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Localized U(1) Gauge Fields, Millicharged Particles, and Holography

Brian Batell, Tony Gherghetta

TL;DR

The paper analyzes how a U(1) gauge field propagating in a slice of AdS5 can have its zero mode localized on the UV or IR brane by tuning bulk and brane mass terms, enabling millicharged fermions and distinct electroweak realizations without requiring fermion delocalization.A 5D Stückelberg framework maintains gauge invariance while incorporating bulk and boundary masses, and a detailed analysis of the photon propagator shows that localization decides whether the photon is elementary or composite in the dual 4D theory.Using AdS/CFT, the authors map the 5D results to a dual 4D CFT with a source field, deriving how millicharges arise from coupling to a composite photon and how kinetic mixing between U(1)s is enhanced by the conformal sector, with consistent high- and low-energy behavior.Electroweak implications are discussed, showing that localizing the U(1)_Y gauge boson near the IR brane can avoid stringent constraints, and the framework provides a coherent mechanism for millicharges and emergent photon phenomena in a holographic context.

Abstract

We consider U(1) gauge fields in a slice of AdS_5 with bulk and boundary mass parameters. The zero mode of a bulk U(1) gauge field can be localized either on the UV or IR brane. This leads to a simple model of millicharged particles in which fermions can have arbitrarily small electric charge. In the electroweak sector we also discuss phenomenological implications of a localized U(1)_Y gauge boson. Using the AdS/CFT correspondence we present the 4D holographic interpretation of the 5D model. In particular the photon is shown to be a composite particle when localized near the IR brane, whereas it is elementary when localized near the UV brane. In the dual interpretation the ``millicharge'' results from an elementary fermion coupling to a composite photon via a vector current with large anomalous dimension.

Localized U(1) Gauge Fields, Millicharged Particles, and Holography

TL;DR

The paper analyzes how a U(1) gauge field propagating in a slice of AdS5 can have its zero mode localized on the UV or IR brane by tuning bulk and brane mass terms, enabling millicharged fermions and distinct electroweak realizations without requiring fermion delocalization.A 5D Stückelberg framework maintains gauge invariance while incorporating bulk and boundary masses, and a detailed analysis of the photon propagator shows that localization decides whether the photon is elementary or composite in the dual 4D theory.Using AdS/CFT, the authors map the 5D results to a dual 4D CFT with a source field, deriving how millicharges arise from coupling to a composite photon and how kinetic mixing between U(1)s is enhanced by the conformal sector, with consistent high- and low-energy behavior.Electroweak implications are discussed, showing that localizing the U(1)_Y gauge boson near the IR brane can avoid stringent constraints, and the framework provides a coherent mechanism for millicharges and emergent photon phenomena in a holographic context.

Abstract

We consider U(1) gauge fields in a slice of AdS_5 with bulk and boundary mass parameters. The zero mode of a bulk U(1) gauge field can be localized either on the UV or IR brane. This leads to a simple model of millicharged particles in which fermions can have arbitrarily small electric charge. In the electroweak sector we also discuss phenomenological implications of a localized U(1)_Y gauge boson. Using the AdS/CFT correspondence we present the 4D holographic interpretation of the 5D model. In particular the photon is shown to be a composite particle when localized near the IR brane, whereas it is elementary when localized near the UV brane. In the dual interpretation the ``millicharge'' results from an elementary fermion coupling to a composite photon via a vector current with large anomalous dimension.

Paper Structure

This paper contains 17 sections, 84 equations, 1 figure.

Table of Contents

  1. Introduction
  2. Abelian gauge fields in warped space
  3. Massless bulk gauge fields
  4. Massive bulk gauge fields
  5. Localizing the photon
  6. Modification of the propagator
  7. Phenomenological implications
  8. Millicharged particles
  9. Kinetic mixing
  10. Electroweak model
  11. Electroweak constraint
  12. Holographic interpretation
  13. $\alpha_-$ branch holography
  14. Interactions with external fermions
  15. $\alpha_+$ branch holography
  16. ...and 2 more sections

Figures (1)

  • Figure 1: The Feynman diagrams in the 4D dual theory responsible for the corrections to the propagator. The source field $A_\mu \,$, interacts with the CFT contribution, indicated by the blob.