D=4 chiral string compactifications from intersecting branes

Abstract

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.

Chiral D-brane Models with Frozen Open String Moduli

“…Indeed, in orientifold models cancellation of RR tadpoles goes beyond homology constraints of the form (23), and also implies the cancellation of K-theory torsion charges [81]. These extra constraints have proven to be non-trivial in intersecting/magnetized D-brane models [38,83], rendering some of the known models in the literature inconsistent.…”

“…In particular, important issues, such as the pattern of soft SUSY breaking terms [11][12][13][14][15] and the signatures and stability of cosmic strings (see [16][17][18] for some reviews) formed at the end of brane inflation [19,20], depend critically on how the Standard Model (SM) is embedded. In the past few years, the intersecting brane (and, in the T-dual picture, magnetized D-brane) world scenario [21][22][23][24][25][26] has proven to be a promising framework to construct semi-realistic, D = 4, N = 1 chiral string models [27][28][29][30][31][32]. 1 More importantly, it has recently been realized [11,37,38] that by turning on background NSNS and RR fluxes in magnetized D-branes models, SUSY can be softly broken while satisfying the supergravity equation of motion (hence the usual problem of the classical instability in non-supersymmetric models due to non-vanishing NSNS tadpoles is absent).…”

Type I strings with F- and B-flux

“…In principle, this can be reconciled by lowering the string scale to the electroweak regime in the way of a large volume compactification. As was pointed out in [14,15] for the case of type II theory, such intersecting brane world models have appealing phenomenological implications, as for instance the absence of perturbative proton decay and a possible hierarchy of the Yukawa couplings. However, for D-branes at angles there generically appear tachyons in the spectrum, which in type II theory may trigger a decay into the vacuum.…”

“…This will work just as in the type II scenario of [14,15], where, moreover, the first problem did not arise at all. Another general feature of all the non-supersymmetric vacua is, that there remains an uncancelled NSNS tadpole which leads to a shift in the background, as shown in [16].…”

Constraining the string scale: from Planck to weak and back again

“…The first semirealistic models were constructed in [24] and soon after in [25] and [26] (see [27] for some related technical developments). These initial models presented additional gauge symmetries or matter content beyond the ones in the SM.…”