A quark ( or ) is an elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei.
Due to a phenomenon known as color confinement, quarks are never directly observed or found in isolation; they can be found only within baryons or mesons.
For this reason, much of what is known about quarks has been drawn from observations of the hadrons themselves.
There are six types of quarks, known as flavors: up, down, strange, charm, bottom, and top.
Up an...