Towards the Standard Model on Rigid D-Branes
Seminar Room 2, Newton Institute Gatehouse
D-brane worlds have provided a geometrically intuitive approach for Standard Model (SM) engineering within string theory compactifications. While the types and ranks of gauge groups and the chiral matter content are tackled in a straightforward manner, it remains on the one hand an open question if the SM can be obtained on rigid D-branes - without continuous gauge symmetry breaking along flat directions and with non-trivial D-brane instanton contributions to the effective action - and on the other hand if such models are consistent with low-energy data. The T6/Z(2)xZ(6') orbifold with discrete torsion admits a plethora of left-right symmetric `local models' without exotic matter in the adjoint, symmetric or antisymmetric representation. However, `global' completions usually come at the cost of introducing new exotic states in bifundamental representations. It furthermore turns out that USp(2) and SO(2) in place of U(2) symmetries can at most provide one particle generation. Last but not least, it turns out that for any T6/Z(2)xZ(2M) orbifold with discrete torsion and tilted tori, orientifold invariant stacks of D6-branes carry USp(2N) or SO(2N) gauge groups depending on their relative position to the exotic O6-plane. The new classification reduces the number of K-theory constraints. If time permits, I will briefly comment on the drastic change in the pattern of Yukawa couplings on orbifolds compared to the six-torus using a T6/Z6' example. As a result, quark Yukawas can provide large mixings while lepton Yukawas are flavour diagonal at leading order. Also the rule of `one massive generation only' is circumvented.