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Gauge boson

physical science Maturity 9-11 Next article →

In particle physics, a gauge boson is an elementary particle that acts as a force carrier. These particles allow elementary fermions to interact with one another. They do this through a process called the exchange of gauge bosons. Often, these messengers act as virtual particles during these interactions.

Standard Model of Elementary Particles
Standard Model of Elementary Particles

All known gauge bosons possess a spin of 1. Because of this specific property, scientists also call them vector bosons. This distinguishes them from other types of bosons found in nature.

There are several ways to categorize different kinds of bosons. First, there are fundamental scalar bosons, such as the Higgs boson. The Higgs boson is unique because it has a spin of zero. Second, there are mesons, which are composite bosons made of quarks. Third, there are larger composite bosons that do not carry forces. An example of these larger particles would be certain types of atoms. Gauge bosons are distinct because they are the primary messengers of fundamental forces.

The Standard Model recognizes four specific types of gauge bosons. Photons are the gauge bosons that carry the electromagnetic interaction. W and Z bosons are responsible for carrying the weak interaction. Finally, gluons are the particles that carry the strong interaction.

Standard Model of Elementary Particles
Standard Model of Elementary Particles

It is important to note that isolated gluons do not occur in nature. This is because they are color-charged and subject to a process called color confinement.

In a quantized gauge theory, the number of gauge bosons is determined by the gauge field. Specifically, there are as many gauge bosons as there are generators of that field. For example, quantum electrodynamics uses the U(1) gauge group. In this simple case, there is only one gauge boson, which is the photon. Quantum chromodynamics is more complex and uses the SU(3) group. This group has eight generators, which correspond to the eight different gluons.

Theoretical math suggests that all gauge bosons should be massless. This is because gauge invariance requires field equations for massless particles. If they had mass, it would violate gauge symmetry in the Lagrangian. However, experimental evidence shows that the weak and strong interactions have a short range. This creates a conflict with the idea of massless, long-ranged forces. The Standard Model resolves this through the Higgs mechanism.

Standard Model of Elementary Particles
Standard Model of Elementary Particles

The Higgs mechanism explains how W and Z bosons gain mass. In this process, four gauge bosons from the electroweak interaction couple to a Higgs field. This field undergoes spontaneous symmetry breaking due to its interaction potential. This results in a non-zero Higgs vacuum expectation value, or VEV, throughout the universe. This VEV couples to the W, W-, and Z bosons to give them mass. The photon remains massless because it does not couple to the VEV in the same way.

Scientists also look for messengers beyond the current Standard Model. The Georgi–Glashow model predicts additional particles called X and Y bosons. These hypothetical bosons would mediate interactions between quarks and leptons. Such interactions would cause proton decay and violate the conservation of baryon number. These particles would be even more massive than W and Z bosons. So far, data from the Super-Kamiokande neutrino detector has shown no evidence of them.

There is also the question of gravity and its messenger. Gravity is the fourth fundamental interaction in our universe. Some theories suggest it may be carried by a boson called the graviton. However, there is currently no experimental evidence for the graviton. Scientists also lack a mathematically coherent theory of quantum gravity. Because of this, it is unknown if the graviton would truly be a gauge boson. In general relativity, the role of gauge invariance is played by diffeomorphism invariance.

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File:Standard Model of Elementary Particles.svg
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