values cause the rhs to quickly approach smaller values as we descend in energy scale. The above equation then locks to the QCD coupling . This is known as the (infrared) quasi-fixed point of the renormalization group equation for the Yukawa coupling. No matter what the initial starting value of the coupling is, if it is sufficiently large it will reach this quasi-fixed point value, and the corresponding quark mass is predicted. The value of the quasi-fixed point is fairly precisely determined in the Standard Model, leading to a predicted top quark mass of 230 GeV. The observed top quark mass of 174 GeV is slightly lower than the standard model prediction by about 30% which suggests there may be more Higgs doublets beyond the single standard model Higgs boson. Minimal Supersymmetric Standard Model Renomalization group studies in the Minimal Supersymmetric Standard Model (MSSM) of grand unification and the Higgs–Yukawa fixed points were very encouraging that the theory was on the right track. So far, however, no evidence of the predicted MSSM particles has emerged in experiment at the Large Hadron Collider. See also Banks–Zaks fixed point Callan–Symanzik equation Quantum triviality References Further reading Peskin, M and Schroeder, D.; An Introduction to Quantum Field Theory, Westview Press (1995). A standard introductory text, covering many topics in QFT including calculation of beta functions; see especially chapter 16. Weinberg, Steven; The Quantum Theory of Fields, (3 volumes) Cambridge University Press (1995). A monumental treatise on QFT. Zinn-Justin, Jean; Quantum Field Theory and Critical Phenomena, Oxford University Press (2002). Emphasis on the renormalization group and related topics. Renormalization group Scaling symmetries