# Why are fermion masses considered free parameters of the standard model rather than Yukawa coupling constants?

From what I understand, fermion masses in the standard model are due to Yukawa couplings to the Higgs field. Nevertheless, when listing free parameters of the standard model it is the fermion masses that are listed, rather than the coupling constants. I understand that these are essentially equivalent, given the values of the other free parameters, but still the Yukawa coupling constants seem the more fundamental quantities.

Why is it then that the fermion masses are usually listed?

• This is largely arbitrary. Most observables and theoretical quantities in the SM can be expressed as functions of others. For example, rest mass and a Yukawa for a SM fermion are proportional to each other. Which one of related quantities to call fundamental and which to call derived is a question of style, even though the number of degrees of freedom can't be reduced in the process. Mar 11 '19 at 21:15

## 1 Answer

Specifying the particle mass as the free parameter instead of the Yukawa coupling makes the theory more relatable to experiment. The thing you ultimately measure at the end of day is the mass of the physical particle, not some parameter within an abstract theory. Different theories may come up with their own expressions for what the electron mass ought to be, but whatever that expression is it needs to give me the actual electron mass at the end of the day.