Particle physicists treat all matter as being composed of elementary particles; the most complicated (and most whimsically-named) of these are the six quarks. The two lightest quarks make up most of the ordinary matter in the universe, while the other four combine to make exotic, short lived particles only found using particle accelerators and in the earliest instants following the Big Bang. Despite their ubiquity, we never see quarks on their own, or 'bare', rather, they appear in composite particles called 'hadrons'. The most common hadrons are the proton and the neutron, which together compose the nuclei of atoms.

The History of the Quark

Early in the development of subatomic physics, during the 1930s, the world of elementary particles was a simple one. We had the proton, the electron, and the neutron, and these three particles made up everything we see, both in our ordinary lives and in the laboratory. The addition of antiparticles by Dirac was a relatively minor complication, and the addition of the neutrino by Pauli seemed to have no practical benefit outside of nuclear physics. Alas, this simple picture of the universe was not to last.

The 1940's saw the discovery of the first meson, predicted by Yukawa to bind the nucleus together. Or rather, the first two mesons: the pi meson or pion, which is the meson that Yukawa expected, and the mu meson or muon, which turned out to have nothing to do with the Yukawa meson and was instead a particle just like the electron, but 200 times more massive. Nobody expected to find heavy electrons; Rabi famously asked, "Who ordered that?". Soon, dozens of unstable, apparently 'elementary', particles were being discovered, mostly using with the burgeoning new technology of the particle accelerator.

This bewildering array of particles was a source of frustration to many physicists, as there initially seemed to be very little pattern to these new particles. Many, remembering the analogous case of the chemical elements and the periodic table, hoped that a system of classification would eventually present itself and lead, like the periodic table, to an explanation of the apparent complexity of the universe in terms of a much simpler set of objects.

Murray Gell-Mann finally developed a classification system in the early 1960s. By separating the known particles into groups by their intrinsic spin, and then ordering them into a two-dimensional grid by two quantum numbers, 'isospin' and 'strangeness', he found that each group (or 'multiplet') had a geometrical form predicted by the mathematics of group theory. While this is much less straightforward than the periodic table, which fell into nice columns using only the mass, it is built on powerful mathematics that were useful for making predictions. In particular, as with the periodic table a century before, there were conspicuous gaps whose notional occupants had well-defined properties. The simplest of these theorized new particles was named the 'omega-minus' by Gell-Mann, and the discovery of this particle in the laboratory was a decisive proof of the validity of Gell-Mann's "Eightfold Way", winning him the 1969 Nobel Prize in Physics.

The mathematical representation used by Gell-Mann in the Eightfold Way had a curious property; the most fundamental multiplet consisted of three particles, none of which had ever been observed. This multiplet was fundamental in the sense that all the particles in the larger multiplets could be 'assembled' from combinations of these three particles and their antimatter counterparts. Gell-Mann hypothesized that this process of assembly was the true origin of the particles described by the Eighfold Way, and named the three fundamental particles 'quarks', after a passage from James Joyce's novel Finnegans Wake:

Three quarks for Muster Mark!
Sure he hasn't got much of a bark
And sure any he has it's all a beside the mark.

But O, Wreneagle Almighty, wouldn't un be sky of a lark
To see that old buzzard whooping about for uns shirt in the dark
Or he huntin' round uns speckled trousers along by Palmerstown Park!"
The three quarks that Gell-Mann hypothesized were given the labels up, down, and strange, the latter name coming because it was the only quark with a nonzero strangeness.

For many years this 'quark model' wasn't taken very seriously in the larger world of particle physics. Experiments continued to fail at producing particles matching the descriptions of the three quarks, leading many to argue that they were merely a mathematical abstraction and an artifact of the theory of the Eightfold Way. Even the theorists that proposed a fourth quark, in all seriousness, had no objections to naming it the 'charmed' or 'charm' quark. (Some people stolidly insisted on calling the strange quark the 'sideways' quark, but no analogous term arose for the charm quark.) All of this changed with the 1974 discovery of a particle, the J/ψ meson, whose properties strongly suggested that it was composed of charm quarks. Once these properties were verified, the scientists involved concluded not only that the quark model was correct, but also that they'd discovered a new one!

Once the quark model was thus confirmed, the floodgates opened to the discovery of new particles, but with a comforting framework for understanding their properties: hadrons are made of quarks. An additional pair of quarks was postulated: the 'truth' and 'beauty' quarks, though sadly the new seriousness of the endeavour caused them to be renamed 'top' and 'bottom', respectively. The bottom quark was discovered promptly, in 1977, but the heavy top quark remained unseen until 1995. Since the discovery of the top quark, most physicists have believed that the set of quarks is now complete, and that these six quarks and their antiparticles are adequate to describe any hadron that could possibly be produced, returning the number of truly elementary particles to a manageable quantity.

Properties of Quarks

The six known quarks are, in order of mass, the down, up, strange, charm, bottom, and top quarks. The up, charm, and top quarks have a positive electric charge, 2/3 of the proton charge, while the other three quarks have a negative charge that is 1/3 of the electron charge. The quarks are paired up into three 'generations', each containing a positive and negative quark; the two quarks in each generation are relatively close in mass and they are intrinsically connected by the weak nuclear force. (There are subtleties here, see Cabibbo Kobayashi Maskawa matrix for more details)

Quarks are bound together into hadrons by the strong nuclear force, which is by far the strongest of the four fundamental forces; the next closest is the electromagnetic force at 1/137 of the strength. The strong nuclear force acts between particles that have 'colour charge', which can be one of three 'colours', usually named red, blue, and green. (We can call them whatever we want because they have nothing to do with the colours of light, despite the name.) Each quark has a single unit of colour charge, in one of the three colours. Antiquarks have negative colour charge, or 'anticolour'. All other matter particles have a colour charge of zero.

The strong nuclear force binds together quarks in combinations where the amount of each colour is the same. The two simplest way of doing this are to have one quark of each colour, or to have a quark and antiquark of the same colour. The former results in a combination of three quarks (or three antiquarks), called a 'baryon', while the latter combination is referred to as a 'meson'. Protons and neutrons are baryons: the proton is the combination of two up quarks and a down quark, and the neutron contains two down quarks and an up quark.

The binding provided by the strong nuclear force is very strong; although the force is weak between two particles when they are close together, it increases drastically with increasing distance. The energy required to remove a quark from a hadron is so large that at a certain point it will, through Einstein's equation E = mc2, produce a quark/antiquark pair from the vacuum. One of these joins the original hadron and the other joins with the quark being removed to form a new hadron. This property is called 'quark confinement'. The single exception to this situation is the top quark, which has such a short lifetime that by the time the strong force is able to 'dress' it in other quarks it has already decayed.

Quark Physics and You

For most of the activities that we, as humans, perform with matter, the existence of quarks is irrelevant. Even most of nuclear physics works just the same with the 1930s picture of the proton and neutron as elementary particles. Nevertheless, one important phenomenon that we all depend on is thought to be a consequence of quark-level physics. This phenomenon is the strange imbalance between matter and antimatter in the observable universe, namely that everything we see is made out of matter and antimatter is an exotic material produced in particle accelerators.

This is puzzling to particle physicists and cosmologists because the laws of physics are, for the most part, symmetric between matter and antimatter. Two fundamental forces are known to be completely symmetric between matter and antimatter: the strong nuclear force and the electromagnetic force. Where asymmetry between matter and antimatter appears is in the weak nuclear force; this asymmetry is referred to as CP violation.

The CP-violating part of the weak force acts directly at the quark level, and has been observed with both strange quarks and bottom quarks. This preference for quarks over antiquarks, though, is a small and subtle effect, far from the gross violations that would be necessary to produce a matter-filled universe. Violations involving other elementary particles, mainly neutrinos, have been postulated to fill this gap, but no definite evidence has yet been found.

Sources include my undergraduate and graduate particle physics courses, conversations with my thesis supervisor, and David Griffiths's text Introduction to Elementary Particles.
This writeup is copyright 2006 D.G. Roberge and is released under the Creative Commons Attribution-NoDerivs-NonCommercial licence. Details can be found at .