Speaker
Prof.
Harald W. Griesshammer
(Institute for Nuclear Studies, George Washington University)
Description
Compton scattering from protons and neutrons probes the two-photon response of the nucleon in electric and magnetic fields at fixed photon frequency and multipolarity. As these fields induce radiation multipoles by displacing the target constituents, the angular and energy dependence of the emitted radiation provides detailed tests of the symmetries and strengths which govern the interactions of the constituents with each other and with photons. At low energies, the process is parameterised by six energy-dependent dipole polarisabilities. Their zero-energy limit are the static electric and magnetic scalar dipole polarisabilities $\alpha_{E1}$ and $\beta_{M1}$, and the four spin-polarisabilities. Differences between proton and neutron values stem from isospin-breaking interactions, exploring the interplay between chiral symmetry as well as the pattern of its breaking, and short-distance Physics. The information is for convenience often compressed in the static scalar dipole polarisabilities, i.e.~the extrapolated values at zero photon energy. In combination with emerging lattice QCD determinations, they provide stringent tests for our theoretical description of hadron structure and are thus fundamental quantities in their own right. Moreover, they are crucial to the neutron-proton mass difference, the proton charge radius puzzle, and the Lamb shift of muonic hydrogen.
Recently, a new extraction of the static electric and magnetic scalar dipole polarisabilities of the proton and neutron from all published elastic data below 300~MeV was performed in Chiral Effective Field Theory. ChiEFT is ideal for that purpose since it provides reliable theoretical uncertainties by a model-independent estimate of higher-order corrections and encodes the correct low-energy dynamics of QCD, including, for few-nucleon systems, consistent nuclear currents, rescattering effects and wave functions. The proton and neutron values turn out to be identical within the accuracy of available data.
After reviewing context, ingredients and results of this analysis, this talk discusses the following topics: statistical consistency of the world Compton dataset; status of one- and few-nucleon Compton theory; desirable improvements of the database; elastic and inelastic Compton scattering on deuterons and ${}^3$He; and predictions for high-intensity experiments with polarised targets an polarised beams. The goal is to extract not only scalar nucleon polarisabilities, but also the so-far poorly explored spin-polarisabilities, which parametrise the stiffness of the nucleon spin in external electro-magnetic fields (nucleonic bi-refringence/Faraday effect).
Primary author
Prof.
Harald W. Griesshammer
(Institute for Nuclear Studies, George Washington University)
Co-authors
Prof.
Daniel R. Phillips
(Institute for Nuclear and Particle Physics and Department of Physics and Astronomy, Ohio University, Athens OH, USA)
Prof.
Judith A. McGovern
(School of Physics and Astronomy, The University of Manchester, UK)
