Standard Model Building from Intersecting D-Branes
  We provide a general overview of the current state of the art in four dimensional three generation model building proposals - using intersecting D-brane toroidal compactifications [without fluxes] of IIA, IIB string theories - which have only the SM at low energy. In this context, we focus on these model building directions, where non-supersymmetric constructions - based on the existence of the gauge group structure $SU(3)_c \times SU(2)_L \times U(1)_Y$, Pati-Salam $SU(4)_C \times SU(2)_L \times SU(2)_R$, SU(5) and flipped SU(5) GUTS - appear at the string scale $M_s$. These model building attempts are based on four dimensional compactifications that use orientifolds of either IIA theory with D6-branes wrapping on $T^6$, $T^6/Z_3$ and recently on $T^6/Z_3 \times Z_3$ or of IIB theory with D5-branes wrapping on $T^4 \times C/Z_N$. Models with D5-branes are compatible with the large extra dimension scenario and a low string scale that could be at the TeV; thus there is no gauge hierarchy problem in the Higgs sector. In the case of flipped SU(5) GUTS - coming from $T^6/Z_3$ - the special build up structure of the models accommodates naturally a see-saw mechanism and a new solution to the doublet-triplet splitting problem. Baryon number is a gauged symmetry and thus proton is naturally stable only in models with D5 branes or in models with D6-branes wrapping toroidal orientifolds of type IIA. Finally, we present new RR tadpole solutions for the 5- and 6- stack toroidal orientifold models of type IIA which have only the Standard Model with right handed neutrinos at low energy. 
Christos Kokorelis