D-brane Standard Model

TL;DR

This work embeds the Standard Model in a non-supersymmetric Type I/I' D-brane configuration with strong and electroweak sectors on separate brane stacks, allowing a TeV-scale string scale $M_s$ that naturally yields the observed value of $\sin^2\theta_W$. Hypercharge emerges as a linear combination of three $U(1)$ factors, with two viable embeddings that fix the relation among gauge couplings and predict $\sin^2\theta_W(M_s)$ matching data for $M_s \sim 6$–$8$ TeV; this framework necessitates two Higgs doublets to generate realistic fermion masses. Anomalous $U(1)$ cancellation via the Green-Schwarz mechanism renders the corresponding gauge bosons massive, leaving baryon number as a perturbatively exact global symmetry that protects proton stability. The model also predicts additional TeV-scale vector bosons and, depending on the hidden-brane sector, the possibility of a very light mediator (a fifth force) with characteristic ranges, all grounded in a concrete string-theoretic construction.

Abstract

The minimal embedding of the Standard Model in type I string theory is described. The SU(3) color and SU(2) weak interactions arise from two different collections of branes. The correct prediction of the weak angle is obtained for a string scale of 6-8 TeV. Two Higgs doublets are necessary and proton stability is guaranteed. It predicts two massive vector bosons with masses at the TeV scale, as well as a new superweak interaction.

Figures (1)

• Figure 1: The experimental value of $\sin^2\theta_W$ (thick curve), together with the theoretical predictions (\ref{['sintheta12']}) with $c_3=-1/3$ (dashed line) and (\ref{['sintheta13']}) with $c_3=2/3$ (dotted-dashed), are plotted as functions of the string scale $M_s$.