D-Branes at Singularities :
The Standard Model
We propose a bottom-up approach to the building of particle physics models from string theory.
Our building blocks are Type II D-branes which we combine appropriately to reproduce desirable features of a particle theory model: 1) Chirality ; 2) Standard Model group ; 3) N=1 or N=0 supersymmetry ; 4) Three quark-lepton generations.
We start such a program by studying configurations of D=10, Type IIB D3-branes located at singularities. We study in detail the case of Z_N, N=1,0 orbifold singularities leading to the SM group or some left-right symmetricextension.
In general, tadpole cancellation conditions require the presence of additional branes, e.g. D7-branes. For the N=1 supersymmetric case the unique twist leading to three quark-lepton generations is Z_3, predicting \sin^2\theta_W=3/14=0.21$.
The models obtained are the simplest semirealistic string models ever built. In the non-supersymmetric case there is a three-generation model for each Z_N, N>4, but the Weinberg angle is in general too small.
One can obtain a large class of D=4 compact models by considering the above structure embedded into a Calabi Yau compactification.
We explicitly construct examples of such compact models using Z_3 toroidal orbifolds and orientifolds, and discuss their properties.
In these examples, global cancellation of RR charge may be achieved by adding anti-branes stuck at the fixed points, leading to models with hidden sector gravity-induced supersymmetry breaking.
More general frameworks, like F-theory compactifications, allow completely \NN=1 supersymmetric embeddings of our local structures, as we show in an explicit example.
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