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.
(CC)
This writeup is copyright 2006 D.G. Roberge and is
released under the Creative Commons
Attribution-NoDerivs-NonCommercial licence. Details can be found at
http://creativecommons.org/licenses/by-nd-nc/2.5/ .