||Dr C N Booth
Fundamental Interactions - 2) Weak Interactions
Three properties of certain decays led to the postulation of the weak interaction;
The original Fermi theory of beta decay involved a point like "contact
interaction" with a coupling constant
G = 1.166×10−5 GeV−2 ≃ 10−5/mp2
(i.e. weak compared with 1/137).
Low rates or long lifetimes.
Violation of certain "conservation laws" - changes in parity, strangeness
Frequent involvement of neutrinos, particles which are not involved in
any other sort of interactions.
This value of G is compatible with all the observed decay rates
for the above processes. However, if we calculate neutrino electron scattering
ν e → ν e, it is found that the cross-section rises with neutrino energy E as
tending to infinity with E!
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
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
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
At low energies, the cross section is proportional to
so if mW is large, the weak coupling g is not
necessarily very small.
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
νμ N → νμ X
(where N is a nucleus and X a hadronic system) and
νμ e → νμ e.
However, this exposed a new theoretical problem.
At sufficiently high energy, it should be possible to produce real W+
W− pairs in e+ e−
annihilation through the Z0, and a calculation shows that at
very high energies this cross section again becomes unphysically large.
The diagram for this process would be cancelled by that corresponding to
electromagnetic annihilation to W+ W−
through a photon, but this cancellation will only occur if the weak coupling
constant g is equal to the electromagnetic coupling,
(Note that at such very high energies, the mass of the Z0 becomes
Although this argument was originally a theoretical one, experimental evidence from
W+ W− production at the large electron-positron
collider, LEP, many years later confirmed this prediction, and such results
will be presented in the lectures.
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 d u̅ → W− → e− ν̅e
were observed in the UA1 and UA2 experiments, while the
next year the rarer but cleaner signature
q q̅ → Z0 → e+ e−
In the late 1980s, a
large e+ e− collider, LEP, was built
at CERN, allowing the "mass production" of Z0 bosons.
By colliding the particles with exactly the right energy to supply the
Z mass, a large resonance in the cross section occurs, and now several
million Z0 decays have been observed, shared between the 4 large
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.
Thus, in e+
e− annihilation, the Z0 is seen with a width
in mass of about 2.5 GeV (FWHM).
Decays of the Z into neutrinos have two effects.
Each species of neutrino decreases the number of visible
Z decays by 13% and also shortens the lifetime, and hence increases the
width by 0.176 GeV.
Measurements by the ALEPH collaboration at LEP show that the data imply
Nν = 2.994 ± 0.012 i.e. 3!
There are therefore no
new generations of lepton and neutrino (unless the mass of the new neutrino
is greater than 45 GeV/c2 - very different from the three
known species which are almost massless).
Cross sections for e+ e− → hadrons
as a function of centre-of-mass energy.
The Standard Model predictions for
Nν = 2, 3 and 4 are shown.
Supplementary material on the weak interaction,
mainly of a popular or non-technical nature, can be obtained from a number
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,
(The above chapter references are for edition 3.
For edition 2, consult chapters 13, 19 and 25.)
- the weak interaction and beta decay - chapter 12
- W bosons - chapter 18
- the search for the W and Z - chapter 24.
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