Spin polarisabilities of the nucleon at NLO in the chiral expansion

Abstract

We present a calculation of the fourth-order (NLO) contribution to spin-dependent Compton scattering in heavy-baryon chiral perturbation theory, and we give results for the four spin polarisabilities. No low-energy constants, except for the anomalous magnetic moments of the nucleon, enter at this order. For forward scattering the fourth-order piece of the spin polarisability of the proton turns out to be almost twice the size of the leading piece, with the opposite sign. This leads to the conclusion that no prediction can currently be made for this quantity. For backward scattering the fourth-order contribution is much smaller than the third-order piece which is dominated by the anomalous scattering, and so cannot explain the discrepancy between the CPT result and the current best experimental determination.