Neutron Polarisabilities from Deuteron Compton Scattering in \chiEFT

Abstract

Chiral Effective Field Theory is for photon energies up to 200 MeV the tool to accurately determine the polarisabilities of the neutron from deuteron Compton scattering. A multipole analysis reveals that dispersive effects from an explicit Delta(1232) prove in particular indispensable to understand the data at 95 MeV measured at SAL. Simple power-counting arguments derived from nuclear phenomenology lead to the correct Thomson limit and gauge invariance. At next-to-leading order, the static scalar dipole polarisabilities are extracted as identical for proton and neutron within the error-bar of available data: \alpha^n=11.6\pm1.5_stat\pm0.6_Baldin, \beta^n=3.6\mp1.5_stat\pm0.6_Baldin for the neutron, in units of 10^-4 fm^3, compared to \alpha^p=11.0\pm1.4_stat\pm0.4_Baldin, \beta}^p=2.8\mp1.4_stat\pm0.4_Baldin for the proton in the same framework. New experiments e.g. at MAXlab (Lund) will improve the statistical error-bar.