1 5 O ct 2 00 7 Neutron Polarisabilities from Deuteron Compton Scattering in χ EFT

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 ∆(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: α = 11.6± 1.5stat ± 0.6Baldin, β n = 3.6∓ 1.5stat ± 0.6Baldin for the neutron, in units of 10 fm, compared to α = 11.0 ± 1.4stat ± 0.4Baldin, β p = 2.8 ∓ 1.4stat ± 0.4Baldin for the proton in the same framework. New experiments e.g. at MAXlab (Lund) will improve the statistical error-bar. 1 The Problem with Neutron Polarisabilities As the nucleon is not a point-like spin2 target with an anomalous magnetic moment, the photon field displaces in low-energy Compton scattering γN → γN its charged constituents, inducing a non-vanishing multipolemoment. These long-known nucleon-structure effects are for static external fields parameterised by the electric polarisability α and its magnetic counter-part β. For the proton, the generally accepted static values are α ≈ 12× 10 fm, β p ≈ 2× 10 fm, with error-bars of about 1. 1 Does the neutron react similarly under deformations, α ≈ α, β p ≈ β n ? Different types of experiments report a range of values α ∈ [−4; 19]: Coulomb scattering of neutrons off lead, and deuteron Compton-scattering γd → γd with and without breakup, see e.g. [1] for a list. The latter should be It is customary to measure the scalar dipole-polarisabilities in 10 fm, so that the units are dropped in the following. Notice that the nucleon is quite stiff.