The Standard Model

Table of the elementary particles The standard model of particle physics accounts for everything we've seen in atom-smashing experiments.

The electron

Hopefully you've heard of electrons. They're the particles that, in most normal situations, carry electric charge as it moves around to do the many things that electronic equipment gets up to. They're the light-weight part of atoms that can break free of the rest of the atom and thereby make chemistry complex enough that anything interesting can happen in the world. To the best of our present knowledge, they're actually elementary – that is, they aren't made up of some combination of other things, they're irreducible. We'll never split the electron, as we did the atomic nucleus a few decades ago. The electron is usually denoted by the letter e, also used to denote the (positive) magnitude of the (negative) electric charge it carries.

Like each of the particles I'll be discussing below, the electron has an associated anti-particle. Because it was the first such to be discovered, the electron's anti-particle has its own name, positron.


More usually, the anti-particle of an x (for any x) is simply named anti-x. An anti-x and an x can always react to annihilate one another, releasing the whole of their mass as energy – typically at least including some photons (i.e. a flash of light, albeit possibly in the form of gamma rays) but it can be in any form at all, including pairs of a particle and its anti-particle, which typically get lots of kinetic energy (i.e. they fly apart fast). For any particle x, the anti-x's anti-particle is the x, so anti-anti-x is x; for example, the anti-electron is the positron and the anti-positron is the electron.

A particle and its anti-particle necessarily have equal mass. If a particle has charge, its anti-particle has the same amount of charge but of the opposite sign; this applies not only to the electric charge with which I can hope you're familiar but also to the charge associated with each of the other forces I'll be mentioning later. This gives rise to a common convention for distinguishing a particle from its anti-particle; if they carry electric charge, giving each a superscript indicating the charge distinguishes them. Thus, for example, the electron is e and the positron, its anti-particle, is e+. In other cases, principally when the particle has no charge, a bar is written over the top of the symbol for a particle, to denote the anti-particle.

Because a particle and its anti-particle are so very alike, it's often not interesting to care about the distinction: anything a particle can do in some context, its anti-particle can do in a suitably anti-ed context – each of the other particles involved has to be replaced by its anti-particle, naturally; any fields (e.g. electric and magnetic, but not gravitational) involved probably need reversed; and you might have to reflect things in a mirror to make it all work out right. So I tend not to remember (hence bother to make) the distinction between a particle and its anti.


The electron is one of three charged leptons – the other two are called the muon and the tau. As the electron is denoted by e, the muon is denoted by μ and the tau by τ. Each of these has the same charge as the electron, but they are more massive. Associated with each charged lepton is an uncharged lepton, called a neutrino, denoted ν with a subscript indicating the associated charged lepton. It's possible neutrinos have no mass; personally, I suspect they do have mass, but the evidence we presently have indicates that their mass, if they have any, is tiny even when compared to that of the electron. The charged leptons and their associated neutrinos are all elementary: like the electron, they can't be subdivided.

The combination of a charged lepton and its associated neutrino, or the anti-particle of this neutrino (I can never remember which), can transform into the corresponding combination for one of the other lepton pairs (charged and neutrino). More usually, a muon or tau can emit its associated anti-neutrino (or neutrino, if my preceding sentence needed the anti-particle) to decay into an electron and its associated neutrino (or anti, as before). Because the muon and tau are much more massive than the electron (and the neutrino mass is tiny compared even to the electron), such decays release a lot of energy.


I mentioned that electrons are the light-weight (well, strictly, low-mass) parts of atoms; the bulk of the mass of an atom is in its nucleus. The nuclei of atoms of normal matter can usually be well-described in terms of protons and neutrons. The proton, denoted p, has positive charge equal in amount to the charge on the electron (so it's p+ and the anti-proton is p); the neutron, denoted n, has no charge (so the anti-neutron is denoted by an n with a bar over it). The proton and neutron have nearly equal masses; the difference is about one part in 727; as multiples of the electron's mass, the neutron's mass is 1838.66 and the proton's is 1836.1, to the limits of precision known to me. However, the proton and neutron are not elementary, like the electron: they are made up of quarks, which are (to the best of our knowledge) elementary.

Specifically, the proton and neutron are made up of up and down quarks, two of one and one of the other, in each case. The down quark has charge one third that of the electron (which is negative); the up quark has charge two thirds that of the proton (so minus two times the charge on the down). The neutron is thus made of one up and two down; the proton of two up and one down. Just as the electron has analogous μ and τ, the up (denoted u) and down (denoted d) quarks have analogues: corresponding to μ, we get charm (denoted c) and strange (denoted s) quarks; corresponding to τ, we get top (sometimes called truth, denoted t) and bottom (sometimes called beauty, denoted b). Like up, charm and top carry two thirds of the charge on a proton; like down, strange and bottom carry one third of the charge on an electron.


These three families, each comprising a pair of leptons and a pair of quarks, form the matter part of the bestiary of elementary particles. This bestiary is usually presented as a table with four rows – two for the two kinds of lepton and two for the two kinds of quark – and three columns. All of these elementary particles are fermions; no two, of a given type, can ever be in exactly the same state. The interactions between these particles are mediated by four more kinds of particle, all of which are bosons, of which arbitrarily many can be in any given state (which is how a laser works).

Electrical and magnetic interactions are mediated by massless chargeless particles of light known as photons and denoted γ (because, in nuclear physics, the photons one usually encounters are gamma rays). Iteractions between quarks, the strong nuclear force, are mediated by gluons (so-called because they glue nuclei together, overcoming the strong electric repulsion between the protons) of eight kinds. These interact with a sort of strong charge carried by quarks, known as colour (it has nothing to do with the phenomenon of that name in the real world), which has three variants that add up to neutral (unlike electric charge, which has two). This complicates things more than I'm willing to go into here; but suffice it to say that the resulting quantum chromodynamics (QCD) yields eight gluons which collectively embrace one another's anti-particles.

The reactions I sketched above, that turn a charged lepton and a neutrino into a different charged lepton and neutrino, have analogues for column-changing (in the standard diagram) reactions involving neutrinos and quarks; these, together with row-changing reactions, are mediated by a charged boson called W, whose charge has the same magnitude as that on the electron. Finally, we have the chargeless Z boson, which mediates purely mechanical interactions (very similar to those mediated by photons, except that the particles involved don't need electric charge) among the assorted particles. These four kinds of boson are commonly included as a fourth column alongside the above-mentioned table of leptons and quarks; however, do not let this mislead you into thinking each of them corresponds to one row of that table ! The Z and γ are their own anti-particles; W and its anti-particle have opposite charge; with the eight gluon varieties we have twelve bosons in all.


Collectively, the twelve elementary fermions, their anti-particles and the twelve bosons (of four kinds) constitute the standard model. Given the masses of these particles, the standard model explains all of particle physics. Its one failing is that it can't account for the masses: it must be told those by some other means.

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