| Force | Gravity | Strong | Weak | Electromagnetic |
|---|---|---|---|---|
| Carrier | Graviton (?) | Gluon | W, Z | Photon |
| Charge | 0 | 0 | +1, -1 (W) | 0 |
| 0 (Z) | ||||
| Mass (GeV/c2) | 0 ? | 0 ? | 80.22 (W) | 0 |
| 91.18 (Z) |
Current thinking says that quarks and anti-quarks cannot exist on their own. They are bound into composite particles such as protons and neutrons, by the strong nuclear force - the strength of which increases with distance. The strong force does limit the number of quarks or anti-quarks clumping together to three, or a pair of one quark and anti-quark:
Since there are six types of quark there are quite a variety of hadrons involving different combinations of up, down, strange, charm, bottom and top.