FIGURE 6.3

Schematic diagram of the principle of the first experiment to discover that reflection symmetry is violated by the weak interaction. A ⁶⁰Co nucleus has many of the properties of a spinning top. If the electron from the beta decay of ⁶⁰Co were to be emitted along one sense of the axis of the spinning nucleus, then the situation would be just the opposite in a mirror. Nuclear physics experiments have clearly demonstrated that reflection symmetry is not obeyed by the weak interaction.

ordinary beta decay of muons and nuclei are compared to the exact Standard Model predictions. Nuclear physicists have invented novel techniques for polarizing nuclei and measuring the positron helicities to accomplish these tests. The neutron is also an excellent system for study, because it is possible to achieve a high degree of neutron polarization and the neutron is simple and more easily understood theoretically than are more complex nuclei. Presently, the most precise experiments exploit muon beta decay and ordinary nuclear beta decay. The muon experiments have significant prospects for improvement because of new initiatives for intense low-energy muon sources at meson factories. For nuclear beta decay, the rates are usually high; improving the sensitivity is a matter of eliminating systematic uncertainties. The latest experiments attempt to study the beta decay of radioactive atoms in the extremely clean environments of optical traps. Optical trapping is a way of confining atoms to a small region of space by using the radiation pressure of laser light. These experiments promise enormous improvements, but the level of precision of existing experiments is good enough to rule out theories with extra exchange gauge particles as heavy as several