Quarks
Fundamental spin-1/2 elementary particles that experience all four fundamental interactions and combine via the strong force to form hadrons.
Governing formula
q = +⅔ e (u, c, t), q = -⅓ e (d, s, b)
SI unit
Fractional Elementary Charge e
In depth
Quarks exist in six flavors (Up, Down, Charm, Strange, Top, Bottom) and carry fractional electric charges (+2/3 e or -1/3 e) along with color charge (Red, Green, Blue). Due to color confinement, quarks are never observed isolated in nature.
Examples in the real world
A proton composed of two up quarks and one down quark (uud), and a neutron composed of one up quark and two down quarks (udd).