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Principles of Physics

Quarks

by Marianne Moss Madsen, MS

Fields of Study

Quantum Electrodynamics; Quantum Field Theory; Quantum Mechanics; Relativity; String theory; Superstring theory

Summary

A quark is a fundamental particle. It has an anti-particle called an antiquark. Quarks join together to form hadrons, such as protons and neutrons. Quarks are not independent; they are always in combination with other quarks. Because they are never alone, quarks have never actually been seen, but their behavior has been observed. There are six types, or “flavors,” of quarks: Up, down, strange, charm, bottom, and top.

Principal Terms

  • color charge: a property of quarks that distinguishes quarks and gluons from each other.

  • color force: the force of the strong interaction that operates on the quark level.

  • elementary particle: one of the fundamental constituents of matter.

  • fermion: one of two main classes of particles, characterized by adherence to Fermi-Dirac statistics; includes quarks, leptons, and any particles that contain an odd number of quarks or leptons, such as protons and neutrons.

  • hadron: a subatomic particle that is made of either three quarks or one quark and one antiquark and held together by the strong force.

  • quantum chromodynamics: a quantum field theory that describes the interactions of quarks and gluons, subatomic particles that are responsible for the strong interaction.

  • spin: an intrinsic form of angular momentum carried by elementary particles, composite particles (hadrons), and atomic nuclei.

  • strong force: the force that holds quarks together.

  • strong interaction: the fundamental process of particle interaction that binds quarks into hadrons and hadrons into nuclei.

Discovery of Quarks

In 1964, Murray Gell-Mann and George Zweig postulated that the hundreds of particles known at that time, that were thought to be the foundation of the universe, could all be made from combinations of only three elementary particles, fundamentals that would form the basis of everything and were so small that they couldn’t be broken up into anything smaller. They chose the word “quark” based on a nonsense word from James Joyce’s novel, Finnegan’s Wake (“Three quarks for Muster Mark!”). To make the math regarding the electrical charges that they were observing work correctly, they assigned fractional electrical charges, which was a new concept at the time, to the quarks. For quite some time, quarks were thought to be just an imaginary way to explain something that had no other explanation. Eventually, experiments proved that quarks do exist, and that there are at least six types of quarks. The last to be discovered was the top quark in 1995.

Quarks were first observed at the Stanford Linear Accelerator National Laboratory in 1968. There, scientists used particle accelerators to smash subatomic particles into small bits by accelerating them to incredible speeds, then smashing the particles into a target material. These high-speed crashes used so much energy that they made the protons and neutrons inside the atoms break apart. Scientists could then observe particle “tracks” that scattered in just the way they had predicted that quarks would behave. Scientists believe that one can’t see a quark in isolation because of the color force holding them together so tightly. If one uses enough energy to separate quarks from each other, they form quark/antiquark pairs before they are far enough apart to be seen as a separate entity.

Types and Properties of Quarks

The study of quarks is called quantum chromodynamics. Quarks are only one of the types of the particles which make up matter. They belong to a class of particles called fermions. There are six quarks, which are known by their “flavor,” but since they are always paired, scientists usually talk about them in three groups: Up/down, charm/strange, and top/bottom. Most matter is made of protons and neutrons, which are made from quarks. For each quark, there is also an anti-quark.

Hadrons are composite particles made of quarks. The two types of hadrons are as follows:

  • Baryons: a hadron made of three quarks, such as protons (made of two up quarks and one down quark) and neutrons (made up of one up quark and two down quarks);

  • Mesons: a hadron made of one quark and one antiquark, such as a pion (made of an up quark and a down antiquark, a very unstable combination).

In 2013, scientists claimed to have found a new type of particle: One made of four quarks. Now there are even claims of a five-quark particle, which, through further testing, appear to be false. None of these particles fit the theory of the standard model of physics, which is the framework that physicists use to describe all the currently known elementary particles, at least in theory.

Quarks have mass, but because they are never observed alone, it is impossible to isolate them and measure the mass of each flavor of quark in a direct manner. The mass of quarks is implied from the scattering experiments described above.

Quarks have a fractional electric charge, which is unusual. For example, an up quark has a charge of +2/3 and a down quark has a charge of -1/3. Therefore, a proton, which is made of two up quarks (+2/3) + (+2/3) and a down quark (-1/3) has a positive electrical charge of +1. Quarks also have spin. Their spin is ½, which means they are fermions, as anything with a spin of 1 is a boson.

Quarks have color charge, though one should not confuse this naming convention with the traditional understanding of color. This is just a way to explain how quarks can behave according to a well-known principle stating that no two identical objects can occupy the same place (the Pauli Exclusion Principle). Quarks can be red, blue, or green, and antiquarks can be anti-red, anti-blue, or anti-green. Quarks making up the same hadron must have different colors; for example, all three quarks in a baryon are different colors, and a meson contains a colored quark and its corresponding anti-colored quark. This means that a baryon, with all three colors, is color neutral, just the way that if red, green, and blue light are emitted together, the light is white. Mesons are color neutral because they contain one color and its corresponding anticolor. According to the standard model, quarks can only combine in certain ways based on their charge and color.

The strong force holds quarks together with the strong interaction. The electrical charges of the quarks work along with gluons (the particles that carry the strong force) to keep the hadrons such as protons and neutrons, and, therefore, the entire universe, together. Gluons also have color charge, but composite particles made of quarks and gluons are color neutral. When two quarks get close enough, they exchange gluons, which binds them together with a very strong color force field. Surprisingly, as quarks get further apart, the bond becomes stronger and stronger, and as they move closer together, the bond becomes weaker. Quarks are constantly changing their color charges as they continuously exchange gluons with other quarks.

Why Are Quarks Important?

Quarks are one of the basic particles that make up our universe. By understanding them and the forces that hold them together and the other properties that they exhibit, we can have a better understanding of our universe as a whole. Scientist do not know if there is anything even smaller than a quark, but the search continues.

Citation Types

MLA 9th
Madsen, Marianne Moss. "Quarks." Principles of Physics, edited by Donald R. Franceschetti, Salem Press, 2016. Salem Online, online.salempress.com/articleDetails.do?articleName=POP_0100.
APA 7th
Madsen, M. M. (2016). Quarks. In D. R. Franceschetti (Ed.), Principles of Physics. Salem Press. online.salempress.com.
CMOS 17th
Madsen, Marianne Moss. "Quarks." Edited by Donald R. Franceschetti. Principles of Physics. Hackensack: Salem Press, 2016. Accessed September 20, 2026. online.salempress.com.