Remember that the masses of the u and d quarks are only a few MeV, yet the “constituent” quark mass is 940/3 or around 310 MeV. Also, we emphasize again that the standard model theories of strong, weak and electromagnetic interactions start with massless quarks and leptons. Their mass is generated later by coupling to the Higgs field. But the fact that the basic equations of the Standard Model involve massless particles creates yet another symmetry,  chiral symmetry. Chirality is a Lorentz-invariant generalization of the concept of helicity, or handedness, which we have also previously discussed briefly.  Massless particles are eigenstates of helicity, and have a definite chirality. When the Higgs coupling is turned on, the chiral symmetry is also broken. In the same way that the Higgs mechanism generates mass, the chiral symmetry breaking also generates additional mass. The proton and neutron are full of virtual quark-antiquark pairs, but before chiral symmetry breaking the vacuum expectation value of the mass of a virtual quark-antiquark pair is zero.  After symmetry-breaking, it is not zero. Mass generation by the chiral symmetry breaking is most significant only for the lightest quarks, the u, d and s. But notice that the mass generated for the u and d quarks is the principal source of almost all the mass of ordinary matter, 98% of it to be more precise!  In other words, the virtual quark-antiquark pairs within the nucleon now make a major contribution to its total mass.

The telltale signs of the symmetry breaking in our universe today, apart from nucleon mass,  are a distinctive splitting by 500 MeV of the orginally-degenerate “chiral partner” states of mesons and baryons, and the appearance of pseudo-Goldstone bosons (the pions).