A D-brane alternative to the MSSM

The success of SU(5)-like gauge coupling unification boundary conditions$g_3^2=g_2^2=5/3 g_1^2$ has biased most attempts to embed the SM interactionsinto a unified structure. After discussing the limitations of the orthodoxapproach, we propose an alternative that appears to be quite naturally impliedby recent developments based on D-brane physics. In this new alternative: 1)The gauge group, above a scale of order 1 TeV, is the minimal left-rightsymmetric extension $SU(3)\times SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ of theSM; 2) Quarks, leptons and Higgs fields come in three generations; 3) Couplingsunify at an intermediate string scale $M_s= 9\times 10^{11}$ GeV with boundaryconditions $g_3^2=g_L^2=g_R^2=32/3 g_{B-L}^2$. This corresponds to the naturalembedding of gauge interactions into D-branes and is different from thestandard SO(10) embedding which corresponds to $k_{B-L}=8/3$. Unification onlyworks in the case of three generations; 4) Proton stability is automatic due to the presence of $Z_2$ discreteR-parity and lepton parities. A specific Type IIB string orientifold model withthe above characteristics is constructed. The existence of three generations isdirectly related to the existence of three complex extra dimensions. In this model the string scale can be identified with the intermediate scaleand SUSY is broken also at that scale due to the presence of anti-branes in thevacuum. We discuss a number of phenomenological issues in this model including Yukawacouplings and a built-in axion solution to the strong-CP problem. The presentframework could be tested by future accelerators by finding the left-rightsymmetric extension of the SM at a scale of order 1 TeV.Comment: 50 pages, 7 figures. References adde