178
R. Barrett and P. P. Delsanto
Within the hadron, the colour force is unlike the other forces, and does not
drop off with distance. This property is responsible for the confinement of the
quarks within the hadron, and is the reason no free quarks are observed in
laboratory experiments. Indeed, the colour force, produced by the exchange
of gluons, is so strong that quark-antiquark pair production will occur before
the quarks can be separated. However, at short separation distances the colour
force is weak, and the quarks can move freely, until they start to get too far
apart. Then the colour force kicks in and brings them back together, rather
like a sheep dog rounding up lambs that have strayed too far from the flock.
Colour is the analogue in the colour force to charge in the electromagnetic force. However, whereas two values were needed for the electric charge,
denoted by convention “positive” and “negative”, three values are needed for
the colour. These three values of the colour “charge” have been given the
names of the three primary colours: red, green and blue. Clearly the colour
charges have nothing to do with visible light, or the perception of colours.
They are just another example of the Humpty-Dumpty influence that we
mentioned earlier.
Without the existence of colour, the Pauli Exclusion Principle would not
allow, for instance, two up quarks to be present inside a hadron. However, if
each up quark is of a different colour, they are non-identical, and therefore
can be present together.
Having got our heads around the basic concepts of the Standard Model,
we are now at last in a position to see how hadrons are put together.
There are some limitations on how these particles are constructed. First, only
“colourless” hadrons exist in nature. These may be baryons comprising three
quarks of the three different colours (in the same manner that white light
is produced by combining the three primary colours of red, green and blue
light), or mesons made from a quark-antiquark pair. Antiquarks have an anticolour, so that a quark-antiquark pair is colourless. In addition, as no particles
with fractional electronic charges exist in nature, baryons must have an electric charge that is an integer multiple (disregarding the sign) of the electronic
charge. As they are fermions, they must also have half-odd integer spin.
Now would seem to be an appropriate time to introduce a table of hadrons,
showing how they are all constructed from their constituent quarks. However,
thanks to the diligence of experimental high-energy physicists, there are literally hundreds of them, and studying such a list would, for the non-specialist,
be about as entertaining as reading the Greater London Telephone Directory,
or perhaps even Whitehead and Russell’s Principia (see Chap. 3). For illustrative purposes, we discuss here only the proton and the pion, as examples
of a baryon and a meson.
R. Barrett and P. P. Delsanto
Within the hadron, the colour force is unlike the other forces, and does not
drop off with distance. This property is responsible for the confinement of the
quarks within the hadron, and is the reason no free quarks are observed in
laboratory experiments. Indeed, the colour force, produced by the exchange
of gluons, is so strong that quark-antiquark pair production will occur before
the quarks can be separated. However, at short separation distances the colour
force is weak, and the quarks can move freely, until they start to get too far
apart. Then the colour force kicks in and brings them back together, rather
like a sheep dog rounding up lambs that have strayed too far from the flock.
Colour is the analogue in the colour force to charge in the electromagnetic force. However, whereas two values were needed for the electric charge,
denoted by convention “positive” and “negative”, three values are needed for
the colour. These three values of the colour “charge” have been given the
names of the three primary colours: red, green and blue. Clearly the colour
charges have nothing to do with visible light, or the perception of colours.
They are just another example of the Humpty-Dumpty influence that we
mentioned earlier.
Without the existence of colour, the Pauli Exclusion Principle would not
allow, for instance, two up quarks to be present inside a hadron. However, if
each up quark is of a different colour, they are non-identical, and therefore
can be present together.
Having got our heads around the basic concepts of the Standard Model,
we are now at last in a position to see how hadrons are put together.
There are some limitations on how these particles are constructed. First, only
“colourless” hadrons exist in nature. These may be baryons comprising three
quarks of the three different colours (in the same manner that white light
is produced by combining the three primary colours of red, green and blue
light), or mesons made from a quark-antiquark pair. Antiquarks have an anticolour, so that a quark-antiquark pair is colourless. In addition, as no particles
with fractional electronic charges exist in nature, baryons must have an electric charge that is an integer multiple (disregarding the sign) of the electronic
charge. As they are fermions, they must also have half-odd integer spin.
Now would seem to be an appropriate time to introduce a table of hadrons,
showing how they are all constructed from their constituent quarks. However,
thanks to the diligence of experimental high-energy physicists, there are literally hundreds of them, and studying such a list would, for the non-specialist,
be about as entertaining as reading the Greater London Telephone Directory,
or perhaps even Whitehead and Russell’s Principia (see Chap. 3). For illustrative purposes, we discuss here only the proton and the pion, as examples
of a baryon and a meson.