High-Accuracy Analysis of Compton Scattering in Chiral EFT: Status and Future
Harald Griesshammer (The George Washington University, USA und Forschungszentrum Jülich)

Compton scattering from protons and neutrons provides important insight into the structure of the nucleon. A new extraction of the static electric and magnetic dipole polarisabilities \(\alpha_{E1}\) and \(\beta_{M1}\) c of the proton and neutron from all published elastic data below 300~MeV in Chiral Effective Field Theory shows that within the statistics-dominated errors, the proton and neutron polarisabilities are identical, i.e.~no iso-spin breaking effects of the pion cloud are seen. Particular attention is paid to the precision and accuracy of each data set, and to an estimate of residual theoretical uncertainties. ChiEFT is ideal for that purpose since it provides a model-independent estimate of higher-order corrections and encodes the correct low-energy dynamics of QCD, including, for few-nucleon systems used to extract neutron polarisabilities, consistent nuclear currents, rescattering effects and wave functions. It therefore automatically respects the low-energy theorems for photon-nucleus scattering. The \(\Delta(1232)\) as active degree of freedom is essential to realise the full power of the world's Compton data.
Its parameters are constrained in the resonance region. In view of ongoing effort at HI\(\gamma\)S, MAMI and MAXlab, desirable improvements of the database are discussed. IN particular single- and doubly-polarised observables can be used to extract not only scalar nucleon polarisabilities, but also the so-far poorly explored spin-polarisabilities. They parametrise the stiffness of the nucleon spin in external electro-magnetic fields.