Quantum Chromodynamics

FIELDS OF STUDY: Quantum Field Theory; Particle Physics; Quantum Physics

ABSTRACT: Quantum chromodynamics (QCD) is the quantum field theory underlying the strong interaction, one of the four fundamental forces in the standard model of particle physics. The strong interaction holds quarks together to form hadrons, and it also holds together protons and neutrons in atomic nuclei. This article will explain the principles of QCD and its context within particle physics.

principal terms

• color force: the force of the strong interaction that operates on the quark level.
• 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.
• gluon: the elementary particle that carries the strong interaction between quarks.
• hadron: a composite particle consisting of either three quarks or one quark and one antiquark, held together by the color force.
• quark: an elementary fermion that combines with other quarks to form a baryon, such as a proton or a neutron, or with an antiquark to form a particle called a meson.
• quantum field theory: a theory that explains interactions between subatomic particles as the result of a field extending between them.
• standard model: the generally accepted theory of particle physics that deals with the strong, weak, and electromagnetic interactions.
• strong interaction: the fundamental process of particle interaction that binds quarks into hadrons and hadrons into nuclei.

Fundamental Forces

All interactions between matter are governed by forces that act on specific properties universal to all matter. There are four fundamental interactions, or forces, in the standard model of physics: the strong interaction, the weak interaction, gravity, and electromagnetism. The strong interaction is what binds quarks together to form hadrons and holds together the hadrons—specifically, protons and neutrons—in atomic nuclei. It is the most powerful of the fundamental forces, able to overcome the electromagnetic repulsion of the positively charged protons.

Each of the four fundamental interactions is believed to operate according to a quantum field theory. A quantum field theory is a theory in which particles are treated not as individual entities but as excitations in an underlying physical field. Quantum chromodynamics (QCD) is the quantum field theory of the strong interaction. When the strong interaction operates on the level of quarks, it is also known as the color force. The force that holds protons and neutrons together is a residual effect of the color force and is often called the "residual strong force" or the "strong nuclear force."

"Color" in QCD has nothing to do with visual color. It simply refers to one of the main properties of quarks, along with spin and electric charge. The existence of this property, known as "color charge," was suggested in 1964 to explain how quarks with otherwise identical properties could coexist in the same hadron without violating the Pauli exclusion principle. (This principle states that no two fermions, of which quarks are one type, that occupy the same particle can have completely identical properties.) The color analogy was chosen because color charge has three possible values, just as the human eye can detect three primary colors of light. These colors—red, green, and blue—are also the names of the three color charges. An antiquark, or antimatter particle of a quark, has the anti-color charge of the quark to which it corresponds: anti-red, anti-green, or anti-blue.

The function of color charge in QCD is analogous to that of electric charge in quantum electrodynamics (QED), the quantum field theory of electromagnetism. In order to be affected by the strong interaction, a particle must have a color charge, just as a particle must have an electric charge to be affected by electromagnetism. The strong interaction is carried by massless particles known as gluons, so called because they "glue" quarks together. Gluons have color charge but no electric charge or mass, meaning that they are also affected by the very interaction that they carry. This is in contrast to photons, their QED counterparts, which carry electromagnetism but have no electric charge themselves.

The Standard Model

In order to understand the context of QCD, it is necessary to have a basic understanding of the standard model of particle physics.

Particles can be characterized in a number of different ways. An elementary particle is one that cannot be broken down further into component particles. The two classes of elementary particles are fermions, which consist of quarks and leptons, and bosons, which consist of gauge bosons and scalar bosons. The difference between the two classes is the rules that govern their behavior. Fermions obey a set of rules called Fermi-Dirac statistics, while bosons follow Bose-Einstein statistics. Gauge bosons are the particles that carry the fundamental interactions. They travel between the particles affected by those interactions, transmitting the energy that makes up their respective fields (color field, electromagnetic field, etc.). The gluon and the photon are both gauge bosons.

All other particles are composite particles, formed from some combination of elementary particles (or antiparticles). Composite particles can also be classified as either fermions or bosons. If a particle contains an odd number of fermions, it too is a fermion; if it contains an even number of fermions, it is a boson. For example, a hadron is any composite particle that consists of quarks held together by the color force. There are two types of hadrons: mesons and baryons. Mesons contain one quark and one antiquark, so they are bosons. Baryons contain three quarks, so they are fermions. Protons and neutrons are both types of baryons. The type of meson or baryon is determined by the types, or "flavors," of the quarks they contain.

Color Charge and the Residual Strong Force

Quarks and gluons are the only types of particles that have color charge. Even though hadrons are made of quarks, all hadrons must be "colorless"—that is, the colors of their constituent quarks must cancel one another out. In a meson, this means that the quark and the antiquark must be a color and its anti-color, such as red and anti-red. A baryon, on the other hand, must contain one quark of each color. The three colors combined neutralize each other, much like the combination of all visible colors of light appears white to the human eye.

While quarks have a single color charge and antiquarks have a single anti-color charge, gluons carry both a color charge and an anti-color charge. A gluon’s color charge does not necessarily have to correspond to its anti-color charge. Quarks within the same hadron are constantly exchanging gluons between them, which causes the quarks to change color. This is always done in such a way that the overall colorlessness of the hadron is maintained. For example, if a red quark emits a red/anti-blue gluon, its color will change to blue, while the blue quark will absorb the gluon and become red.

Although hadrons have zero color charge, the protons and neutrons in a nucleus are still held together by the strong interaction. This is where the residual strong force comes in. Some of the color force between the quarks in a hadron "leaks" out and affects the quarks in nearby hadrons. This residual force is much weaker than the color force and has a much shorter range. However, within the confines of an atomic nucleus, it is still stronger than the electromagnetic repulsion that would otherwise drive the protons apart.

Color Confinement

One particularly puzzling aspect of QCD is the apparent fact that particles with a color charge cannot be observed in isolation. There is no known way to separate a hadron into its constituent quarks. A proton and an electron can be pulled apart because the electromagnetic force between them decreases exponentially with distance. In contrast, the extremely strong color force between quarks remains constant regardless of distance. Any attempt to pull a quark from a meson, for example, will "stretch" the gluon field between the quark and the antiquark more and more, increasing the amount of energy it contains. Eventually the field will "snap" and convert the excess energy into a new quark and antiquark. These particles will immediately combine with the two that have been separated, producing two new mesons to replace the original.

This phenomenon is known as "color confinement" because quarks must always be confined within hadrons. While not fully understood, it is believed to be the result of gluons having color charge as well as carrying the color force. Color confinement is the reason why no free quarks have ever been found.

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