Chiral symmetry breaking

In particle physics, chiral symmetry breaking is the spontaneous symmetry breaking of a chiral symmetry usually by a gauge theory such as quantum chromodynamics, the quantum field theory of the strong interaction. Yoichiro Nambu was awarded the 2008 Nobel prize in physics for describing this phenomenon ("for the discovery of the mechanism of spontaneous broken symmetry in subatomic physics").

Experimentally, it is observed that the masses of the octet of pseudoscalar mesons (such as the pion) are much lighter than the next heavier states such as the octet of vector mesons, such as rho meson.

This is traceable to a consequence of spontaneous symmetry breaking of chiral symmetry in a fermion sector of QCD with 3 flavors of light quarks, u, d and s. Such a theory, for idealized massless quarks, has global SU(3) × SU(3) chiral flavor symmetry. Under SSB, this is spontaneously broken to the diagonal flavor SU(3) subgroup, generating eight Nambu–Goldstone bosons, which are the pseudoscalar mesons transforming as an octet representation of this flavor SU(3).

Beyond this idealization of massless quarks, the actual small quark masses also break the chiral symmetry explicitly as well (providing non-vanishing pieces to the divergence of chiral currents). The masses of the pseudoscalar meson octet are specified by an expansion in the quark masses which goes by the name of chiral perturbation theory. The internal consistency of this argument is further checked by lattice QCD computations, which allow one to vary the quark mass and confirm that the variation of the pseudoscalar masses with the quark masses is as dictated by chiral perturbation theory, effectively as the square-root of the quark masses.

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