FIGURE 6.2

The apparatus used at the Indiana University Cyclotron Facility (IUCF) to measure small violations of charge symmetry in nuclear forces. The experiment was a high-precision study of the scattering of a spin-polarized neutron beam by a spin-polarized proton target. The scattered neutrons and protons were detected in the counters shown. (Courtesy Indiana University Cyclotron Facility.)

and reversal of time (the process evolving in reverse)—are symmetries of nature. It was quickly realized that charge-conjugation symmetry is violated as badly as reflection symmetry.

Experiments suggest that reflection symmetry is broken as badly as it could be in processes like beta decay. This observation is incorporated into the Standard Model, but what is the reason for maximal parity violation? There are alternative models to the Standard Model—for example, attractive left-right symmetric models that contain an extra set of intermediate-vector bosons with the other sense of parity violation. Consistency with low-energy experiments is obtained if the extra bosons have masses sufficiently large not to have an effect. Nuclear physicists are instrumental in testing this plausible class of Standard Model extensions. Precise measurements of basic parity-violating effects in the