GUTs without guts

Abstract

The structure of a Standard Model family is derived in a class of brane models with a U (M)× U (N) factor,from two mil dlyan thropic requirements: a massless photon and a universe that does not turn into a plasma of massless charged particles. If we choose M=3 and N=2, the only option is shown to be the Standard Model with an undetermined number of families. We do not assume the U (1) embedding, charge quantization, family repetition, nor the fermion representations; all of these features are derived, assuming a doublet Higgs. With as lightly stronger as sumption even the Higgs representation is determined. We also consider a more general class, requiring an asymptotically free strong SU (M) (with M 3) interaction from the first factor and an electromagneti cU (1) embedded in both factors. We allow Higgs symmetry breaking of the U (N)× U (1) flavor group by at most one Higgs boson in any representation, combined with any allowed chiral symmetry breaking by SU (M). For M=3 there is a large number of solutions with an unbroken U (1). In all of these, “quarks” have third-integral charges and color singlets have integer charges in comparison to leptons. Hence Standard Model charge quantization holds for any N. Only for N= 2 these models allow an SU (5) GU Textension, but this extension offers no advantages what so ever for understanding the Standard Model; it only causes complications, suchas the doublet–triplets plitting problem. Although all these models have amassless photon, all except the Standard Model are rule dout by these condan thropic requirement. In this class of brane models the Standard Model is realized as a GUT with its intestines removed, to keep only the good parts: a GUT without guts.