In part I, we discussed the fundamental forces in physics and the dual nature of particles and fields. We also talked about how the elementary particles of the Standard Model are divided into fermions and bosons. In part II, we went over the essential characteristics of the elementary fermion family, and today we cover the elementary bosons. As before, we’re chopping these up into very short mini-lessons in order to combat the problem of people being too busy to read something like this if we’d crammed it all into one long lesson.
Unlike fermions, which have ½ integer spin quantum numbers, bosons always have either zero or integer spins; they do not obey Pauli’s aforementioned exclusion principle; and their statistical distribution throughout space is described by different mathematical functions than the fermions (Young and Freedman 1521). As we mentioned in our previous installment, bosons obey Bose-Einstein statistics, whereas fermions obey Fermi-Dirac statistics, but an actual mathematical explanation of what those distributions entail is beyond the scope of this lesson. In addition to the Higgs boson, which we’ll reserve for the next installment, all the particles which mediate the interactions of the four forces are also classified as bosons (Griffiths 55). The strong interaction is described by a theory called quantum chromodynamics, and is mediated by particles called Gluons (Griffiths 55). There are eight different Gluons (Carroll 296). The weak interaction is mediated by three particles: the neutrally charged Z boson, a positively charged W+ boson, and a negatively charged W- boson (Carroll 296). The electromagnetic force is mediated by the photon, and gravity is mediated by the graviton (Griffiths 56). Although the W and Z bosons have mass, photons, gravitons and gluons are massless (Griffiths 301).
Two additional things are worth noting here. The first is that, unlike gluons, photons and gravitons, the W and Z bosons have mass. This fact was one of the considerations which triggered the development of the Higgs field theory. The Higgs boson, which was discovered experimentally at the LHC at CERN in 2012, is the evidence for the existence of the Higgs field. We’ll cover the Higgs in more detail in the installment after next, but for now just understand that the Higgs field imbues certain particles with mass.
The second is that the graviton is merely hypothetical at this point. It hasn’t been confirmed experimentally. To add insult to injury, our leading theory of gravitation, Einstein’s General Theory of Relativity, has not been successfully re-formulated to conform to the framework of quantum mechanics. In fact, one of the most highly publicized goals of many theoretical physicists is to construct a theory that subsumes all of the successes of both GR and QM in a single framework: a theory of quantum gravity.