quark, any member of a group of elementary subatomic particles that interact by means of the strong force and are believed to be among the fundamental constituents of matter. Quarks associate with one another via the strong force to make up protons and neutrons, in much the same way that the latter particles combine in various proportions to make up atomic nuclei. There are six types, or flavours, of quarks that differ from one another in their mass and charge characteristics. These six quark flavours can be grouped in three pairs: up and down, charm and strange, and top and bottom. Quarks appear to be true elementary particles; that is, they have no apparent structure and cannot be resolved into something smaller. In addition, however, quarks always seem to occur in combination with other quarks or with antiquarks, their antiparticles, to form all hadrons—the so-called strongly interacting particles that encompass both baryons and mesons.
| quark type | baryon number | charge | strangeness** | charm** | bottom** | top** | mass (MeV) |
|---|---|---|---|---|---|---|---|
| down (d) | 1/3 | −(1/3)e | 0 | 0 | 0 | 0 | 5–15 |
| up (u) | 1/3 | +(2/3)e | 0 | 0 | 0 | 0 | 2–8 |
| strange (s) | 1/3 | −(1/3)e | −1 | 0 | 0 | 0 | 100–300 |
| charm (c) | 1/3 | +(2/3)e | 0 | 1 | 0 | 0 | 1,000–1,600 |
| bottom (b) | 1/3 | −(1/3)e | 0 | 0 | −1 | 0 | 4,100–4,500 |
| top (t) | 1/3 | +(2/3)e | 0 | 0 | 0 | 1 | 180,000 |
| *Note that antiquarks exist for all flavours of quark and have opposite values for all the quantum numbers listed here. | |||||||
| **These are quantum numbers that must be assigned to the quarks to differentiate the various flavours. |