This project will address some of the current challenges in our understanding of the proton: the proton radius puzzle, whereby the highly precise muonic hydrogen measurement of the radius is seven standard deviations from the currently accepted value from electron-based measurements; and measurement of the scalar and spin polarizabilities of the proton.
Recent theoretical calculations of the proton scalar polarizabilities describe the data as well as dispersion-relation-based analyses, but produce a magnetic polarizability twice that of the dispersion analyses. The spin polarizabilities are predicted within many theoretical frameworks, but, until recently, experimental measurement of these properties was unfeasible. We will perform a series of singly- and doubly-polarized Compton scattering measurements to provide a high quality extraction of the scalar polarizabilities, and a world-first independent extraction of all four spin polarizabilities. The measurement program will be carried out within the framework of the A2 Collaboration at the MAinzer MIkrotron accelerator in Mainz, Germany, where the PI is a lead PI on the polarizability program.
The MUon Scattering Experiment (MUSE) at the Paul Scherer Institute (PSI) in Switzerland will access the proton radius for the first time via muon scattering, achieving a level of precision comparable with the current electron scattering data. The beam, a mixture of muons, pions and electrons / positrons, will be employed in both charge states and electron and muon scattering data will be taken in parallel. This will allow for an extraction of the radius from both proton and muon elastic scattering to a level of precision comparable with that currently obtained in electron scattering experiments. In so doing, it provides a thorough test of lepton universality, two-photon-exchange corrections relevant to the extraction of the radius from the scattering data and accesses regions where possible beyond standard model physics is postulated to occur. The two photon exchange corrections will be further tested in a complementary dilepton photoproduction experiment at the High Intensity Gamma Source (HIGS) at the Triangle Universities Nuclear Laboratory (TUNL) in North Carolina.
All visible matter with which we interact is formed of atoms, which are in turn composed of neutrons, protons and electrons. We know that the nucleons: the protons and neutrons themselves have substructure: components known as quarks. As these quarks are so strongly bound together, it is impossible for us to pull them apart in order to examine the internal structure and binding of the proton. Instead we have to form theories of how they might be bound together, and make predictions based on those theories which we can test experimentally in order to gain understanding of these building blocks of our everyday lives. It is a little like looking at a catalogue of Lego models and trying to infer the shape and binding mechanisms of the bricks from the possible configurations in the catalogue. We will investigate two properties of the proton in this proposal: its radius and polarizabilities. In so doing, we aim to gain a better understanding of this fundamental building block of the universe and, though it, to better understand the strengths and limitations of current theoretical models of nuclear physics. These models influence many modern technologies such as those involved in nuclear power or radiotherapy treatment, and determine how the stars burn in the sky.
The involvement of undergraduate and graduate research students in international research offers excellent training opportunities both in physics and in international relationships. The leadership of a young female PI will hopefully encourage more individuals from underrepresented groups to consider a career in the natural sciences.
