Elastic Compton Scattering from 3He and the Role of the Delta

We report observables for elastic Compton scattering from 3He in Chiral Effective Field Theory with an explicit $ \Delta(1232)$ degree of freedom ( $ \chi$ EFT) for energies between 50 and 120MeV. The $ \gamma$ 3He amplitude is complete at N $ ^{3}$ LO, $ {O}(\mathrm{e}^2\delta^3)$ , and in general converges well order by order. It includes the dominant pion-loop and two-body currents, as well as the Delta excitation in the single-nucleon amplitude. Since the cross section is two to three times that for deuterium and the spin of polarised 3He is predominantly carried by its constituent neutron, elastic Compton scattering promises information on both the scalar and spin polarisabilities of the neutron. We study in detail the sensitivities of 4 observables to the neutron polarisabilities: the cross section, the beam asymmetry and two double asymmetries resulting from circularly polarised photons and a longitudinally or transversely polarised target. Including the Delta enhances those asymmetries from which neutron spin polarisabilities could be extracted. We also correct previous, erroneous results at N $ ^{2}$ LO, i.e. without an explicit Delta, and compare to the same observables on proton, neutron and deuterium targets. An interactive Mathematica notebook of our results is available from hgrie@gwu.edu.