a gauge boson
/ gayj BOH-son /
Ask why particles push and pull on each other at all, and the modern answer is startling: force is the price of a symmetry. Demand that you be free to redefine the phase of a quantum field independently at every point in spacetime, and consistency forces you to introduce a new field whose quanta are the gauge bosons, the particles that carry the forces. The photon that lights this page, the gluons that glue nuclei together, the W and Z that drive radioactive decay: all are gauge bosons, messengers demanded by the mathematics of local symmetry.
A gauge boson is a spin-1 (vector) particle that mediates a fundamental interaction and arises from the requirement of local gauge invariance. The kind of symmetry group fixes how many there are and how they behave: the U(1) of electromagnetism gives one photon; the SU(2) of the weak interaction gives three (W+, W-, Z); the SU(3) of the strong interaction gives eight gluons. Because gauge symmetry in its unbroken form forbids a mass term, gauge bosons want to be massless, and the photon and gluons are. The W and Z are massive only because the Higgs field spontaneously breaks the electroweak symmetry, giving them mass while leaving the photon untouched.
A key distinction is whether the gauge group is abelian (its transformations commute) or non-abelian. The photon's U(1) is abelian, so photons carry no charge and do not interact with each other directly. The gluon's SU(3) and the weak SU(2) are non-abelian, so those gauge bosons carry the very charge they mediate, gluons carry color, the W carries weak charge, and they interact among themselves. That self-interaction is the mathematical root of why the strong force confines quarks, a feature electromagnetism entirely lacks.
Two electrons repel not by touching but by exchanging a photon, the gauge boson of electromagnetism; drawn as a Feynman diagram, one electron emits a photon and the other absorbs it, and the recoil is the force.
In gauge theory a force is literally the exchange of its gauge boson between the interacting particles.
The gauge bosons of an unbroken symmetry must be massless, so the mere fact that the W and Z are heavy is direct evidence that the electroweak symmetry is broken, the problem the Higgs mechanism was invented to solve.