Progress in D‐brane model building

Abstract

The state of the art in D-brane model building is briefly reviewed, focusing on recent achievements in the construction of D = 4 N = 1 type II string vacua with semi-realistic gauge sectors. Such progress relies on a better understanding of the spectrum of BPS D-branes, the effective field theory obtained from them and the explicit construction of vacua. We first consider D-branes in standard Calabi-Yau compactifications, and then the more involved case of compactifications with fluxes. We discuss how the non-trivial interplay between D-branes and fluxes modifies the previous model-building rules, as well as provides new possibilities to connect string theory to particle physics. MotivationAs often emphasized in the literature, string theory gained its popularity among physicists in the mid-80's, as it was recognized as an outstanding candidate for a unified theory of all forces in nature. Not only would it incorporate quantum gravity, but also supersymmetry and non-Abelian gauge interactions. With such ingredients it was soon realized that, upon compactification, one could conceive D = 4 N = 1 effective theories remarkably close to natural extensions of the Standard Model [1].Further developments of the theory showed that the set of string theory vacua is quite complex even at a qualitative level. On the one hand we learned that seemingly different vacua could be dual to each other and, in fact, should be considered to be the same. On the other hand, it has been estimated that the set of semi-realistic vacua is enormous. This latter point has recently raised the question of whether string theory can reproduce almost any D = 4 effective theory that one may think of and, in particular, many possible physics beyond the Standard Model. If that was the case, one may drop the classical strategy to focus on a particular string theory model by means of some vacuum selection principle, and instead try to extract physical information out of the ensemble of realistic vacua via an statistical approach [2]. Now, while string theory has proven to be an excellent generator of phenomenologically interesting scenarios, a string vacuum reproducing the physics observed at particle accelerators is yet to be found. Individual vacua analyzed so far not only fail to describe fully realistic D = 4 physics because of some technical details like extra massless particles or unrealistic couplings, but also because of deep conceptual issues like gauge coupling unification, hierarchy problems and supersymmetry breaking. Moreover, recent cosmological observations have slightly changed our perspective on how a semi-realistic vacuum should be obtained and, in general, agreement with cosmological data puts even more constraints on realistic string constructions.Pointing out these facts is not intended to discourage those interested in the field of model building. On the contrary, there has recently been huge progress (some of which the present review is based on) in constructing vacua which are more and more realistic, and we expect much more p...

Production of light stringy states

“…[31][32][33][34][35][36][37][38][39][40][41][42][43][44]). 3 Such a scenario is displayed in figure 1, where the Standard Model sector is localized on a small region of the full Calabi-Yau manifold. Note that the cycles wrapped by the SM D-brane sector are small, thus reproducing the observed SM gauge couplings, while the overall volume of the Calabi-Yau is large, allowing for a string scale in the TeV region opening the possibility for the direct detection of string excitations [45,46].…”

“…2 In principle after moduli stabilization one may even end up with a Calabi-Yau manifold that large, that the string scale M s is of the order of a few TeV [7][8][9]. Such a scenario provides a solution to the hierarchy and cosmological constant problems but may also lead to interesting signatures observable at the LHC such as signs 1 For recent reviews on D-brane model building as well as specific MSSM D-brane constructions, see [1][2][3][4] and references therein. 2 As shown in [5,6] under specific circumstances the Standard model D-branes do not have to wrap small cycles, since due to 1-loop corrections to the gauge couplings, there can be large cancellations among tree level and one-loop contributions, leading still to a gauge coupling compatible with observations.…”

Sparticle spectra and LHC signatures for large volume string compactifications

“…A second source of uncertainty is the spectrum itself. We know that typical quasi-realistic D-brane models (see [69] for a recent review) usually have extra particles beyond the MSSM and that the hypercharge does not have a canonical normalisation. We took into account these effects by varying the hypercharge normalisation and finding observables, such as the ratios of gaugino masses, which do not change if there are extra fields beyond the MSSM.…”

Rigid D6-branes on $ {{{{T^6}}} \left/ {{\left( {{Z_2} \times {Z_{2M}} \times \Omega \mathcal{R}} \right)}} \right.} $ with discrete torsion

“…There are several good reviews on this kind of model building containing also references to the original literature (including the T-dual constructions of magnetised branes in IIB string theory), e.g. [21][22][23][24][25][26][27]. Geometrically, intersecting D6-brane constructions are quite intuitive.…”