27
1.3. Particle interactions in the Standard Model
FIGURE 1.8
Angular distribution of two-jet events in p¯ p collisions (Arnison et al. 1985)
as a function of cos θ, where θ is the CMS scattering angle. The broken curve
is the prediction of QCD, obtained in the lowest order of perturbation theory
(one-gluon exchange); it is virtually indistinguishable from the Rutherford
(one-photon exchange) shape sin
−4 θ/2. The full curve includes higher order
QCD corrections.
distribution is a clear signal that a massless quantum is being exchanged – in
this case, the gluon.
It might then seem to follow that, as in the case of QED, the QCD interaction has infinite range. But this cannot be right: the strong forces do not
extend beyond the size of a typical hadron, which is roughly 1 fm. Indeed, the
QCD force is mediated by the massless spin-1 gluon, and QCD is also a gauge
theory; but the form of the QCD interaction, though somewhat analogous to
QED, is more complicated, and the long range behaviour of the force is very
different.
As we have seen, each quark comes in three colours, and the QCD force
is sensitive to this colour label: the gluons effectively ‘carry colour’ back and
forth between the quarks, as shown in the one-gluon exchange process of figure 1.9. Because the gluons carry colour, they can interact with themselves,
like the W’s and Z’s of the GSW theory. As in that case, these gluonic
self-interactions cause the QCD interaction strength to decrease at short distances (or high energies), ultimately tending to zero, the property known as
1.3. Particle interactions in the Standard Model
FIGURE 1.8
Angular distribution of two-jet events in p¯ p collisions (Arnison et al. 1985)
as a function of cos θ, where θ is the CMS scattering angle. The broken curve
is the prediction of QCD, obtained in the lowest order of perturbation theory
(one-gluon exchange); it is virtually indistinguishable from the Rutherford
(one-photon exchange) shape sin
−4 θ/2. The full curve includes higher order
QCD corrections.
distribution is a clear signal that a massless quantum is being exchanged – in
this case, the gluon.
It might then seem to follow that, as in the case of QED, the QCD interaction has infinite range. But this cannot be right: the strong forces do not
extend beyond the size of a typical hadron, which is roughly 1 fm. Indeed, the
QCD force is mediated by the massless spin-1 gluon, and QCD is also a gauge
theory; but the form of the QCD interaction, though somewhat analogous to
QED, is more complicated, and the long range behaviour of the force is very
different.
As we have seen, each quark comes in three colours, and the QCD force
is sensitive to this colour label: the gluons effectively ‘carry colour’ back and
forth between the quarks, as shown in the one-gluon exchange process of figure 1.9. Because the gluons carry colour, they can interact with themselves,
like the W’s and Z’s of the GSW theory. As in that case, these gluonic
self-interactions cause the QCD interaction strength to decrease at short distances (or high energies), ultimately tending to zero, the property known as