Spontaneous Global Symmetry Breaking
The preceding chapter has shown how the observed partial conservation of the axial isospin current can be understood as the consequence of the approximate chiral symmetry of the strong interaction Lagrangian. A problem that has been ignored so far, however, is the nonzero mass of the nucleons so that their Lagrangian is at first sight clearly not chirally invariant. A way out of this dilemma is offered by making a careful distinction between the symmetries of the Lagrangian and those of the actual state of the physical system. The discussion of the consequences of this distinction is the purpose of this chapter.
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