Quarks are elementary particles and fundamental building blocks of matter in the Standard Model. Protons and neutrons are not elementary: each is made from quarks bound together by the strong interaction. Quarks also appear in many other short-lived composite particles produced in high-energy experiments.
The six quark flavors
Physicists recognize six flavors of quark: up, down, charm, strange, top, and bottom. They are organized into three generations. Up and down quarks make up nearly all the quark content of ordinary stable matter. The heavier flavors are unstable and are produced in energetic processes before decaying into lighter particles.
Quarks have fractional electric charges. Up-type quarks carry +2/3 of the elementary electric charge, while down-type quarks carry -1/3.
What is color charge?
Quarks also carry a property called color charge. The name is unrelated to visible color; it is the charge associated with the strong interaction described by quantum chromodynamics. Gluons carry the strong force between quarks and themselves carry color information.
Composite particles observed in nature are color-neutral overall. A baryon such as a proton contains three quarks whose color charges combine to a neutral state. A meson contains a quark and an antiquark.
Why do we not see isolated quarks?
Quarks exhibit confinement. As quarks are pulled apart, the energy stored in the strong field does not simply fade away in the manner of some long-range forces. At sufficient energy, new quark-antiquark pairs can be produced, leading to new hadrons rather than a freely isolated quark.
High-energy collisions therefore produce sprays of hadrons called jets. By measuring the energy and direction of those particles, physicists can infer the behavior of the underlying quarks and gluons.
How were quarks discovered?
The quark model was developed to organize the growing number of hadrons discovered in particle experiments. Later scattering experiments revealed point-like constituents inside protons, and quantum chromodynamics provided the modern theory of the strong interaction. The top quark, the heaviest known quark, was discovered at Fermilab in 1995.
Quarks and other elementary particles
Quarks are one of the two broad families of matter particles in the Standard Model. The other family is the leptons, which includes electrons and neutrinos. The Standard Model also includes force-carrying bosons and the Higgs boson.
