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

Abstract

We report observables for elastic Compton scattering from ${}^3$He in Chiral Effective Field Theory with an explicit $\Delta (1232)$ degree of freedom ($\chi$EFT) for energies between 50 and 120 MeV. The $\gamma {}^3$He amplitude is complete at N3LO, $\mathcal{O}(e^2{\delta }^3)$, 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 ${}^3$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 N2LO, 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 [email protected].

arXiv comment37 pages LaTeX2e (pdflatex) including 16 figures as .pdf files using includegraphics; minor corrections, triggered by referee comments, plus typographical errors; text-identical to version published as EPJA 54 (2018) 125

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