High-Accuracy Analysis of Compton Scattering in Chiral EFT: Status and Future

Harald W. Grießhammer (GWU)

Compton scattering from protons and neutrons provides important insight into the structure of the nucleon.  We present a new extraction of the static electric and magnetic dipole polarisabilities of the proton and neutron from all published elastic data below 300 MeV in Chiral Effective Field Theory.  Including the Δ(1232) as explicit degrees of freedom is particularly important for the proton above about 100 MeV and for deuteron Compton scattering at > 90 MeV as measured at SAL and MAXlab.  Consistency arguments dictate including the np-rescattering states and automatically render the correct Thomson limit.  We show that the static electric and magnetic scalar polarisabilities of the proton and neutron are identical within the accuracy of available data.  In view of ongoing effort at HIγS, MAMI and MAXlab, we address in detail single- and doubly-polarised observables.  Several of them can be used to extract not only scalar nucleon polarisabilities, but also the so-far practically un-determined spin-dependent polarisabilities which parametrise the stiffness of the nucleon spin in external electro-magnetic fields.  Amongst the questions addressed are: convergence of the expansion when including the Δ and the rôle of np-rescattering in few-nucleon systems.  The next steps in the programme are outlined.

(Seminarraum II)