A New High-Accuracy Analysis of Compton Scattering in Chiral EFT: Neutron Polarisabilities*

Low-energy Compton scattering tests the symmetries and interaction strengths of a target's internal degrees of freedom in the electric and magnetic fields of a real, external photon. In the single-nucleon sector, information is often compressed into the static scalar dipole polarisabilities which are experimentally not directly accessible but encode information on the pion cloud and the $\Delta(1232)$ excitation. The interaction of the photon with the charged pion-exchange also provides a conceptually clean probe of few-nucleon binding. After demonstrating the statistical consistency of the world's $\gamma$d dataset including the new data from the MAX-IV collaboration described in the preceding talk, we present a new extraction of the neutron polarisabilities in Chiral Effective Field Theory: $\alpha_n=[11.55\pm1.25(\rm{stat})\pm0.2(\rm{BSR})\pm0.8(\rm{th})]$ and $\beta_n=[3.65\mp1.25(\rm{stat})\pm0.2(\rm{BSR})\mp0.8(\rm{th})]$, in $10^{-4}$ fm${}^3$, with $\chi^2=45.2$ for $44$ degrees of freedom. The new data reduced the statistical uncertainties by $30$\%. We discuss data accuracy and consistency, the role of the $\Delta(1232)$, and an estimate of residual theoretical uncertainties. Within statistical and systematic errors, proton and neutron polarisabilities remain identical.