Los puntos clave no están disponibles para este artículo en este momento.
A simple theorem is proved about the - parameter in grand unified models with spontaneous CP symmetry breaking. This theorem states two conditions on such models which, if fulfilled, imply that - is zero at tree level. Models which fulfill these conditions are generalizations of a recent model of Nelson and are easy to construct.
S. M. Barr (Mon,) studied this question.