Electroweak Interactions in Few Nucleon Systems

Abstract

Despite having been known as the constituents of atomic nuclei for over a century, the structure of both protons and neutrons, collectively known as nucleons, is still not fully understood. Here we explore two aspects of nucleon structure using electroweak probes. The first of these is the charge radius of the proton: experiments involving muonic hydrogen found a ∼ 4% reduction in the size of the proton from what was expected at the time, with no mechanism in place to explain the ∼ 5σ discrepancy with previous mea- surements from both electron spectroscopy and electron-proton scattering. This became known as the proton radius puzzle. We re-examine the extraction of the proton charge radius from electron-proton scat- tering, with particular emphasis on what we believe to be the major challenges. We then present potential means to overcome these issues through the use of Bayesian inferences. We then apply this methodology to extract the charge radius from ‘legacy data’ from the 2010 run at MAMI, Mainz, as well as ‘new age’ data from the 2019 PRad collaboration. In doing so, we hope to shed some light on the proton radius puzzle. We also examine the CP-odd content of the nucleon that is induced by the so called θ-term of quantum chromodynamics (QCD). In terms of electromagnetic interactions, this leads to not only an electric dipole moment (EDM), but also to CP-odd electromagnetic polarisabilities. These latter objects describe the consequence of the internal deformation of the nucleon when subject to incident radiation, which is then re-radiated. We compute these polarisabilities to O(p^3) in the momentum expansion of Chiral Perturbation Theory (χPT), before exploring potential experimental access. We also explore how the CP-odd content of the nucleon can lead to an induced EDM in atomic systems. Aspects of the interaction open up the possibility for coherent enhance- ment in heavy nuclei. As such, this provided a novel means of constraining fundamental CP violating parameters.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Michael Birse (Supervisor) & Judith McGovern (Supervisor)