Electroweak Interactions 121Universe are not and will never be in thermal equilibrium on a cosmic timescale. This
result permits us to solve the adiabatic equations of cold matter and hot radiation
separately, as we in fact have.
6.3 Electroweak Interactions
Inquantum electrodynamics(QED) the electromagnetic field is mediated by photons
which are emitted by a charged particle and absorbed very shortly afterwards by
another. Photons with such a brief existence during an interaction are calledvirtual,
in contrast to real photons.
Virtual particles do not travel freely to or from the interaction region. Energy is not
conserved in the production of virtual particles. This is possible because the energy
imbalance arising at the creation of the virtual particle is compensated for when it
is annihilated, so that the real particles emerging from the interaction region possess
the same amount of energy as those entering the region. We have already met this
argument in the discussion of Hawking radiation from black holes.
However, nature impedes the creation of very huge energy imbalances. For exam-
ple, the masses of thevector bosonsW±and Z^0 mediating the electroweak interactions
are almost 100GeV. Reactions at much lower energies involving virtual vector bosons
are therefore severely impeded, and much less frequent than electromagnetic interac-
tions. For this reason such interactions are calledweak interactions.
Real photons interact only with charged particles such as protons p, electrons e−
and their oppositely chargedantiparticles,theanti-protonpandthepositrone+.An
example is the elasticCompton scatteringof electrons by photons:
훾+e±→훾+e±. (6.30)As a result of virtual intermediate states neutral particles may exhibit electromagnetic
properties such as magnetic moment. When an electron is captured by a free proton,
they form a bound state, a hydrogen atom which is a very stable system. An electron
and a positron may also form a bound atom-like state calledpositronium.Thisisa
very unstable system: the electron and positron are antiparticles, so they rapidly end
up annihilating each other according to the reaction
e−+e+→훾+훾. (6.31)Since the total energy is conserved, the annihilation results in two (or three) photons
possessing all the energy and flying away with it at the speed of light.
The reverse reaction is also possible. A photon may convert briefly into a virtual
e−e+pair, and another photon may collide with either one of these charged particles,
knocking them out of the virtual state, thus creating a free electron–positron pair:
훾+훾→e−+e+. (6.32)This requires the energy of each photon to equal at least the electron (positron) mass,
0.51MeV. If the photon energy is in excess of 0.51MeV the e−e+pair will not be created
at rest, but both particles will acquire kinetic energy.