There is a remarkable infrared fixed point of the coupling constants that determine the masses of very heavy quarks. In the Standard Model, quarks and leptons have "Yukawa couplings" to the Higgs boson which determine the masses of the particles. Most of the quarks' and leptons' Yukawa couplings are small compared to the top quark's Yukawa coupling. Yukawa couplings are not constants and their properties change depending on the energy scale at which they are measured, this is known as "running" of the constants. The dynamics of Yukawa couplings are determined by the renormalization group equation: