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R. Barrett and P. P. Delsanto
the rather uninspiring names: up (u), down (d), charm (c), strange (s), top
(t) and bottom (b). For a while, “truth (t)” and “beauty (b)” were floated
as possible names for the last two, but banality ultimately prevailed. Most
physicists pronounce “quark” to rhyme with “Mark”; Gell-mann pronounced
it “kwork”.
At first, considerable effort was expended trying to detect free quarks in
cosmic rays and accelerator data. Quarks have an electric charge of 1/3 or 2/3
that of the electron, which should make them distinctive. Despite a few false
alarms [5], it is now generally accepted that quarks are confined within the
hadron, and cannot be seen on their own. We shall discuss this idea further
shortly.
In a conventional zoo, it is customary to put a label on the animals’ cages
bearing the names of the beasts within. For instance, the tigers’ cage would
have the tag: Panthera tigris tigris if it contains a Bengal tiger, or Panthera tigris
sumatrae if there is a Sumatran tiger lurking somewhere back in the shadows.
The three Latin names are the genus, species and sub-species of that strain of
tiger. In our particle zoo, quantum numbers serve the same purpose, i.e. they
enable us to differentiate between particles. (We have already encountered
the lepton number, which is one example of a quantum number.) Unlike the
Latin scientific names of animals, however, they also limit the interactions
that particles can engage in, via various conservation laws.
Very early in nuclear physics, in 1937, a quantum number called isospin
was introduced by Eugene Wigner, in analogy with ordinary spin, to distinguish protons from neutrons. Particles that are affected equally by the strong
force, with similar properties, apart from their charge (e.g. neutrons and
protons), are considered to be different states of the same particle. They
are said to have different isospin. Later, physicists introduced the additional quantum numbers of baryon number, charm, strangeness, topness, and
bottomness to distinguish the newer members of the particle zoo. Not all
physicists had Gell-man’s erudite knowledge of literature to draw on when
choosing names. A table of the six quarks and their associated quantum
numbers is included in Appendix 9.2.
We are now almost in a position to see how the individual hadrons are put
together. However, simply forming all possible combinations of the known
quarks would result in far more hadrons than are actually observed. Clearly
there is another selection process at work here. This involves what has been
called colour, although it has nothing to do with the visual colours that we
know so well.
Just as the electromagnetic (or more correctly, the electroweak) force binds
the electrons to the atomic nucleus, colour (or the colour force) binds the
R. Barrett and P. P. Delsanto
the rather uninspiring names: up (u), down (d), charm (c), strange (s), top
(t) and bottom (b). For a while, “truth (t)” and “beauty (b)” were floated
as possible names for the last two, but banality ultimately prevailed. Most
physicists pronounce “quark” to rhyme with “Mark”; Gell-mann pronounced
it “kwork”.
At first, considerable effort was expended trying to detect free quarks in
cosmic rays and accelerator data. Quarks have an electric charge of 1/3 or 2/3
that of the electron, which should make them distinctive. Despite a few false
alarms [5], it is now generally accepted that quarks are confined within the
hadron, and cannot be seen on their own. We shall discuss this idea further
shortly.
In a conventional zoo, it is customary to put a label on the animals’ cages
bearing the names of the beasts within. For instance, the tigers’ cage would
have the tag: Panthera tigris tigris if it contains a Bengal tiger, or Panthera tigris
sumatrae if there is a Sumatran tiger lurking somewhere back in the shadows.
The three Latin names are the genus, species and sub-species of that strain of
tiger. In our particle zoo, quantum numbers serve the same purpose, i.e. they
enable us to differentiate between particles. (We have already encountered
the lepton number, which is one example of a quantum number.) Unlike the
Latin scientific names of animals, however, they also limit the interactions
that particles can engage in, via various conservation laws.
Very early in nuclear physics, in 1937, a quantum number called isospin
was introduced by Eugene Wigner, in analogy with ordinary spin, to distinguish protons from neutrons. Particles that are affected equally by the strong
force, with similar properties, apart from their charge (e.g. neutrons and
protons), are considered to be different states of the same particle. They
are said to have different isospin. Later, physicists introduced the additional quantum numbers of baryon number, charm, strangeness, topness, and
bottomness to distinguish the newer members of the particle zoo. Not all
physicists had Gell-man’s erudite knowledge of literature to draw on when
choosing names. A table of the six quarks and their associated quantum
numbers is included in Appendix 9.2.
We are now almost in a position to see how the individual hadrons are put
together. However, simply forming all possible combinations of the known
quarks would result in far more hadrons than are actually observed. Clearly
there is another selection process at work here. This involves what has been
called colour, although it has nothing to do with the visual colours that we
know so well.
Just as the electromagnetic (or more correctly, the electroweak) force binds
the electrons to the atomic nucleus, colour (or the colour force) binds the