Nucleon Spin Polarisabilities from Polarised Deuteron Compton Scattering

TL;DR

This paper calculates deuteron Compton scattering with polarized beams and targets using Chiral Effective Field Theory, aiming to extract nucleon spin-dependent and independent polarisabilities from experimental observables.

Contribution

It provides a detailed theoretical framework including np-rescattering effects and polarization observables to determine nucleon polarisabilities, especially the spin-dependent ones.

Findings

Polarization observables can help extract nucleon polarisabilities.

np-rescattering contributions are crucial at leading order.

The approach guarantees current conservation and correct Thomson limit.

Abstract

We present recent results on elastic deuteron Compton scattering calculations for polarised beans and targets up to next-to-leading order within Chiral Effective Field Theory in the Small Scale Expansion variant to implement a dynamical \Delta(1232) degree of freedom. A simple power-counting argument discloses that np-intermediate rescattering states must be explicitly included at leading order already. This automatically results in the correct Thomson limit and guarantees current conservation. In view of ongoing effort at MAXlab, proposals at HIGS and plans at MAMI, we address in detail single- and double-polarised observables with linearly or circularly polarised photons on both unpolarised and vector-polarised deuterons. Our results indicate that several of the polarisation observables can be instrumental to extract not only spin-independent nucleon polarisabilities, but also the so-far practically un-determined spin-dependent polarisabilities which parameterise the stiffness of the nucleon spin in external electro-magnetic fields. Amongst the questions addressed are: convergence of the expansion for including the \Delta, the role of the np-rescattering contributions, and sensitivity to the deuteron wave function. An interactive Mathematica 7.0 notebook of these findings is available from the authors.