D-brane instanton effects in four-dimensional string vacua

by Timo Weigand (Pennsylvania University)

Tuesday 24 Jul 2007, 14:00 → 15:00 Europe/Zurich

TH AMPHITHEATRE (CERN)

We discuss D-brane instantons in four-dimensional string compactifications with special emphasis on Eucliden D2-branes in Type IIA orientifolds with spacetime filling D6-branes. Among other things, these can induce superpotential couplings involving the open string fields which are forbidden at the perturbative level since they violate some of the global U(1) symmetries generically present in string theory. Phenomenologcially important couplings of this type include Majorana mass terms for right-handed neutrinos and mu-terms in the Higgs sector of the MSSM. The exponential suppression of such non-perturbative terms avoids the notorious fine-tuning problem associated with the hierarchies characteristic of these couplings. D-brane instantons are also essential to generate the important 10 10 5 Yukawa coupling in SU(5) GUT models based on intersecting D6-branes. After discussing the general philosophy, we outline the prescription for the CFT computation of such non-perturbative superpotential couplings. A technical challenge, both conceptually and for concrete model building, is the uplifting of unwanted zero modes of instantons not invariant under the orientifold projection. We discuss strategies to achieve this aim. As an application, we demonstrate the generation of intermediate Majorana mass scales in a local intersecting brane setup.