Hadron structure at low Q^2

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.

55 pages, 47 figures, review

References in corpus (17)

• Unitary Isobar Model - MAID2007
• Nucleon Electromagnetic Form Factors
• Electromagnetic excitation of the Delta(1232)-resonance
• Global analysis of proton elastic form factor data with two-photon exchange corrections
• Chiral Perturbation Theory and Baryon Properties
• Dispersion analysis of the nucleon form factors including meson continua
• Nucleon electromagnetic form factors
• Two-Photon Physics in Hadronic Processes
• Measurement of the proton electric to magnetic form factor ratio from \vec ^1H(\vec e, e'p)
• Axial, induced pseudoscalar, and pion-nucleon form factors in manifestly Lorentz-invariant chiral perturbation theory
• Measurement of the Rate of Muon Capture in Hydrogen Gas and Determination of the Proton's Pseudoscalar Coupling $g_P$
• Electroweak structure of the nucleon, meson cloud and light-cone wavefunctions
• Proton spin polarizabilities from polarized Compton scattering
• Chiral effective-field theory in the Delta(1232) region: II. radiative pion photoproduction
• Determination of quadrupole strengths in the gamma*p-->Delta(1232) transition at Q2= 0.20 (GeV/c)2
• Separated cross sections in π^0 electroproduction at threshold at Q^2 = 0.05 GeV^2/c^2
• Beam-helicity asymmetry in photon and pion electroproduction in the Delta(1232) resonance region at Q^2= 0.35 (GeV/c)^2

Cited by in corpus (2)

• Delta resonances, quark models, chiral symmetry and AdS/QCD
• Hadron structure at small momentum transfer