Masses and mixing in unified theories

The authors discuss quark and lepton masses and the weak mixing angle with particular emphasis on O(10) unified theories of strong, weak and electromagnetic interactions. It is found that a quark-lepton mass relation which predicts msub(μ)/msub(e) = 9 msub(s)/msub(d) can be implemented in O(10) and agrees with data. The possibility of modifying the gauge hierarchy structure in O(10) is discussed to obtain sin2 thetasub(W) = 0.23. The most attractive such scheme gives a good prediction for msub(b)/msub(tau) with a realistic value for the strong coupling constant. In this case, the prediction for the proton lifetime is the same as in an SU(5) theory. (Auth.)