摘要
It is generally believed that the QCD Lagrangian has N_f^2 conserved axial currents except for quark mass terms. In the world of zero bare quark masses for the first L flavours, the corresponding conservation laws are spontaneously broken and L^2 pseudo-Goldstone bosons are thought to be generated. For three quark flavours the absence of the ninth light pseudoscalar in the real world is a well-known puzzle first pointed out by Glashow and studied subsequently by Wienberg, Kogut and Susskind and many others.
