We consider the E 8 ×E 8 heterotic string theory compactified on Calabi-Yau manifolds with bundles containing abelian factors in their structure group. Generic low energy consequences such as the generalised Green-Schwarz mechanism for the multiple anomalous abelian gauge groups are studied. We also compute the holomorphic gauge couplings and induced Fayet-Iliopoulos terms up to one-loop order, where the latter are interpreted as stringy one-loop corrections to the Donaldson-Uhlenbeck-Yau condition. Such models generically have frozen combinations of Kähler and dilaton moduli. We study concrete bundles with structure group SU (N ) × U (1) M yielding quasi-realistic gauge groups with chiral matter given by certain bundle cohomology classes. We also provide a number of explicit tadpole free examples of bundles defined by exact sequences of sums of line bundles over complete intersection Calabi-Yau spaces. This includes one example with precisely the Standard Model gauge symmetry.

Continuing our recent work hep-th/0411173, we study the statistics of fourdimensional, supersymmetric intersecting D-brane models in a toroidal orientifold background. We have performed a vast computer survey of solutions to the stringy consistency conditions and present their statistical implications with special emphasis on the frequency of Standard Model features. Among the topics we discuss are the implications of the K-theory constraints, statistical correlations among physical quantities and an investigation of the various statistical suppression factors arising once certain Standard Model features are required. We estimate the frequency of an MSSM like gauge group with three generations to be one in a billion.

We investigate the statistics of the phenomenologically important D-brane sector of string compactifications. In particular for the class of intersecting D-brane models, we generalise methods known from number theory to determine the asymptotic statistical distribution of solutions to the tadpole cancellation conditions. Our approach allows us to compute the statistical distribution of gauge theoretic observables like the rank of the gauge group, the number of chiral generations or the probability of an SU (N ) gauge factor. Concretely, we study the statistics of intersecting branes on T 2 and T 4 /Z Z 2 and T 6 /Z Z 2 × Z Z 2 orientifolds.Intriguingly, we find a statistical correlation between the rank of the gauge group and the number of chiral generations. Finally, we combine the statistics of the gauge theory sector with the statistics of the flux sector and study how distributions of gauge theoretic quantities are affected. 11/2004

Using previous results on the statistics of intersecting D6-brane models on T 6 /Z ′ 6 we analyse solutions that resemble the gauge group and matter content of the supersymmetric standard model. In particular the structure of the hidden sector, the gauge coupling constants and chiral exotic matter content are computed and classified for all possible standard model-like configurations on this background. It turns out that the number of chiral exotics, Higgses and values of gauge couplings are strongly correlated. Some examples with the chiral spectrum of the supersymmetric standard model plus vector-like states and a massless U(1) B−L are discussed in detail.fgmeiner@nikhef.nl Gabriele.Honecker@cern.ch

In this paper, globally N 1 supersymmetric configurations of intersecting D6-branes on the Z 6 orientifold are discussed, involving also fractional branes. It turns out rather miraculously that one is led almost automatically to just one particular class of five stack models containing the standard model gauge group, which all have the same chiral spectrum. The further discussion shows that these models can be understood as exactly the supersymmetric standard model without any exotic chiral symmetric/ antisymmetric matter. The superpartner of the Higgs finds a natural explanation and the hypercharge remains massless. However, the nonchiral spectrum within the model class is very different and does not in all cases allow for a N 2 low energy field theoretical understanding of the necessary breaking U1 U1 ! U1 along the Higgs branch, which is needed in order to get the standard Yukawa couplings. Also the left-right symmetric models belong to exactly one class of chiral spectra, where the two kinds of exotic chiral fields can have the interpretation of forming a composite Higgs. The aesthetical beauty of these models, involving only nonvanishing intersection numbers of an absolute value three, seems to be unescapable.