On the SO(5) Effective Field Theory of High T_c Superconductors

We construct the low-energy effective theory for the SO(5) model of high-T_c superconductivity, recently proposed by S.C. Zhang (cond-mat/9610140). This permits us to develop a systematic expansion for low-energy observables in powers of the small symmetry-breaking interactions. The approximate SO(5) symmetry predicts relations amongst these observables, which are model-independent consequences of Zhang's proposed symmetry-breaking pattern.