The renormalization-group equations describing the evolution of fermion-Higgs-boson Yukawa coupling constants down from $M_X$ in a grand unified theory possess fixed points which may lead to universal predictions for fermion masses independent of symmetry considerations at $M_X$. Our analysis predicts ≈ 240 GeV for the fixed-pint $t$-quark mass. Alternatively, a sufficiently heavy fourth SU(5) generation cannot be ruled out by existing bounds on the number of fermion generations, $n_f$, and we find fixed-point mass predictions of $m_T\approx 219$ GeV, $m_B\approx 215$ GeV, and $m_E\approx 60$ GeV.

• Received 19 January 1981

DOI:https://doi.org/10.1103/PhysRevD.24.691