Chiral Symmetry Breaking In Quantum Chromodynamics Research Articles

Compton scattering off pions and electromagnetic polarizabilities

The electric [Formula: see text] and magnetic [Formula: see text] Compton polarizabilities of both the charged and the neutral pion are of fundamental interest in the low-energy sector of quantum chromodynamics (QCD). Pion polarizabilities affect the shape of the [Formula: see text] Compton scattering angular distribution at back scattering angles and [Formula: see text] absolute cross sections. Theory derivations are given for the [Formula: see text] Compton scattering differential cross section, dispersion relations, and sum rules in terms of the polarizabilities. We review experimental charged and neutral polarizability studies and theoretical predictions. The [Formula: see text] polarizabilities were deduced from DESY Crystal Ball [Formula: see text] data, but with large uncertainties. The charged pion polarizabilities were deduced most recently from (1) radiative pion Primakoff scattering [Formula: see text] at CERN COMPASS, (2) two-photon pion pair production [Formula: see text] at SLAC Mark II, and (3) radiative pion photoproduction [Formula: see text] from the proton at MAMI in Mainz. A stringent test of chiral perturbation theory (ChPT) is possible based on comparisons of precision experimental charged pion polarizabilities with ChPT predictions. Only the CERN COMPASS charged pion polarizability measurement has acceptably small uncertainties. Its value [Formula: see text] agrees well with the two-loop ChPT prediction [Formula: see text], strengthening the identification of the pion with the Goldstone boson of chiral symmetry breaking in QCD.

The origin of mass and experiments on future high-energy accelerators

The observable world is the one consisting of nucleons and electrons. The mass of the nucleon arises from chiral symmetry breaking in quantum chromodynamics (QCD), so that high-energy accelerator experiments cannot give any clues to the nature of the mass of matter in the observable world. The origin of the mass of matter will be clarified when the mechanism of chiral symmetry breaking in QCD will be established.

Chiral-symmetry breaking in quantum chromodynamics

We consider chiral-symmetry breaking in a generalization of the Nambu-Jona-Lasinio mechanism to quantum chromodynamics. We derive the pion mass and decay width in the resulting «constituent-quark» picture, and study the interaction of neutrinos with the dynamical quark condensate.