Hi SDF

Quarks can't exist by themselves because to break them out of a nucleon requires energy that the colour-force binding the quarks turns into more particles. The colour force actually gets stronger the further apart the quarks are pulled and this drags more particles from out of the vacuum.

The masses quoted aren't rest masses either, as most of the mass energy actually comes from the relativistic mass of the rotating quarks. And protons (or neutrons) weigh less than 'free' quarks because three quarks within them pull together into a minimum energy configuration that reduces the colour force required to hold them there.

All a bit weird sounding but it matches all the known data and only a few cranks & idiots object to quarks on aesthetic grounds.

Adam