Hadron structure at lowQ2

Abstract

This review deals with the structure of hadrons, strongly interacting many-body systems consisting of quarks and gluons. These systems have a size of about 1 fm, which shows up in scattering experiments at low momentum transfers Q in the GeV region. At this scale the running coupling constant of Quantum Chromodynamics (QCD), the established theory of the strong interactions, becomes divergent. It is therefore highly intriguing to explore this theory in the realm of its strong interaction regime. However, the quarks and gluons can not be resolved at the GeV scale but have to be studied through their manifestations in the bound many-body systems, for instance pions, nucleons and their resonances. The review starts with a short overview of QCD at low momentum transfer and a summary of the theoretical apparatus describing the interaction of hadrons with electrons and photons. In the following sections we present the experimental results for the most significant observables studied with the electromagnetic probe: form factors, polarizabilities, excitation spectra, and sum rules. These experimental findings are compared and interpreted with various theoretical approaches to QCD, such as phenomenological models with quarks and pions, dispersion relations as a means to connect observables from different experiments, and, directly based on the QCD lagrangian, chiral perturbation theory and lattice gauge theory.

Gerasimov-Drell-Hearn sum rule and the discrepancy between the new CLAS and SAPHIR data

“…The latest result calculated from a combination of pion photoproduction and photon absorption processes yields a value of −211 ± 15 µb [18], thus, although it slightly overestimates the sum rule, it is still consistent within the present error bar. However, one can also calculate the I GDH by summing up all possible photoproduction processes.…”

“…It comes as no surprise that the predicted values of γ 0 are quite small compared to the presently known value, i.e., −0.94 × 10 −4 fm 4 [18]. However, it is interesting to compare the contribution from the nπ + η channels, i.e.…”

Baryons as relativistic three-quark bound states

“…There has been much recent interest in a precision determination of the nucleon's polarisabilities, which is reflected in a number of reviews on the topic [441,[638][639][640][641][642][643]. The electric polarisability α and magnetic polarisability β tell us how the nucleon responds to an external electromagnetic field, with current PDG values α = 11.2(4) × 10 −4 fm 3 and β = 2.5(4) × 10 −4 fm 3 for the proton [9].…”

Model analysis of theep→e'pπ+π−electroproduction reaction on the proton

“…The mechanisms of the N π electroproduction have been extensively studied. The most comprehensive recent studies are by the efforts at MAMI [30,31], GWU [32] and JLab [33,34,35,36]. In contrast, the study of N ππ electroproduction mechanisms is still limited.…”