D-Branes at singularities: a bottom-up approach to the string embedding of the standard model

We propose a bottom-up approach to particle physics model building from string theory. We start this program by using configurations of type-IIB D3- and D7-branes located at singularities, combined appropriately to reproduce phenomenologically desirable features. We study in detail _N N = 1,0 orbifold singularities leading to SM or LR symmetric gauge group. The unique N = 1 supersymmetric twist leading to three generations is ₃, predicting sin ²θ_W = 3/14 = 0.21, and yielding the simplest semirealistic string models ever built. In the non-supersymmetric case, each _N, N > 4, leads to three-generation models, but the Weinberg angle is in general too small. One can obtain a large class of D = 4 compact models by embedding the above structure into a Calabi Yau compactification. We construct explicit examples using T⁶₃ orbifolds and orientifolds, with additional antibrane hidden sectors, leading to gravity mediated supersymmetry breaking. We construct an explicit F-theory compactification with unbroken N = 1 supersymmetry,with a three-family LR sector.