constituent vs current quark mass

Ask "how much does an up quark weigh?" and you will get two very different answers, both correct, because the question is ambiguous. There is the mass the quark has on its own as a fundamental particle — tiny — and the much larger effective mass it seems to carry once it is dressed in the roiling strong-force fields inside a proton. These are the current quark mass and the constituent quark mass.

The current quark mass is the bare, intrinsic mass that appears in the equations of QCD — the part that comes from coupling to the Higgs field. For the up and down quarks this is only a few MeV, almost nothing. The constituent quark mass is what you would assign each quark if you pretended a proton were simply three quarks stuck together and demanded their masses add up to the proton's: that gives roughly 300 MeV per quark. The difference — the lion's share — is not the quark itself at all. It is the energy of the gluon fields and the swarm of virtual quarks surrounding the quark, an energy that, via E = mc², shows up as mass.

This distinction carries one of the most important lessons in all of physics: most of the mass of ordinary matter does not come from the Higgs. The Higgs gives the up and down quarks their few MeV, but well over 90% of a proton's mass — and therefore of your own body's mass — is pure strong-force energy, conjured by QCD out of the binding of nearly massless quarks. The constituent picture is a handy bookkeeping device for the low-energy hadron world; the current mass is the honest fundamental parameter.