# What is the problem with the particle masses in the Standard Model?

• A
Gold Member
Summary:
mass hierarchy, top quark mass, neutrino masses
Hi,
Several times I encounter the argument that there is a "problem" with the masses in the Standard Model that we try to "understand". From the one side, you have people who ask why the neutrino masses are so small, and from the other side they ask why the top quark mass is so large. The smallness of the neutrino mass sounds a reasonable concern (as there is no "known"/"verified" mechanism to give neutrinos masses without for example extending the SM with right handed neutrinos or assigning them a Majorana nature), but for the rest of the particles I am a little confused when I see this argument in use.

So I was wondering, how relevant is a concern about this? Or is it also an argument about having a beautiful single value that governs everything (similar to a GUT)? So, is it because there exist some theories can (somehow) connect the masses together? Or is there something fundamentally suspicious about having a large gap of available masses?

Because, at least in the scope of the SM, if we say that the (bare) masses enter as free parameters, I don't understand what stops them from taking any value. Also, to my understanding, there is no problem associated with fermions having different masses (as for example with the mass of the scalar field like the Higgs which brings about the "hierarchy problem"). They are distinct particles, known for not being excitations of each other (a muon is not an excited electron), nor composite particles (as for example the nuclei or the atoms).

Staff Emeritus