polarization observables, nonlinear contributions to cross sections, the ratio of positron--proton to electron--proton scattering, and spin asymmetries, have been undertaken and are still active in several laboratories. My group is involved in providing theoretical support and guidance to the Two-Photon Exchange (TPEX) and Nuclear Radius Extraction (NREC) collaborations. Precision low--energy experiments in atomic parity--violation and in parity--violating electron-proton scattering are a vital complement to direct tests of the Standard Model. They also give information about the structure of the proton, such as its weak charge and strangeness content. These low--energy measurements have the potential to give constraints on new physics, provided that the critical radiative corrections are understood. In particular, there are uncertainties associated with hadronic physics that are always present, even in purely leptonic processes. A major component of my research is aimed at unravelling these hadronic contributions and their associated uncertainties. A significant advance in recent years is to reduce the model- dependence of these contributions. Our work showed how constraints from high energy deep inelastic scattering, together with new data on parity--violating electron scattering at moderate energies, significantly reduces the uncertainties on the corrections compared to previous estimates. In addition to atomic parity--violation and the weak charge of the proton, this work is significant for superallowed beta--decay and a new generation of upcoming precision experiments at Jefferson Lab and Mainz utilizing parity--violating electron scattering.