Quarks are the fundamental particles that make up protons and neutrons. They are the smallest building blocks of matter that we know of. Quarks come in six flavors: up, down, charm, strange, top, and bottom. Up and down quarks are the lightest and most common. A proton is made of two up quarks and one down quark. A neutron is made of two down quarks and one up quark. Quarks are never found alone. They are always bound together by the strong force, in groups of two or three. That phenomenon is called confinement.

Quarks were proposed independently by Murray Gell-Mann and George Zweig in 1964. The name comes from a line in James Joyce's Finnegans Wake. Experiments at the Stanford Linear Accelerator Center in the late 1960s confirmed their existence. Quarks have fractional electric charges: up, charm, and top quarks have +2/3, while down, strange, and bottom quarks have -1/3. They also have color charge, which is the source of the strong force. The strong force is mediated by gluons, which bind quarks together. Without the strong force, quarks would fly apart and atoms would not exist. Quarks are a cornerstone of the Standard Model of particle physics.

Quark facts

• Six flavors. Up, down, charm, strange, top, bottom.
• Charge. Fractional, +2/3 or -1/3.
• Proton. Two up, one down.
• Neutron. Two down, one up.
• Confinement. Never found alone.

Quarks are not the same as leptons. Quarks feel the strong force. Leptons do not.