## Tuesday, May 17, 2005

### Gary Shiu's models

Gary Shiu gave the last talk at Columbia, and it was about
• the search for realistic vacua in the landscape.

He showed a picture what such a search looks like: a five-year-old boy is searching for something - apparently a needle - in the haystack. In fact, the boy was Burt Ovrut. The picture offers a striking support for the statement that the anthropic landscape is really the anthropic haystack: Nima Arkani-Hamed confirmed this conjecture by pointing out that the landscape looks like a two-dimensional object.

Gary reviewed basics of superstring model building - stabilization of moduli, SUSY breaking that is apparently able to stabilize all the moduli by itself, the black box treatment of SUSY breaking via soft SUSY breaking terms, and the difficulties to obtain the Standard Model in flux compatifications.

Nevertheless his aim was to describe an example: an MSSM from the flux vacuum at the orbifold

His picture how a compactification looks like was an octopus with many throats - such as the MSSM throat, the inflation throat (that also gives us cosmic strings), and so on. Although this separation of different problems may be appealing for many purposes, I generally find such a picture repelling because progress in theoretical physics should reveal new relations between previously unrelated notions instead of simply approving that they are unrelated.

A difficult problem is to obtain a chiral gauge theory. In the context of flux vacua, this goal is achieved by one of two methods:

• D-branes on orbifold singularities
• magnetized D-branes: this catchy phrase describes D-branes with a non-trivial gauge bundle

The type IIB magnetized D-branes are T-dual to intersecting D-branes in type IIA. The type IIA picture is however difficult to deal with because it involves non-Kähler geometry, which is why these people stay in the type IIB context. In the magnetized D-brane framework, the number of chiral generations is essentially counted by the flux. The Pati-Salam models are natural in this context. The dilaton tadpole may be computed as the number of D3-branes but we really want D9-anti-D9 with a bundle that carries the same charges. One must be careful about the Z2-valued K-theoretical charges. They must be cancelled otherwise Witten's anomaly would appear on lower-dimensional D-branes, Gary argued.

How do the fluxes affect the open string moduli? The self-dual fluxes cancel the forces against gravity, via the usual BPS cancellations, while the anti-self-dual ones double it and they stabilize the positions of D-branes. However, the Wilson lines remain flat directions.