Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Pseudo-Goldstone bosons arise in a quantum field theory with an approximate symmetry such that if the symmetry were exact, then there would be spontaneous symmetry breaking (SSB) and the consequent formation of Goldstone bosons. The properties of these pseudo-Goldstone bosons can often be found by an expansion around the symmetric theory in terms of the explicit symmetry breaking parameter. Quantum chromodynamics (QCD) provides the most well-known example: see the article on QCD vacuum for details. Experimentally it is seen that the masses of the octet of pseudoscalar mesons (such as the pion) are very much lighter than the next heaviest states, ie, the octet of vector mesons (such as the rho meson). In QCD this is interpreted as the spontaneous symmetry breaking of a version of QCD with 3 flavours of massless quarks. Such a theory has global SU(3)times SU(3) chiral flavour symmetry.

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