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GUTs without guts

The structure of a Standard Model family is derived in a class of brane models with a U(M) x U(N) factor, from two mildly anthropic 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 a slightly stronger assumption even the Higgs representation is determined. We also consider a more general class, requiring an asymptotically free strong SU(M) (with M geq 3) interaction from the first factor and an electromagnetic U(1) embedded in both factors. We allow Higgs symmetry breaking of the U(N)x 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 quantizatio