Some comments on flipped SU(5)×U(1) and flipped unification in general

Abstract

A general group-theoretical discussion of flipped embeddings is given. In addition to the well-known flipped SU(5) and flipped SO(10), the existence of flipped ${\mathrm{E}}_{6}$ and ${\mathrm{E}}_{7}$ is shown, as well as several families and special cases of flipped embeddings. A possible physical reason, essentially based on the group theory of flipped embeddings, why nature prefers the low-energy group SU(3)\ifmmode\times\else\texttimes\fi{}SU(2)\ifmmode\times\else\texttimes\fi{}U(1) to alternatives such as SU(4)\ifmmode\times\else\texttimes\fi{}U(1) and SU(5) is pointed out.