[Paper Review] D-brane Standard Model

Ignatios Antoniadis, Elias Kiritsis+1 more|Nov 29, 2001

Adaptive optics and wavefront sensing4 citations

TL;DR

This paper proposes a non-supersymmetric D-brane realization of the Standard Model in type I string theory, where SU(3) and SU(2) gauge groups arise from separate D-brane stacks, and hypercharge is embedded via anomaly cancellation. It predicts the correct weak mixing angle at a string scale of 6–8 TeV, ensures proton stability via baryon number conservation, and introduces a TeV-scale massive U(1) gauge boson mediating a new short-range force.

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.

Motivation & Objective

• To construct a realistic, non-supersymmetric embedding of the Standard Model in type I string theory using D-branes.
• To explain the observed value of the weak mixing angle sin²θW without gauge coupling unification.
• To ensure proton stability through a perturbatively exact global baryon number symmetry.
• To identify the origin of two Higgs doublets and their role in fermion mass generation.
• To explore the phenomenological consequences of anomalous U(1) gauge bosons from extra D-branes.

Proposed method

• Realize the SM gauge group SU(3)×SU(2)×U(1) via three distinct D-brane stacks: U(3) for color, U(2) for weak isospin, and an additional U(1) from a separate D-brane.
• Identify the U(1) factors of U(3) and U(2) with gauged baryon number and weak-doublet number, respectively.
• Use the Green-Schwarz mechanism to cancel anomalies in the U(1) gauge groups, giving mass to the corresponding gauge bosons at the string scale.
• Embed hypercharge via a linear combination of the three U(1) charges, fixing the normalization to reproduce sin²θW ≈ 0.23.
• Require two Higgs doublets to generate masses for all quarks and leptons while preserving baryon number.
• Analyze the couplings and masses of the anomalous U(1) gauge bosons, showing they can mediate a new short-range force with TeV-scale mass.

Experimental results

Research questions

• RQ1 Can the weak mixing angle be naturally predicted in a non-supersymmetric D-brane model of the Standard Model?
• RQ2 How can proton stability be preserved in a non-supersymmetric string-theoretic model with a low string scale?
• RQ3 What is the role of two Higgs doublets in a D-brane construction where fermion masses are generated via Yukawa couplings?
• RQ4 What are the phenomenological signatures of anomalous U(1) gauge bosons arising from extra D-branes?
• RQ5 Can the strong and weak couplings be explained without unification, using different D-brane stacks with distinct gauge couplings?

Key findings

• The model predicts sin²θW ≈ 0.23 for a string scale of 6–8 TeV, matching the observed value without requiring gauge coupling unification.
• Proton stability is guaranteed by a perturbatively exact global baryon number symmetry, arising as a remnant of an anomalous U(1) gauge symmetry.
• Two Higgs doublets are necessary to generate masses for all quarks and leptons, with both acquiring vacuum expectation values.
• The anomalous U(1) gauge boson acquires a mass of order the string scale (6–8 TeV), leading to a new short-range force with a range of ~40 μm if light, or a TeV-scale mass if heavy.
• The model predicts two massive vector bosons at the TeV scale, arising from the anomalous U(1) gauge groups, with couplings fixed by charge assignments and anomalies.
• Instanton effects can explicitly break the Peccei-Quinn-type global symmetry associated with the linear combination of Higgses, avoiding a massless Goldstone boson.