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 Z N N = 1, 0 orbifold singularities leading to the SM group or some left-right symmetric extension. 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 Z 3 , predicting sin 2 θ 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 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 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 N = 1 supersymmetric embeddings of our local structures, as we show in an explicit example.

We construct N = 1 supersymmetric four-dimensional orientifolds of type IIA on T 6 /(Z 2 × Z 2 ) with D6-branes intersecting at angles. The use of D6branes not fully aligned with the O6-planes in the model allows for a construction of many supersymmetric models with chiral matter, including those with the Standard Model and grand unified gauge groups. We perform a search for realistic gauge sectors, and construct the first example of a supersymmetric type II orientifold with SU (3) C × SU (2) L × U (1) Y gauge group and three quark-lepton families. In addition to the supersymmetric Standard Model content, the model contains right-handed neutrinos, a (chiral but anomalyfree) set of exotic multiplets, and diverse vector-like multiplets. The general class of these constructions are related to familiar type II orientifolds by small instanton transitions, which in some cases change the number of generations, as discussed in specific models. These constructions are supersymmetric only for special choices of untwisted moduli. We briefly discuss the supersymmetry breaking effects away from that point. The M-theory lift of this general class of supersymmetric orientifold models should correspond to purely geometrical backgrounds admitting a singular G 2 holonomy metric and leading to four-dimensional M-theory vacua with chiral fermions.

Intersecting Dp-branes often give rise to chiral fermions living on their intersections.We study the construction of four-dimensional chiral gauge theories by considering configurations of type II D(3 + n)-branes wrapped on non-trivial n-cycles on T 2n × (R 2(3−n) /Z N ), for n = 1, 2, 3. The gauge theories on the four non-compact dimensions of the brane world-volume are generically chiral and non-supersymmetric. We analyze consistency conditions (RR tadpole cancellation) for these models, and their relation to four-dimensional anomaly cancellation. Cancellation of U (1) gauge anomalies involves a Green-Schwarz mechanism mediated by RR partners of untwisted and/or twisted moduli. This class of models is of potential phenomenological interest, and we construct explicit examples of SU (3) × SU (2) × U (1) three-generation models. The models are non-supersymmetric, but the string scale may be lowered close to the weak scale so that the standard hierarchy problem is avoided. We also comment on the presence of scalar tachyons and possible ways to avoid the associated instabilities. We discuss the existence of (meta)stable configurations of D-branes on 3-cycles in (T 2 ) 3 , free of tachyons for certain ranges of the six-torus moduli.

We construct the first three family N 1 supersymmetric string model with standard model gauge group SU͑3͒ C 3 SU͑2͒ L 3 U͑1͒ Y from an orientifold of type IIA theory on T 6 ͑͞Z 2 3 Z 2 ͒ and D6-branes intersecting at angles. In addition to the minimal supersymmetric standard model particles, the model contains right-handed neutrinos, a chiral (but anomaly-free) set of exotic multiplets, and extra vectorlike multiplets. We discuss some phenomenological features of this model. DOI: 10.1103/PhysRevLett.87.201801 PACS numbers: 11.10.Kk, 11.25.Mj, 12.60.Jv, 98.80.Cq The space of classical string vacua is highly degenerate, and at present we are unable to make definitive statements about how the string vacuum describing our universe is selected. Nonetheless, one can use phenomenological constraints as guidelines to construct semirealistic string models and explore, with judicious assumptions, the resulting phenomenology. The purpose of such explorations is, of course, not to find the model which would fully describe our world, but to examine the generic features of these string derived solutions.Until a few years ago, such explorations were carried out mainly in the framework of weakly coupled heterotic string theory. Indeed, a number of semirealistic string models have been constructed and analyzed [1]. However, an important lesson from string duality is that these models represent only a corner of M theory -the string vacuum describing our world may well be in a completely different regime in which the perturbative description of heterotic string theory breaks down [2]. Fortunately, the advent of D-branes allows for the construction of semirealistic string models in another calculable regime, as illustrated by the various four-dimensional N 1 supersymmetric type II orientifolds ([3-12] and references therein) constructed using conformal field theory techniques. However, the constraints on supersymmetric fourdimensional models are rather restrictive, leading to not fully realistic gauge sectors and matter contents. Motivated by the search for standard model-like solutions, several discrete or continuous deformations of this class of models have been explored. They include the following: (i) blowing-up of orientifold singularities [13,14], (ii) locating the branes at different points in the internal space (see, e.g., [9,12,15]) which in a T -dual picture corresponds to turning on continuous or discrete Wilson lines, (iii) introduction of discrete values for the Neveu-Schwarz -Neveu-Schwarz (NS-NS) B field [7,16] which in the T-dual picture corresponds to tilting the compactification tori, (iv) introduction of gauge fluxes in the D-brane world volumes (see [17] for an earlier discussion, and [18,19] for supersymmetric D 6 models), which in the T -dual version corresponds to D-branes intersecting at angles (hence closely related to models in [20,21]).An appealing feature of (iv) is that generically, there exists chiral fermions where D-branes intersect [22]. Their multiplicity is hence determined by a topological q...

It is known that chiral fermions naturally appear at certain intersections of branes at angles. Motivated by this fact, we propose a string scenario in which different standard model gauge interactions propagate on different (intersecting) brane worlds, partially wrapped in the extra dimensions. Quarks and leptons live at brane intersections, and are thus located at different positions in the extra dimensions. Replication of families follows naturally from the fact that the branes generically intersect at several points.Gauge and Yukawa couplings can be computed in terms of the compactification radii.Hierarchical Yukawa couplings appear naturally, since amplitudes involving three different intersections are proportional to e −A ijk , where A ijk is the area of a string world-sheet extending among the intersections. The models are non-supersymmetric but the string scale may be lowered down to 1-10 TeV. The proton is however stable due to a set of discrete symmetries arising from world-sheet selection rules, exact to all orders in perturbation theory. The scenario has some distinctive features like the presence of KK, winding and other new excited states ('gonions'), with masses below the string scale and accessible to accelerators. The models contain scalar tachyons with the quantum numbers of standard SU (2) × U (1) Higgs doublets, and we propose that they induce electroweak symmetry breaking in a somewhat novel way. Specific string models with D4-branes wrapping on T 2 × (T 2 ) 2 /Z N , leading to three-family semirealistic spectra, are presented in which the above properties are exemplified.