Quark–gluon plasma

Lower case, note the en dash

Any of a number of subatomic particles carrying a fractional electric charge, postulated as building blocks of the hadrons. Quarks have not been directly observed but theoretical predictions based on their existence have been confirmed experimentally.

Quarks have the different states up, down, top, bottom, charm and strange.  Use a space after the state:

Of all the quarks I have studied, I find the up quark the most intriguing.

Unless they are used as compound adjectives:

A quench is an abnormal termination of magnet operation that happens when part of the superconducting coil goes back to its normal (resistive) state. This can happen if the field inside the magnet or the rate of change of field is too large (causing eddy currents and heating in the copper support matrix), or due to a combination of the two. More rarely, a defect in the magnet can cause a quench.