Quarks and Leptons Elementary Particles in the Standard Model

The Standard Model (SM) of particle physics describes the elementary particles which make up the universe, as well as the interactions between them. Aspects of the standard model have been tested to incredible precision – for example, quantum electrodynamics (QED) is the most precisely tested theory in physics, to date – and while the theory is not complete, it provides a framework for the understanding of a great many particle interactions and physical phenomena. The standard model describes three kinds of elementary particle; quarks, leptons and gauge bosons. We’ll start with quarks.

Quarks are elementary particles (i.e. particles which are not composed of other, smaller particles, but are believed to be a single, indivisible entity) which possess electric charge and one of six quark flavours. The flavour is just a quantum mechanical marker that determines the “kind” of quark. Quark flavours are given the names up (u), down (d), charm (c), strange (s), top (t) and bottom (b) (or beauty). They form three generations:

( u, d ) ( c, s ) ( t, b )

The first generation, consisting of the up and down quark, are the quarks found in the nuclei of the atoms making up the universe around us, while the other two generations are simply heavier (more massive) versions of the up and down quark. The up quark has electric charge of +2/3 and the down has charge -1/3. The charm and top quarks have the same charge as the up quark, and similarly with the down, strange and bottom quarks.

Each quark also carries a quantity known as colour. The colour of a quark is not related to any kind of visible colouration, but to the so-called strong interaction, described by quantum chromodynamics (QCD). QCD dictates that free particles (those not part of a bound state) should be colourless. This mean that quarks cannot exist in isolation, but must be combined together to make colourless combinations. Quarks can possess one of three colours (commonly referred to as red, green and blue).

Quarks can be combined in one of two ways; as bound states of three quarks, where they make particles known as baryons, or as bound states of quark and anti-quark, where they make particles known as mesons.

Baryons include the proton, composed of two up quarks and one down quark, and the neutron, composed of two down quarks and one up. Adding together the charges on each quark provide the well-known properties that the proton has electric charge +1, and the neutron has zero charge. Baryons make colourless combinations of quarks by binding together one red quark, one green quark, and one blue quark. The result is an overall “white” or colourless state. It doesn’t matter which quark flavour carries which colour; in fact the nature of the strong interaction means that the colour of a quark is constantly changing as it interacts with the quarks around it!

Mesons are more unusual; they consist of a quark and an anti-quark, which is just the same as a quark but with opposite colour and opposite electric charge (in fact, all of the internal quantum numbers have the opposite sign, but that is not important here). Mesons can be made from quark and anti-quark of different flavour, but the colours must be exactly opposite, for example a red quark and an anti-red quark, or a green and anti-green.

The second set of elementary particles are called leptons. These alo exist in three generations, but here one partner in each generation ha no charge, and a very tiny mass (compared to the masses of the other elementary particles). The charged leptons are the electron, muon and tau, while the neutral leptons are called neutrinos. There is one neutrino flavour for each charged lepton, so you get the electron neutrino, the muon neutrino and the tau neutrino. Of course, each one of these has an anti-particle with the opposite properties too!

The leptons, then, look like this:

( e, v_e ) ( mu, v_mu ) ( tau, v_tau )

where the v_ variety represents the neutrino (usually denoted by the Greek letter nu).

Unlike the quarks, leptons do not combine together to form composite particles.

Finally, the standard model prescribes a number of gauge bosons. These particles are responsible for transmitting (mediating) the interactions between quarks and leptons. There are four fundamental interactions:

– Strong, described by quantum chromodynamics
– Electromagnetic, described by quantum electrodynamics – Weak, described by electroweak theory – Gravitational, not yet a part of the standard model

The gravitational interaction is problematic as its strength is much weaker than the other three (even though everyday experience tells us that gravity is quite strong). For the time being, most particle physicists ignore gravity since it doesn’t come into play at the energy-scales involved. It is thought that the gravitational interaction may be mediated by particles called gravitons.

The strong interaction occurs only between quarks, and is mediated by particles called gluons. There are eight different kinds of gluon, each one carrying a different combination of colours. This is why quarks continually change colour when they are in bound states; when a quark interacts with another quark by emitting a gluon, the gluon carries away one unit of the colour the first particle had, plus one unit of an “anti-colour”. This gives the quark the colour corresponding to the anti-colour. Eight different quarks are needed to represent all of the colour plus anti-colour combinations.

The electromagnetic interaction occurs between any particles which carry electric charge. The mediator is the photon, and the electromagnetic interaction is responsible for most of the forces in everyday life, including friction, tension and compressive forces when we push or pull materials against each other.

The weak interaction is special; it allows quarks and leptons to change flavour, giving rise to a lot of interesting physics. It is a short-range interaction mediated by one of three particles, two charged (known as the W+ and the W-) and one neutral (known as the Z0).

There is a lot more to the standard model of particle physics than a description of the elementary particles, provided above. It is a fascinating field of ongoing research, and is under continual improvement and evolution. However, the standard model is not perfect, and there are several proposed extensions, many of which introduce new elementary particles such as the now-famous Higgs boson.

To summarise, the elementary particles consist of six quarks (up, down, charm, strange, top, bottom), three charged leptons (electron, muon, tau), three neutrinos (electron neutrino, muon neutrino, tau neutrino) and a number of gauge bosons (eight gluons, the photon, two W bosons, one Z boson, and possibly gravitons). Extensions include the Higgs boson which is thought to be responsible for giving mass to the other particles.

Of course, each of the particles here has an anti-particle twin, with all internal quantum numbers switching sign; this means, for example, that the electron has a positively-charged anti-particle called the positron.