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

S. A. Abel, J. Santiago

TL;DR

The paper investigates whether and how the string scale can be lowered from the Planck scale toward accessible energies, surveying weak/strong heterotic and brane-world constructions. It clarifies how vacuum degeneracy and large extra dimensions can decouple gravity from gauge interactions, and it analyzes intermediate scenarios with kinetic mixing that affect SUSY breaking and flavor phenomenology, including millicharged states. It then explores TeV-scale string models built from intersecting branes, highlighting their potential to realize MSSM-like spectra and distinctive flavor signatures. Overall, it argues that current flavor and precision measurements already probe string-scale physics in many setups, while concrete brane constructions offer falsifiable predictions for future experiments.

Abstract

String and field theory ideas have greatly influenced each other since the so called second string revolution. We review this interrelation paying particular attention to its phenomenological implications. Our guiding principle is the radical shift in the way that we think about the fundamental scale, in particular the way in which string models have been able to accommodate values from the Planck $M_\mathrm{Pl}\sim 10^{18}$ GeV down to the electroweak scale $M_{EW}\sim $ TeV.

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

TL;DR

The paper investigates whether and how the string scale can be lowered from the Planck scale toward accessible energies, surveying weak/strong heterotic and brane-world constructions. It clarifies how vacuum degeneracy and large extra dimensions can decouple gravity from gauge interactions, and it analyzes intermediate scenarios with kinetic mixing that affect SUSY breaking and flavor phenomenology, including millicharged states. It then explores TeV-scale string models built from intersecting branes, highlighting their potential to realize MSSM-like spectra and distinctive flavor signatures. Overall, it argues that current flavor and precision measurements already probe string-scale physics in many setups, while concrete brane constructions offer falsifiable predictions for future experiments.

Abstract

String and field theory ideas have greatly influenced each other since the so called second string revolution. We review this interrelation paying particular attention to its phenomenological implications. Our guiding principle is the radical shift in the way that we think about the fundamental scale, in particular the way in which string models have been able to accommodate values from the Planck GeV down to the electroweak scale TeV.

Paper Structure

This paper contains 8 sections, 24 equations, 12 figures, 1 table.

Table of Contents

  1. Vacuum degeneracy: vice or virtue?
  2. The road to $M_s = M_W$
  3. From $M_s \sim M_{\mathrm{Pl}}$ to $M_s \sim M_{\mathrm{GUT}}$: weakly and strongly coupled heterotic models
  4. Intermediate models
  5. Kinetic Mixing and Millicharged particles
  6. Kinetic mixing and SUSY breaking
  7. Other aspects
  8. $M_s \sim$TeV: Branes at angles

Figures (12)

  • Figure 1: Brane world picture with 4 large flat dimensions represented as a plane and extra small dimensions determining the different scales of nature.
  • Figure 2: M-theory road map
  • Figure 3: Horava-Witten construction for the strong coupling limit of the $E_8\times E_8$ heterotic string. The green planes represent the 10-dimensional boundaries of the orbifold $S^1/Z_2$ where each $E_8$ factor lives, 11-dimensional supergravity propagates in the bulk.
  • Figure 4: D-brane realization of a U(2) gauge group.
  • Figure 5: Schematic picture of the bottom-up approach. The small blue points represent the local configuration of D-branes leading to the MSSM whereas the large green blob represent the global structure, less important from a phenomenological viewpoint.
  • ...and 7 more figures