CRITICAL INSTABILITY AS AN ORIGIN OF LARGE ISOSPIN BREAKING

Critical instability, as implied by the fine tuning of a 4-fermi coupling constant, is proposed as to the origin of the large isospin breaking in the third generation quark masses. We suggest two possible scenarios with new physics beginning at scales LAMBDA-approximately 20 TeV or LAMBDA-approximately 10(16)-10(19) GeV. In the low LAMBDA-scenario, a fourth generation is necessary and its isodoublet is naturally predicted to be almost mass degenerate. The high LAMBDA-scenario is discussed in the framework of the E6 GUT. We also give a formula describing how the critical value of the 4-fermi coupling is shifted by the presence of general gauge interactions.