Semi-simple unification on T6/Z12 orientifold in the type IIB supergravity

The semi-simple unification model based on SU(5)GUT×U(3)H gauge group is an interesting extension of the minimal SU(5)GUT grand unification theory (GUT), since it solves the two serious problems in the standard GUT: the triplet–doublet splitting problem and the presence of dangerous dimension five operators for proton decay. Here, the extra U(3)H gauge interaction plays a crucial role on the GUT breaking. In this Letter, we show that the full multiplet structure of the U(3)H sector required for the desired GUT breaking is reproduced naturally on T6/Z12 orientifold in the type IIB supergravity with a D3–D7 system. The SU(5)GUT vector multiplet lives on D7-branes and the U(3)H sector resides on D3-branes. We also show that various interesting features in the original SU(5)GUT×U(3)H model are explained in the present brane-world scenario. A possible extension to the type IIB string theory is also discussed.