Quark

Quarks are elementary particles that serve as the basic building blocks of matter. They come together to form composite particles known as hadrons, with the most familiar examples being protons and neutrons—the particles that make up atomic nuclei. The ordinary matter we encounter every day consists entirely of up quarks, down quarks, and electrons. Due to a property called color confinement, quarks are never observed alone; they exist only inside hadrons (such as baryons like protons and neutrons, or mesons) or in a state of matter called a quark–gluon plasma.

Quarks possess several intrinsic traits, including electric charge, mass, color charge, and spin. In the Standard Model of particle physics, they are the only elementary particles that take part in all four fundamental forces—electromagnetism, gravity, the strong nuclear force, and the weak nuclear force. They are also the only known particles whose electric charges are not whole-number multiples of the elementary charge.

There are six distinct types, or flavors, of quarks: up, down, charm, strange, top, and bottom. The up and down quarks have the smallest masses. Heavier quarks quickly transform into up and down quarks through particle decay, moving from a higher mass state to a lower one. As a result, up and down quarks are stable and the most abundant in the universe, while strange, charm, bottom, and top quarks appear only in high-energy collisions, such as those from cosmic rays or particle accelerators. Each quark flavor has a corresponding antiparticle, called an antiquark, which is identical except that certain properties—like electric charge—have the same magnitude but opposite sign.

The quark model was proposed independently by Murray Gell-Mann and George Zweig in 1964. Initially, quarks were introduced as a way to organize hadrons, and there was little direct evidence for their physical reality until deep inelastic scattering experiments at the Stanford Linear Accelerator Center in 1968. Subsequent accelerator experiments confirmed all six flavors. The top quark, discovered at Fermilab in 1995, was the last to be found.

In the Standard Model, quarks are classified as spin-½ particles, meaning they are fermions subject to the Pauli exclusion principle—no two identical fermions can share the same quantum state. This sets them apart from bosons, which have integer spin and can occupy the same state. Unlike leptons, quarks carry color charge, which allows them to participate in the strong interaction. This force attracts different quarks to one another, forming hadrons.

Hadrons are built from valence quarks, which determine their quantum numbers, but they can also contain an indefinite number of virtual "sea" quarks, antiquarks, and gluons that do not affect these numbers. Hadrons come in two families: baryons, made of three valence quarks, and mesons, made of a valence quark and an antiquark. The most common baryons are protons and neutrons, the core components of atomic nuclei. Many other hadrons exist, distinguished by their quark content and the properties those quarks confer. Exotic hadrons with more valence quarks, such as tetraquarks (four quarks) and pentaquarks (five quarks), were predicted from the start of the quark model but were not observed until the early 21st century.

Elementary fermions are arranged into three generations, each containing two leptons and two quarks. The first generation includes up and down quarks, the second includes strange and charm quarks, and the third includes bottom and top quarks. Searches for a fourth generation have failed, and strong indirect evidence suggests only three generations exist. Particles in higher generations are generally heavier and less stable, decaying into lower-generation particles via the weak interaction. Only first-generation quarks (up and down) are common in nature. Heavier quarks are created only in high-energy collisions, such as those involving cosmic rays, and decay rapidly; however, they are thought to have been present in the first moments after the Big Bang, during the quark epoch. Studies of these heavier quarks are conducted in artificial conditions, like those in particle accelerators.

Quarks are the only known elementary particles that engage in all four fundamental interactions—electromagnetism, gravity, the strong force, and the weak force. Gravity is too weak to affect individual particle interactions except at extreme energies and distances, and because there is no successful quantum theory of gravity, it is not included in the Standard Model.