Speaker
Description
I review the theory side of the synergetic international effort of experimentalists and theorists in Compton scattering on one- and few-nucleon systems. It probes the symmetries and strengths of nucleonic and nuclear interactions and relates them to lattice-QCD computations of fundamental hadronic properties. The polarisabilities parametrise the stiffness of charge distributions against deformations. Their spin components encode the stiffness of the spin in external electro-magnetic fields (nucleonic Faraday effect) and probe the spin-dependent component of the pion-nucleon interaction. As the scalar components are crucial for the proton-neutron mass difference, we focus on the prediction and extraction of their neutron values from few-nucleon experiments which also test the chiral symmetry of the charged pion-exchange contribution to nuclear binding. Precision theory is available for the proton and deuteron. For the first-ever descriptions of Compton scattering on $^3$He, $^4$He and $^6$Li, the transition-density formalism is highly efficient for computing reactions with perturbative probes. One- and two-body transition densities that encode the nuclear structure of the target are evaluated once and stored. They are then convoluted with an interaction kernel to produce observables, exploiting factorisation between nuclear structure and interaction kernel in Chiral EFT. The same densities can be used with different kernels, like pion-photoproduction and pion scattering. We summarise the prospects and identify kinematics and observables for the likely biggest improvement of the polarisability values.
