|PHY304||Particle Physics||Dr C N Booth|
Very general scattering theory provides a limit on the maximum possible elastic cross-section, purely based on the conservation of probability, or "unitarity", and this is violated when . A solution to this problem, as we have already suggested, is to replace the 4-fermion contact interaction with boson exchange, as in QED. The exchange of a massless boson would not be compatible with low energy behaviour (where the cross section does rise with E), and the exchanged "weak intermediate vector boson", W, must have a considerable mass. The propagator that this introduces is , and this acts as a constant at low energy (or low q2), but falls as 1/q2 at very high q2.
The effective strength of the weak interaction at low energies thus depends both on the coupling of the W to fermions, g, and upon its mass. At low energies, the cross section is proportional to
The weak interaction responsible for beta decay involves a charged
(or charge-changing) current, and the W must exist as W+ and
This suggested there might also be a weak neutral
current, propagated by a third boson, the Z0.
for this was first observed at CERN in 1973 in the form of neutrino interactions
In the 1960's, Glashow, Salam and Weinberg proposed the unification
of the electromagnetic and weak interactions as a single gauge theory with
a common coupling constant.
This implied that the mass of the W and
Z must be about 90 GeV/c2.
(At low energies, the W and Z cannot be produced as real particles, and the
electromagnetic and weak interactions appear as separate processes.)
The first real weak bosons were produced at the CERN antiproton-proton
collider, which effectively allowed antiquarks and quarks to interact.
In 1982, processes such as
were observed in the UA1 and UA2 experiments, while the
next year the rarer but cleaner signature
In the late 1980s, a
The Z couples to all fermion-antifermion pairs, including neutral neutrinos.
The decays to neutrinos are basically undetectable, as νs
leave no tracks and can pass through large amounts of material without
interacting, but nonetheless they modify the properties of the Z, and results
on ν production
were some of the most important to emerge in the early days of LEP!
The Z is a very short-lived particle, with a lifetime of only 2.6x10−25
s. The uncertainty principle implies a relationship between the uncertainty
in energy (and hence mass) and that in time.
Cross sections for
Supplementary material on the weak interaction, mainly of a popular or non-technical nature, can be obtained from a number of sources. You may wish to consult some of the following information on the Web:
Supplementary Reading Material
For further non-technical discussion of weak interactions, you might like to consult The Ideas of Particle Physics, as follows:
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