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R. Barrett and P. P. Delsanto
Gluons
Before quitting our zoo, we should say a little about the final components of the Standard Model, the unfortunately named gluons, carriers of the
colour (or interquark) force. Gluons carry out the same role for the colour
force between quarks as photons perform for the electromagnetic interaction
between electrons. However, they are also carriers of the colour charge, unlike
photons, which do not carry the electromagnetic charge. They are massless,
like the photon, but unlike the photon, they are never observed outside the
confines of the hadron. They have spins of 1 unit, and are therefore bosons.
Gluons carry both a colour and an anticolour. This gives a possible nine
combinations. An exchange of a gluon between quarks converts a quark from
one colour to another. The analogy of this process with photon exchange has
led to the name Quantum Chromodynamics (QCD) being coined for this
aspect of the Standard Model.
9.5 Beyond the Standard Model
In the last chapter we have made much of the incredible agreement between
QED predictions and experimental measurements for physics involving the
electron-electron interaction. Naturally physicists hoped to achieve the same
precision with the strong force analogue to QED, i.e. QCD. As we shall
see, they have been only partially successful. Various reasons have been put
forward for this, and suggestions made to extend the Standard Model. As
this is currently a highly active research field, we should bear in mind that
the situation can change very quickly.
In both QED and QCD, the force is produced by the exchange of carrier
particles. This is illustrated by the Feynman diagrams below in Fig. 9.5.
These are the lowest order diagrams for three processes. As we mentioned in
Chap. 8, there are higher order diagrams, and all possible diagrams should,
in principle, be included in the theoretical calculations.
In (a) the decay of a neutron by means of the weak interaction into a
proton, an electron and an antineutrino is displayed. The process is mediated
by the exchange of an Intermediate Vector Boson.
In (b) an example of the colour force in action is shown. A green quark
and a blue quark swap colours by the exchange of a green-antiblue gluon.
The last example in (c) is the scattering of a neutron and a proton inside
the nucleus by the exchange of a pion. This is the process that was proposed
by Yukawa when he predicted the existence of the meson to explain the strong
nuclear interaction between nucleons inside the nucleus. Now, if the strong
R. Barrett and P. P. Delsanto
Gluons
Before quitting our zoo, we should say a little about the final components of the Standard Model, the unfortunately named gluons, carriers of the
colour (or interquark) force. Gluons carry out the same role for the colour
force between quarks as photons perform for the electromagnetic interaction
between electrons. However, they are also carriers of the colour charge, unlike
photons, which do not carry the electromagnetic charge. They are massless,
like the photon, but unlike the photon, they are never observed outside the
confines of the hadron. They have spins of 1 unit, and are therefore bosons.
Gluons carry both a colour and an anticolour. This gives a possible nine
combinations. An exchange of a gluon between quarks converts a quark from
one colour to another. The analogy of this process with photon exchange has
led to the name Quantum Chromodynamics (QCD) being coined for this
aspect of the Standard Model.
9.5 Beyond the Standard Model
In the last chapter we have made much of the incredible agreement between
QED predictions and experimental measurements for physics involving the
electron-electron interaction. Naturally physicists hoped to achieve the same
precision with the strong force analogue to QED, i.e. QCD. As we shall
see, they have been only partially successful. Various reasons have been put
forward for this, and suggestions made to extend the Standard Model. As
this is currently a highly active research field, we should bear in mind that
the situation can change very quickly.
In both QED and QCD, the force is produced by the exchange of carrier
particles. This is illustrated by the Feynman diagrams below in Fig. 9.5.
These are the lowest order diagrams for three processes. As we mentioned in
Chap. 8, there are higher order diagrams, and all possible diagrams should,
in principle, be included in the theoretical calculations.
In (a) the decay of a neutron by means of the weak interaction into a
proton, an electron and an antineutrino is displayed. The process is mediated
by the exchange of an Intermediate Vector Boson.
In (b) an example of the colour force in action is shown. A green quark
and a blue quark swap colours by the exchange of a green-antiblue gluon.
The last example in (c) is the scattering of a neutron and a proton inside
the nucleus by the exchange of a pion. This is the process that was proposed
by Yukawa when he predicted the existence of the meson to explain the strong
nuclear interaction between nucleons inside the nucleus. Now, if the strong