It is shown that SU(2)×U(1) can be broken at the tree level, without the occurrence of global potential minima that break U(1)_e.m., in supergravity models that are more general than those proposed by Nilles, Srednicki and Wyler. The study comprises an analysis of models with a general soft supersymmetry-breaking structure as well as those of the Hall-Lykken-Weinberg type.

It is shown that dimension five non-renormalizable interactions can produce light Dirac neutrinos in an extension of the minimal SU(5) GUT containing additional SU(5) singlets and global U(1) symmetries.

CP-violating rate asymmetry can be generated in a process only if its amplitude possesses an absorptive part. It is pointed out that such an absorptive part can be provided ine⁺e⁻ annihilation by the presence of aZ(Z′) resonance of non-zero width. The CP asymmetry in the process$$e^ + e^ - \to l_i \bar l\left( {l_i \bar l_j } \right)$$, wherel_i are charged leptons, is discussed in several models. In a specific two-Z model, large and observable CP asymmetry ine⁺e⁻ →τ⁺e⁻ (τ⁻e⁺) is shown to be possible at LEP/SLC energies.

The observed deficits in the solar and atmospheric neutrino fluxes along with the accelerator results on neutrino oscillations significantly constrain possible mass and mixing patterns among neutrinos. We discuss possible patterns emerging from the experimental results and review theoretical attempts to understand them.