The Standard Model

In the classification of particles in the last chapter, one abundant particle, photon—the particle of light—was conspicuously missing. Photon is neither a hadron nor a lepton; it belongs to a third class of particles called gauge bosons, the byproducts of gauge theories. According to quantum field theory, fundamental particles exert forces on one another by exchanging gauge bosons. The process of repulsion of two electrons, for example, is described as the exchange of photons between the two electrons.