Neutral Currents and the Z0
The W is involved in commonly observed processes such as the decay of the neutron, beta decay, the decay of the pion. The interaction of the Z0 is somewhat more subtle. The interaction of the Z0 with a quark does not change its color, and its interaction with a lepton does not change its charge. The interaction of the Z0 with electrons can be represented by a Feynman diagram.
Referred to as a "neutral current" this Z0 interaction is very difficult to detect because it is masked by the much stronger electromagnetic interaction with which it is compared above.
Theoretically, the neutral current is important since it represents an essential part of the picture of electroweak unification. It was observed in 1973 in neutrino scattering experiments, and that connection has become more important in recent years as a part of the resolution of the solar neutrino problem. The researchers at the Sudbury Neutrino Observatory have addressed this problem with a program to detect the following neutrino interactions.
A look at these interactions in terms of Feynman diagrams can perhaps suggest why the probabilities for these interactions are different, a situation which helps in the modeling of the solar neutrino behavior.
McDonald, Klein & Wark