Compton scattering on He4 with nuclear one- and two-body densities

We present the first ab initio calculation of elastic Compton scattering from
4He. It is carried out to N3LO in the delta expansion of chiral EFT. At this order and for this target, the only free parameters are the scalar-isoscalar electric and magnetic dipole polarisabilities of the nucleon. Adopting current values for these yields a parameter-free prediction. This compares favourably with the world data from HIGS, Illinois and Lund for photon energies 50-120 MeV within our theoretical uncertainties of 10%. We predict a cross section up to 7 times that for deuterium. As in
3He, this emphasises and tests the key role of meson-exchange currents between
np pairs in Compton scattering on light nuclei. We assess the sensitivity of the cross section and beam asymmetry to the nucleon polarisabilities, providing clear guidance to future experiments seeking to further constrain them. The calculation becomes tractable by use of the Transition Density Method. The one- and two-body densities generated from 5 chiral potentials and the AV18+UIX potential are available using the python package provided at https://pypi.org/project/nucdens/.