Elastic Compton scattering from 3He and the role of the Delta

. We report observables for elastic Compton scattering from 3 He in Chiral Effective Field Theory with an explicit Δ ( 1232 ) degree of freedom ( χ EFT) for energies between 50 and 120MeV. The γ 3 He amplitude is complete at N 3 LO, O ( e 2 δ 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 3 He 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.