Protons and neutrons once seemed to be the ultimate indivisible particles
of matter. But a closer inspection reveals an intricate world within, which
proves to be a seething hive of activity. This is the realm of particles
called quarks and the strong force that controls them
WHAT are the fundamental constituents of matter? Once scientists thought
atoms were indivisible, but during the early decades of the 20th century they
discovered that atoms are made of protons, neutrons and electrons. Electrons
really do seem to be “fundamental”, but in the 1960s physicists began to
realise that protons and neutrons are made up of smaller particles, which they
came to call quarks.
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Particle physicists now recognise two families of basic building blocks – the
quarks and the leptons (which include electrons). There seem to be six members
of each family, that is, six kinds of quark and six kinds of lepton. Only two
types of quark, called up and down, are necessary to build the proton and the
neutron, but four other types seem to be needed to build more exotic,
short-lived particles found in cosmic rays and in experiments that study high-
energy particle collisions.
Quarks are quite different from leptons. The main difference is that the
quarks are subject to a fundamental force called the strong force, which
leptons do not feel. The strong force binds quarks together within more
complex particles such as the proton, where they form part of a teeming world
of surprising complexity.
Finding the quark
Finnigans wake
THE IDEA of quarks came originally from studying regularities in the
properties of short-lived particles related to the proton. A similar
systematic study of the properties of the elements had led Dmitri Mendeleev to
produce the periodic table in 1869. The subsequent discovery of the electron
and the atomic nucleus revealed that the regularities in Mendeleev’s table
reflect the internal structure of the atom. Now, a century later, we know that
the patterns of the relationships between subatomic particles in turn reflect
their internal structure.
The proton has many relatives that interact by way of the strong force, one
of the two fundamental forces with a range limited to distances comparable to
the atomic nucleus or smaller (see Inside Science No. 17, “Subatomic Forces”).
These strongly interacting particles are collectively called hadrons (from the
Greek for “strong”). Apart from the proton, which is stable as far as we can
tell, the hadrons are all unstable. An isolated neutron lives for nearly 15
minutes, but other hadrons have much shorter lifetimes.
The short-lived hadrons are not just the artificial products of experiments
in the laboratory. They are produced naturally whenever cosmic rays – high-
energy particles (mainly protons) from outer space – collide with the nuclei
of atoms high in the Earth’s atmosphere. Studies of cosmic-ray collisions
revealed the first evidence for hadrons such as the pion, the kaon, and the
lambda, all of which have lifetimes in the region of 10-8 to
10-10 seconds (and which were generally named after letters of the
Greek alphabet). However, experiments using particle accelerators (see Inside
Science No. 55, “Particle Accelerators”) can mimic the collisions of cosmic
rays under controlled conditions, and allow physicists to make a more
systematic study of the particles created.
One of the first aspects of hadrons that such studies revealed was a new
property, which labels some hadrons as being different from others, and which
has no analogue in the macroscopic world. Because this property leads to
behaviour that originally seemed strange, the property itself became called
strangeness, and particles that possess this property are called strange
particles. Of the particles mentioned so far, the proton and the neutron have
no strangeness, nor does the pion. But the kaon and the lambda have one unit
of strangeness, and another particle called the sigma has two units of
strangeness.
In the early 1960s, the American Murray Gell-Mann and Israeli Yuval Ne’eman
independently worked on classifying the known hadrons according to their
values of charge, strangeness and spin (a particle’s intrinsic angular
momentum). They found that patterns of eight particles (octets) and ten
particles (decuplets) occurred but there were gaps in the patterns. It soon
became clear that they could relate these groups of particles to the theory of
the mathematical symmetry group known as SU(3) (an acronym for “special
unitary group in three dimensions”).
One gap in the SU(3) scheme of classification corresponded to a new
particle with negative charge and three units of strangeness. Physicists
called it the omega-minus. Subsequently, a research group at the Brookhaven
National Laboratory in New York found it, in 1964, confirming that the theory
could be used to make predictions; it also paved the way for the concept of
quarks.
The mathematics of SU(3) shows that the larger groups – the decuplets and
octets – are all built from a basic group of only three members. So it seemed
reasonable to ask if the observed groups of hadrons were likewise based on an
underlying group of three particles. Gell-Mann, and independently another
American George Zweig, proposed that hadrons are indeed built from such basic
entities. Gell-Mann called them quarks, reputedly from a line in James
Joyce’s Finnegans Wake.
However, to create the groups of observed hadrons, it turned out that the
quarks must have fractional electric charges, frac 13 and frac 23 the size
of the electron’s charge, e.
As long ago as the early 19th century, Michael Faraday, in his work on
electrolysis, established that electric charge always exists in multiples of
some “unit” of charge. J.J. Thomson’s discovery of the electron in 1897 – the
first known fundamental constituent of all matter – suggested that this unit
of charge was none other than the charge of the electron. So the idea of
quarks with fractions of this unit of charge at first seemed revolutionary,
and led many physicists to wonder if quarks were artefacts of the mathematics
rather than any real kind of particle.
To build all the hadrons that were known in 1964, only three quarks are
necessary, those we call up (u), down (d) and strange (s), with charges
( frac23;)e, -( frac13;)e and -( frac13;)e, respectively. The up and down
quarks have zero strangeness, while the strange quark has a strangeness value
of -1. By grouping quarks together in threes we can build baryons, that is,
hadrons with spin frac12; (such as the proton, which is duu, or the neutron
which is ddu, or the lambda which is dus) or with spin frac32; (such as the
omega, which is sss). Alternatively we can combine a quark with an antiquark
(with exactly opposite values of charge and strangeness) to make hadrons with
spin 0 or 1, which are called mesons. These include the charged pion (u quark
and d antiquark, or vice versa) and the charged kaon (u quark and s antiquark,
or vice versa).
The idea of quarks was difficult to accept completely, mainly because of
the unusual “fractional” charges. No one had ever detected anything like a
particle with a charge of ( frac13;)e or ( frac23;)e. And if quarks were
supposed to exist inside other particles, why were they not knocked out in
high-energy collisions, just as protons and neutrons can be knocked out of
nuclei?
Inside the proton
Hives of activity
THE EVIDENCE that quarks are indeed locked inside protons and neutrons –
the hadrons that build up the material of the everyday world – comes from
experiments that probe deep within the particles. The principle is to use
uncomplicated particles, namely leptons, to unlock the contents of the more
complex proton. As far as we can tell, the leptons are uncomplicated as they
have no structure and behave like simple points. They do not feel the strong
force, so any interactions that occur can be interpreted in terms of only the
electromagnetic force (between charged particles), and the weak force (the
nuclear force that underlies certain forms of radioactivity).
Another essential feature of these experiments is that the probing leptons
have high energies. Quantum theory tells us that these “particles” also have a
wave nature (see Inside Science No 25). In the wave-particle picture, the
greater a particle’s energy, the shorter the associated wavelength. So the
greater a particle’s energy, the smaller the structures that its interactions
will reveal, or in other words, the “deeper” the investigation. At energies
large enough to be interesting, the collision often disturbs the proton to the
extent that several new particles are formed. Such collisions are called
“inelastic”, in contrast to the more simple “elastic”, billiard-ball like
collisions. Together, these two effects endow the high-energy experiments that
use leptons to probe inside the proton with the name of deep inelastic
scattering.
At the end of the 1960s, physicists at the Stanford Linear Accelerator
Center (SLAC) in California used beams of electrons from the 3-kilometre long
machine in experiments to unravel the structure of the proton. They directed
the beams of electrons at “targets” of liquid hydrogen, where the negatively-
charged electrons interacted via the electromagnetic force with the positively
charged protons in the hydrogen. Detectors measured the energies of the
scattered electrons and the angles at which they emerged. By analysing these
measurements, the physicists could put together a picture of what the protons
looked like to the electrons – a picture that showed how the electric charge
in the protons appeared to the high-energy electrons.
The results revealed that rather than passing through clouds of charge the
size of a proton, the electrons were striking tiny, point-like concentrations
of charge. Indeed, the “lumps” were sufficiently concentrated that sometimes
they would knock the electrons sideways, with large amounts of energy and
momentum transferring from the electron to the proton. Here was evidence that
the proton does indeed contain smaller parts, or “partons” (a name coined by
the American theorist Richard Feynman). But could these partons be identified
as quarks?
To make this connection required a knowledge of the charge of the partons.
Electrons are scattered by the electromagnetic force as they interact with the
charge of the partons. Neutrinos, however, are uncharged and interact only by
way of the weak force. A comparison of the amount of scattering for the two
types of particle reveals the charge on the partons.
The first results of this kind were from an experiment by researchers at
the European research centre for particle physics, CERN, in Geneva, using
Gargamelle, a large bubble chamber. When physicists compared the neutrino
results from Gargamelle with the electron data from SLAC, they found the
results confirmed beautifully that the partons carry charges of ( frac13;)e
and ( frac23;)e. Evidently, the quarks with their unusual fractional charges
are components of protons and neutrons.
Charm and bottom
Matter and antimatter
ANOTHER way to investigate quarks is by making them, rather than looking
for them within other particles. Quarks are created in any high-energy
collision that produces new particles, in particular, mesons which are quark-
antiquark pairs. However, one especially interesting way to create quarks is
in the collisions of electrons with their antiparticles, positrons.
When a particle meets its antiparticle, with the same mass but opposite
properties, such as charge, the result is an act of mutual self-destruction,
called annihilation. The particle and the antiparticle disappear, their
combined mass turning into energy in the form of a photon. (Or, if the total
energy is high enough into a particle called the Z°.) The photon lives for
only an instant, before the energy reforms into a particle and an
antiparticle. But the new particle-antiparticle pair does not have to be of
the same type as the pair that annihilated. Providing it has sufficient
energy, an electron-positron pair can produce a muon-antimuon pair, a tau-
antitau pair, or any matching quark-antiquark pair.
In 1974, experiments studying electron positron annihilations discovered
evidence for a new, heavier, fourth type of quark, which became called charm
(for its existence worked “like a charm” to cure certain theoretical
problems). The experiments at an electron-positron colliding beam machine
built at SLAC revealed the existence of a new particle, which became known as
the J/psi. The J/psi is a meson, consisting of a charm quark bound together
with its anti-quark. When the total energy of the colliding electron and
positron was just sufficient to make the mass of the J/psi – more than three
times the mass of the proton – the experimenters observed a huge increase in
the rate at which particles streamed through their detectors. These were the
products of the decay of the unstable J/psi.
The J/psi was discovered concurrently in another experiment, at the
Brookhaven Laboratory, which looked at pairs of electrons and positrons
produced in the collisions of high-energy protons with a beryllium target. In
this case, the experimenters detected the J/psi via the annihilation of its
quark and antiquark into an electron-positron pair, the reverse of the process
observed at SLAC.
A fifth, still heavier type of quark, called bottom, showed up in a similar
way in 1977 in an experiment at Fermilab in Illinois. Here the experimenters
were studying muon-antimuon pairs produced in the collisions of high-energy
protons with a target. This time they found evidence for a new particle some
10 times heavier than the proton. This could be interpreted as a new heavy
quark, bottom, bound with its antiquark.
Like the strange quark, the charm quark and the bottom quark both seem to
carry their own peculiar property which they endow upon the particles they
help to form. For example “charmed” mesons exist, which contain a charm quark
together with an antiquark of another variety. Quarks can change from one
variety to another via the weak force, so ultimately the bottom, charm and
strange quarks all decay to the u and d quarks of everyday matter.
Coloured quarks
Glue in the proton
MAKING quarks and antiquarks in electron-positron annihilations in this way
has also revealed another property of quarks. The probability for making a
particle-antiparticle pair in an electron-positron annihilation depends on the
square of the charge of the particle. So the ratio of the probability for
producing hadrons – particles containing quarks and antiquarks – to the
probability for producing pairs of muons and anti-muons should reflect the
charges of all the kinds of quark compared with the size of the charge of the
muon, which is simply e.
Experiments of this kind have shown that hadrons are always three times
more likely to be produced than suggested by a calculation based on the number
of quark types and their charges. This indicates that there is another
property that further differentiates the quarks, so that each type of quark
occurs in three states. In analogy with the three primary colours of light,
this property is called colour and its three values are dubbed “red”, “green”
and “blue”, although there is no connection at all with colour in the usual
sense.
Additional evidence for this special property of quarks comes from the
omega-minus particle, which contains three strange quarks (sss). As deep
inelastic scattering experiments have shown, the quarks must have an intrinsic
spin of frac12;. But the omega-minus has spin frac32;, which implies that
the three s quarks must have their spins aligned in the same direction. This
raises a problem, because a consequence of quantum theory, known as the Pauli
exclusion principle, states that identical spin- frac12; particles cannot
occupy the same region of space with spins aligned. The only way that this can
happen in the omega-minus is if the quarks differ in some other way, which
indeed they can if they all have different values of the extra property,
colour.
This property of colour belongs only to quarks, and appears to lie at the
root of the strong force. The leptons, such as the electron, have no colour
and they do not feel the strong force at all. The more complex particles that
quarks form – the baryons and the mesons – have no overall colour either. But
they interact strongly when they come close enough for the coloured quarks
within one particle to feel the effects of the colour within the other
particle.
The colours of the quarks (and the antiquarks) that the hadrons contain
neutralise each other, rather as the negative charges of electrons balance the
positive charges of protons to make an atom that is neutral overall. A baryon,
such as the proton, contains three quarks, each of a different colour (red,
green and blue). A meson contains a quark of one colour (red, say) bound with
an antiquark with the appropriate “anti-colour” (“anti-red”, which we can give
the name of cyan, the colour complementary to red).
Colour is a key element in quantum chromodynamics, the quantum theory of
the strong force (see Inside Science No. 17, “Subatomic Forces”). In this
theory, colour charge is the source of the strong force, just as electric
charge is the source of electromagnetism. Moreover, when coloured quarks
interact they exchange gluons, the particles that carry the strong force. This
is analagous to electromagnetism, in which photons carry the force at the
quantum level. One vital difference, however, is that while photons have no
electric charge, gluons carry combinations of colour, and so change the
colours of the quarks when they pass from one to another. This feature of
gluons has the important consequence that as quarks move apart the strong
force between them becomes stronger. High-energy collisions never knock single
quarks out of particles. Instead, during a collision, energy materialises as
quark-antiquark pairs – in other words, mesons – so creating sprays of
particles which are seen in cosmic-ray collisions.
A sea of quarks
Teeming world within
EXPERIMENTS studying deep inelastic scattering and electron-positron
annihilations have confirmed not only the existence of gluons within the
proton, but also the validity of the theory of quantum chromodynamics. The
early experiments at SLAC and with Gargamelle, for example, measured the
momentum carried by the quarks moving around within the proton. The results
showed that the quarks appear to carry only half the total momentum, implying
that other “partons” lie buried within the proton.
These other partons include not only the gluons, but also quark-antiquark
pairs, which can materialise from a gluon for an instant before annihilating
into a gluon again. So the proton contains its three “valence quarks” embedded
in a mesh of gluons and a “sea” of ephemeral quark-antiquark pairs. In
detailed experiments with high-energy neutrino beams at CERN during the 1980s,
physicists were able to separate out the way that these different classes of
object within the proton contribute to the deep inelastic scattering,
detecting even the presence of the antiquarks.
Though the strong force appears to ensure that single quarks can never be
ejected from the proton, experiments have detected the sprays or “jets” of
particles into which a single energetic quark materialises as it tries to
leave the grip of the strong force. And in 1979, physicists at DESY, the
German centre for particle physics in Hamburg, discovered jets due to gluons
as well as quarks. In these experiments, an electron and a positron
annihilated to produce a quark and an antiquark, one of which radiated a
gluon, rather as an electron can radiate a photon. The quark, the antiquark
and the gluon each generated a spray of particles, producing a characteristic
“event” of three jets.
The latest experiments to explore this teeming world within the proton are
also taking place at DESY, at a new machine, HERA, which causes electrons to
collide with protons head on. At higher collision energies than before, the
experiments at HERA are probing into the proton more deeply than ever. And at
Fermilab, researchers are studying very energetic collisions between protons
and antiprotons to investigate the heaviest quark, top. The underlying
symmetry of the theory of quarks and leptons and their interactions suggested
for several years that there must be six quarks, with a heavy quark of charge
+( frac23;)e to partner the bottom quark with charge -( frac13;)e. In
April 1994, experimenters at Fermilab at last claimed to have found evidence
for the production of top quarks and their antiquarks in the high-energy
collisions of protons with antiprotons. When fully confirmed, this discovery
will be a vital piece in our investigations of the peculiar world of quarks.
Meet the quarks and the friendly leptons
FIVE types of quark are well known, and it seems very likely that a sixth
quark, top, also exist. In April 1994, Fermilab in Illinois produced the
first evidence for the production of top quarks. There are also six types of
lepton. Both sets of particle can be grouped together in pairs. While only the
lightest pair in each group forms the matter of the everyday world, the other
pairs occur at higher energies. (The masses are given in units of mega-
electron volts (MeV). On this scale the proton has a mass of 938 MeV.) The
leptons include two heavier particles which have the same electric charge as
an electron and in fact behave just like an electron. They are called the
muon and the tau. The other three members of the lepton family are the three
neutrinos, one associated with each of the charged leptons, which are
electrically neutral and have very little or no mass.
Another difference between quarks and leptons lies in the values of their
electric charges. The charge of the quarks is either frac13; or frac23; the
size of the charge of the electron, and can be positive or negative, while the
leptons either have the same size (and sign) of charge as the electron, or no
charge at all (the neutrinos).
Further Reading
The Hunting of the Quark, by M. Riordan (Simon and Schuster, 1987) provides
an historical account of the discovery of quarks, in particular the
experiments at SLAC.
The Cosmic Onion, by F. E. Close (Heinemann Educational, 1983) is a good
introduction to the basic ideas of quarks.
Spaceship Neutrino, by C. Sutton (CUP, 1992) includes a more detailed
description of how neutrino experiments tell us about the proton.
The Experimental Foundations of Particle Physics, by R. N. Cahn and G.
Goldhaber (CUP, 1989) includes reprints of papers describing the key
discoveries plus some commentary. “Turning the proton inside out”, N. Harnew
and C. Sutton, 91av, 30 May 1992, describes the new experiments at
the HERA machine in DESY.