Gauge boson

• 1. The Force Carriers Defined

  ›

  In particle physics, a gauge boson is a bosonic elementary particle that acts as the force carrier for elementary fermions. Elementary particles whose interactions are described by a gauge theory interact with each other by the exchange of gauge bosons, usually as virtual particles. Photons, W and Z bosons, and gluons are gauge bosons. All known gauge bosons have a spin of 1 and therefore are vector bosons. For comparison, the Higgs boson has spin zero and the hypothetical graviton has a spin of 2. Gauge bosons are different from the other kinds of bosons. First, fundamental scalar bosons like the Higgs boson exist. Second, mesons are composite bosons made of quarks. Third, larger composite non-force-carrying bosons include certain atoms.

  12

• 2. Four Forces In Action

  ›

  The Standard Model of particle physics recognizes four kinds of gauge bosons. Photons carry the electromagnetic interaction. W and Z bosons carry the weak interaction. Gluons carry the strong interaction. Isolated gluons do not occur because they are color-charged and subject to color confinement. The photon mediates forces between electrically charged particles across vast distances. The W and Z bosons operate only over subatomic ranges due to their mass. Gluons bind quarks together inside protons and neutrons. This binding force prevents individual quarks from ever being observed in isolation. The short range of the weak and strong interactions contrasts sharply with the infinite reach of electromagnetism.

  34

• 3. Mass From Broken Symmetry

  ›

  Gauge invariance requires that gauge bosons are described mathematically by field equations for massless particles. Otherwise, the mass terms add non-zero additional terms to the Lagrangian under gauge transformations, violating gauge symmetry. Therefore, at a naïve theoretical level, all gauge bosons are required to be massless. The conflict between this idea and experimental evidence that the weak and strong interactions have a very short range requires further theoretical insight. According to the Standard Model, the W and Z bosons gain mass via the Higgs mechanism. In the Higgs mechanism, the four gauge bosons of SU(2)×U(1) symmetry couple to a Higgs field. This field undergoes spontaneous symmetry breaking due to the shape of its interaction potential. As a result, the universe is permeated by a non-zero Higgs vacuum expectation value. This VEV couples to three of the electroweak gauge bosons, giving them mass. The remaining gauge boson remains massless as the photon. This theory also predicts the existence of a scalar Higgs boson, which has been observed in experiments at the LHC.

  34

• 4. Mathematics Of Multiplicity

  ›

  In a quantized gauge theory, gauge bosons are quanta of the gauge fields. Consequently, there are as many gauge bosons as there are generators of the gauge field. In quantum electrodynamics, the gauge group is U(1). In this simple case, there is only one gauge boson, the photon. In quantum chromodynamics, the more complicated group SU(3) has eight generators, corresponding to the eight gluons. The three W and Z bosons correspond roughly to the three generators of SU(2) in electroweak theory. The number of force carriers depends directly on the mathematical structure of the underlying symmetry group. A simpler group yields fewer particles. A complex group generates multiple distinct bosons to mediate different aspects of the same fundamental force.

  12

• 5. The Confinement Problem

  ›

  Isolated gluons do not occur because they are color-charged and subject to color confinement. Gluons carry a property called color charge that distinguishes them from photons. Photons have no electric charge and can travel freely through space. Gluons interact with each other because they possess color charge themselves. This self-interaction creates a binding energy that increases with distance. Attempting to separate two quarks requires so much energy that new quark-antiquark pairs form instead. The result is that free gluons never exist outside of hadrons. They remain trapped within composite particles like protons and neutrons. This phenomenon explains why we observe bound states rather than isolated strong force carriers.

  13

• 6. Grand Unification Predictions

  ›

  The Georgi, Glashow model predicts additional gauge bosons named X and Y bosons. The hypothetical X and Y bosons mediate interactions between quarks and leptons, hence violating conservation of baryon number and causing proton decay. Such bosons would be even more massive than W and Z bosons due to symmetry breaking. Analysis of data collected from such sources as the Super-Kamiokande neutrino detector has yielded no evidence of X and Y bosons. These theoretical particles represent a bridge between known forces at extremely high energies. Their existence remains unconfirmed despite decades of searching in deep underground detectors. Proton decay has never been observed, placing strict limits on the mass scale of these potential bosons.

  5

• 7. Gravity And The Missing Particle

  ›

  The fourth fundamental interaction, gravity, may also be carried by a boson, called the graviton. In the absence of experimental evidence and a mathematically coherent theory of quantum gravity, it is unknown whether this would be a gauge boson or not. The role of gauge invariance in general relativity is played by a similar symmetry: diffeomorphism invariance. Unlike photons or gluons, the graviton has not been detected directly. Its spin is predicted to be 2 based on theoretical models. Scientists continue searching for indirect signatures of gravitational waves that might reveal its properties. Current technology cannot yet probe the energy scales where individual gravitons would become detectable.

  13