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:

I was somewhat disappointed by the bottom-quark experiment; it just didn't charm me.

Note: If you need to pronounce the word out loud - in an interview or podcast for example - pronounce it to rhyme with "park" and "dark". The term was originally "quork", but it was changed to rhyme with the name "Mark" by association with the line "Three quarks for Muster Mark" in James Joyce's novel Finnegans Wake (1939).