fermion masses through Yukawa couplings

The Higgs mechanism naturally explains how the force carriers (the W and Z) get their mass, but the matter particles — the electron, the quarks, and their heavier cousins — are a separate story, and they need their own piece of machinery. That piece is the Yukawa coupling: a direct interaction written into the theory between each matter particle and the Higgs field. When the Higgs field switches on to its nonzero vacuum value, this interaction turns into a mass for that particle. So fermions get their mass by the same field, but through a different doorway than the W and Z do.

Concretely, for each matter particle the theory contains a number — its Yukawa coupling — that says how strongly that particle talks to the Higgs field. The particle's mass is simply that Yukawa number multiplied by the Higgs vacuum value. A large Yukawa coupling, like the top quark's (which is close to one), gives a very heavy particle; a tiny Yukawa coupling, like the electron's (which is a few parts in a million), gives a very light one. This is why the same mechanism can produce both the featherweight electron and the hefty top quark: the field is the same, but the coupling numbers differ enormously from one particle to the next.

Yukawa couplings are the source of much of the Standard Model's untidiness. There is one such number for each charged fermion, and they span an astonishing range — the top quark is roughly 340,000 times heavier than the electron — yet the theory does not predict any of them; they are simply measured and plugged in. Why the masses spread across such a vast range, and why there are exactly three generations of ever-heavier copies, is unexplained and is sometimes called the flavor puzzle. A note on neutrinos: their tiny masses do not fit comfortably into this simple Yukawa scheme and may require additional mechanisms, which is itself an active research frontier.