Gluon
Gluons are fundamental particles that serve as the gauge bosons mediating the strong force, which binds quarks together within protons and neutrons. These elementary particles are integral to the structure of matter, as they help form hadrons, the family of particles that includes nucleons. Gluons possess a spin of 1 and unique properties that allow them to interact with each other, enabling the strong force to operate effectively at varying distances. Notably, they carry a property known as color charge, which distinguishes the different types of quarks and facilitates their interactions.
Gluons are not observed individually due to a phenomenon called asymptotic freedom, wherein the strong force strengthens at greater distances. This property leads to the creation of a quark-gluon plasma (QGP) during high-energy collisions, mimicking the early universe conditions seconds after the Big Bang. Ongoing research, such as the GlueX experiment, aims to explore the potential existence of glueballs—hypothetical particles made entirely of gluons. The study of gluons and their interactions not only enhances our understanding of particle physics but also provides insights into the foundational elements of the universe.
Gluon
FIELDS OF STUDY: Particle Physics
ABSTRACT: This article describes the characteristics, discovery, and significance of gluons. Gluons are subatomic particles that maintain the structure of atomic nuclei and possess unique qualities useful in particle physics research.
Principal Terms
- boson: a particle that has integral spin; usually carries force and acts as the "glue" that holds matter together.
- color charge: a property of quarks that distinguishes quarks and gluons from each other.
- hadron: a subatomic particle that is made of quarks and held together by the strong force.
- nucleon: a type of baryon, either a proton or a neutron, that is found in atom’s nucleus.
- quark: an elementary subatomic particle.
- spin: an intrinsic form of angular momentum carried by elementary particles, composite particles (hadrons), and atomic nuclei.
- strong force: the force that binds quarks together.
Holding It All Together
All matter encountered in daily life is composed of elementary particles. Elementary particles are particles that cannot be broken down into smaller constituent parts. There are two families of elementary particles: bosons, which carry force, and fermions, which carry matter. Fermions interact with each other by exchanging bosons. This exchange is what creates the four fundamental forces of nature: gravitation, electromagnetism, the weak force, and the strong force. Each of the four forces is carried by a gauge boson. Photons carry electromagnetism, and W and Z bosons carry the weak force. (The gauge boson of gravitation, called the graviton, is hypothesized but not yet proved to exist.) Gluons are the gauge bosons that mediate the strong force between quarks, which are the elementary particles that make up protons and neutrons in atomic nuclei.
The existence of quarks was first posited in 1964. Two American physicists, Murray Gell-Mann (b. 1929) and George Zweig (b. 1937), independently proposed them as particles that respond to the strong force. This implied that a gauge boson must exist to mediate the strong force between quarks, similar to how photons mediate electromagnetism between electrically charged particles. In 1968, British theoretical physicist Christopher Llewellyn Smith (b. 1942) found the first evidence for the existence of gluons. He noted that quarks account for only about half of a proton’s momentum and theorized that electrically neutral particles must account for the other half.
Gluons were directly observed for the first time in 1979 at the Positron-Electron Tandem Ring Accelerator (PETRA) in Hamburg, Germany. Since their discovery, gluons have been used as a tool to study other phenomena in particle physics. Scientists used them in the Large Hadron Collider (LHC) experiments that helped confirm the existence of the Higgs boson at the European Organization for Nuclear Research (CERN) in 2013.
What Makes a Gluon a Gluon?
All bosons have integral spin values (e.g., 0, 1, 2). Fermions have half-integral spin values (e.g., 1/2, 3/2, 5/2). Gluons have a spin of 1. Typically, particles with a spin of 1 have three polarization states. However, because gluons, like photons and unlike the W and Z bosons, are massless, they have only two polarization states: 1 and −1.
Similar to how protons and electrons possess electrical charge, quarks possess a quality called color charge. Just as electrically charged particles interact by exchanging photons, thus creating electromagnetism, color-charged particles (quarks) interact by exchanging gluons, thus creating the strong force. Individual quarks come in three "colors": red, green, and blue. (These qualities have nothing to do with the actual color of the particles, but are simply labels.) The antiparticles (antimatter particles) of quarks, called antiquarks, come in three "anti-colors": anti-red, anti-green, and anti-blue. Gluons themselves possess one color charge and one anti-color charge.
Gluons can interact with each other as well as with quarks. There are eight different types of gluons, each corresponding to a different combination of color and anti-color. The attraction between quarks of opposite color charge creates the strong force. Gluons and quarks combine to make all of the composite particles in the hadron family: mesons (one quark and one antiquark) and baryons (three quarks), the latter of which include the class of particles known as nucleons. The high-energy condition of many gluons interacting, known as a gluon condensate, could also be partly responsible for giving mass to some hadrons. Nucleons (protons and neutrons) are of particular importance, as they are the building blocks for most observable matter in the universe. The extreme strength of the strong force between gluons and quarks holds atomic nuclei together. This is why fission-powered nuclear weapons produce such huge amounts of energy: the fission, or breaking apart, of nuclei causes that force to be released.
Gluons’ Quirky Behavior
While they form the basis for most "normal" matter humans observe, gluons also have some unusual traits that scientists are exploiting to enhance humankind’s knowledge of the cosmos. Gluons and quarks have never been observed individually because of a quality of the strong force known as asymptotic freedom. Unlike the other forces, the strong force gets stronger as the distance between two quarks increases, making it difficult to pull them apart. Conversely, the strong force gets weaker as quarks are pressed more closely together. This process effectively frees the quarks to move individually. Asymptotic freedom could be responsible for a special condition known as quark-gluon plasma (QGP)—a nearly "perfect" or frictionless liquid. QGP has been created by high-speed particle collisions inside particle accelerators, which break protons and neutrons apart into gluons and quarks at 4–6 trillion degrees Celsius (7–10 trillion degrees Fahrenheit), about 100,000 times hotter than the center of the sun. The resulting hot "soup" is thought to resemble the state of the universe just fractions of a second after the big bang, about 13.8 billion years ago. Insights gained from studying QGP can help scientists learn more about how the universe’s initial conditions evolved to produce galaxies, stars, planets, and, ultimately, life.
Because gluons can carry color charge, it would be theoretically possible to have a particle composed entirely of gluons. Known as "glueballs," such particles are expected to consist of either two or three gluons, have mass, and decay quickly after synthesis into pairs of several possible particles: pions (pi mesons), kaons (K mesons), or eta mesons. The observation of these emitted particles would provide further evidence in support of the standard model of particle physics. Many previously detected particles are candidates for glueballs. They have been the subject of active research for nearly two decades. The GlueX experiment, which began taking data at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia, in 2014, is specifically designed to produce more definitive experimental evidence of glueballs. This will open up further avenues for the advancement of particle physics.
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