SYMMETRIES

Abstract

This chapter discusses time reversal invariance (TRI), parity, and exotic (primarily second-class weak) currents. The primary interest in TRI at present continues to be the desire to understand the CP violation observed in the decay of the K°. Although the present data is consistent with the superweak theory, milliweak theories cannot be ruled out as yet. If the superweak theory is correct, then no effect should be seen in any other weak reactions than those involving the K°. Probably the best and most decisive test of this theory is the measurement of the neutron dipole moment. The chapter discusses the results suggesting that TRI is valid in weak and strong interactions to about three parts per 1000. Electromagnetic tests of TRI in nuclei both at low and high energies also have not revealed any effects to a level of ∼3 × 10 −3 . Such tests attempt to discover TR violations because of either the electromagnetic interaction of hadrons or a millistrong theory.