Polarizability of the nucleon and Compton scattering

One of the central challenges of hadron physics in the regime of strong (non-perturbative) QCD is to identify the relevant degrees of freedom of the nucleon and to quantitatively explain experimental data in terms of these degrees of freedom. Among the processes studied so far Compton scattering plays a prominent role because of the well understood properties of the electromagnetic interaction. Different approaches to describe Compton scattering have been discussed up to now. It will be shown that the most appropriate ones are provided by nonsubtracted dispersion theories of the fixed-t and fixed-θ types, where the properties of these two versions are complementary so that advantage can be taken from both of them. In the frame of fixed-t dispersion theory it was possible to precisely reproduce experimental differential cross sections obtained for the proton in a wide angular range and for energies up to 1 GeV. At energies of the first resonance region and below, precise values for the electromagnetic polarizabilities and spin-polarizabilities have been determined for the proton and the neutron. As a summary we give the following recommended experimental values for the electromagnetic polarizabilities and backward spin-polarizabilities of the nucleon: αp=12.0±0.6, βp=1.9∓0.6, αn=12.5±1.7, βn=2.7∓1.8, in units of 10−4 fm3 and γπ(p)=−38.7±1.8, γπ(n)=58.6±4.0 in units of 10−4 fm4. These data show that diamagnetism is a prominent property of nucleon structure. It will be shown that the largest part of diamagnetism, or equivalently (α−β), is not related to the conventional isobar-meson structure of the nucleon as showing up in meson photoproduction. Rather, the underlying mechanism is a t-channel σ-meson exchange, with the constituent-quark-meson configuration remaining in its ground state. The same is true for the backward spin-polarizability γπ where the relevant meson is the π0. Making the reasonable assumption that the quantities (α−β) and γπ are related to the structure of the nucleon, we come to the conclusion that the σ and π0 intermediate states are part of the structure of the nucleon. It is a challenge for further research to integrate these degrees of freedom into a consistent description of the structure of the nucleon.