New Standard Model Vacua from Intersecting Branes

TL;DR

This work constructs explicit non-supersymmetric Standard Model vacua from five stacks of intersecting D6-branes on a toroidal orientifold, achieving the SM spectrum with right-handed neutrinos at low energy. Hypercharge remains massless through a generalized Green–Schwarz mechanism while extra U(1)s generically acquire mass, with proton stability ensured by gauged baryon number and neutrinos obtaining Dirac masses via a PQ-like symmetry. A supplementary SUSY-based mechanism at a single intersection can further break the remaining anomaly-free U(1)s, potentially yielding an intermediate scale and a Z' boson. Overall, the paper demonstrates a concrete string-theoretic route to realistic SM-like vacua without GUTs, incorporating distinctive neutrino and Higgs-sector features tied to brane angles and tachyon dynamics.

Abstract

We construct new D6-brane model vacua (non-supersymmetric) that have at low energy exactly the standard model spectrum (with right handed neutrinos). The minimal version of these models requires five stacks of branes. and the construction is based on D6-branes intersecting at angles in $D = 4$ type toroidal orientifolds of type I strings. Three U(1)'s become massive through their couplings to RR couplings and from the two surviving anomaly free U(1)'s, one is the standard model hypercharge generator while the extra anomaly free U(1) could be broken from its non-zero couplings to RR fields and also by triggering a vev to previously massive particles. We suggest that extra massless U(1)'s should be broken by requiring some intersection to respect N=1 supersymmetry thus supporting the appearance of massless charged singlets at the supersymmetric intersection. Proton is stable as baryon number is gauged and its anomalies are cancelled through a generalized Green-Schwarz mechanism. Neutrinos are of Dirac type with small masses, as in the four stack standard models of hep-th/0105155, as a result of the existence of a similar PQ like-symmetry. The models are unique in the sense that they predict the existence of only one supersymmetric particle, the superpartner of $ν_R$.

Figures (1)

• Figure 1: Assignment of angles between D6-branes on the five stack type I model giving rise to the SM at low energies. The angles between branes are shown on a product of $T^2 \times T^2 \times T^2$. We have chosen $\beta^1 =\beta^2 =1$, $n_b^1, n_c^1, n_a^2, n_d^2 >0$, $\epsilon = {\tilde{\epsilon}} = 1$.