[Paper Review] AdS/QCD, Light-Front Holography, and Sublimated Gluons

Stanley J. Brodsky, Guy F. de Téramond|arXiv (Cornell University)|Dec 19, 2011

Black Holes and Theoretical Physics3 citations

TL;DR

This paper proposes a light-front holographic framework within AdS/QCD to model nonperturbative QCD dynamics, mapping fifth-dimensional fields in anti-de Sitter space to light-front wavefunctions in 3+1 dimensions. It successfully predicts linear Regge trajectories, form factors, and an effective coupling αₛ(Q²) with an infrared fixed point, consistent with experimental data from the Bjorken sum rule, supporting a sublimated gluon picture of confinement.

ABSTRACT

Gauge/gravity duality leads to a simple, analytical, and phenomenologically compelling nonperturbative approximation to the full light-front QCD Hamiltonian. This approach, called "Light-Front Holography", successfully describes the spectroscopy of light-quark meson and baryons, their elastic and transition form factors, and other hadronic properties. The bound-state Schrodinger and Dirac equations of the soft-wall AdS/QCD model predict linear Regge trajectories which have the same slope in orbital angular momentum L and radial quantum number n for both mesons and baryons. Light-front holography connects the fifth-dimensional coordinate of AdS space z to an invariant impact separation variable zeta in 3+1 space at fixed light-front time. A key feature is the determination of the frame-independent light-front wavefunctions of hadrons -- the relativistic analogs of the Schrodinger wavefunctions of atomic physics which allow one to compute form factors, transversity distributions, spin properties of the valence quarks, jet hadronization, and other hadronic observables. One thus obtains a one-parameter color-confining model for hadron physics at the amplitude level. AdS/QCD also predicts the form of a non-perturbative effective running coupling and its beta-function with an infrared fixed point which agrees with the effective coupling extracted from measurements of the Bjorken sum rule below 1 GeV^2. This is consistent with a flux-tube interpretation of QCD where soft gluons are sublimated into a color-confining potential for quarks. We discuss a number of phenomenological hadronic properties which support this picture.

Motivation & Objective

• To develop a frame-independent, nonperturbative model of hadron structure using light-front holography and AdS/QCD.
• To connect fifth-dimensional AdS fields to physical light-front wavefunctions via an invariant impact variable ζ.
• To reproduce hadronic spectroscopy, including linear Regge trajectories for mesons and baryons, with a single parameter.
• To derive the nonperturbative effective coupling αₛ(Q²) and its β-function from holographic dynamics.
• To provide a phenomenological explanation for confinement and anomalies like the J/ψ→ρπ puzzle through sublimated soft gluons.

Proposed method

• Use gauge/gravity duality to map classical gravity in AdS space to a light-front Hamiltonian in physical space-time.
• Implement light-front holography to relate the fifth-dimensional coordinate z to the invariant impact separation ζ in 3+1 dimensions.
• Solve a single-variable light-front Schrödinger-like equation in z, with a soft-wall dilaton profile exp(κ²z²), to determine hadronic spectra.
• Map the resulting eigenmodes Φ(z) to frame-independent light-front wavefunctions ψ(xᵢ, b⊥ᵢ) in physical space-time.
• Derive the effective coupling αₛ^(AdS)(Q²) and its β-function from the holographic model, matching QCD constraints.
• Use the model to compute form factors, transversity distributions, and spin-dependent observables such as the Sivers effect.

Experimental results

Research questions

• RQ1 Can light-front holography in AdS/QCD reproduce the linear Regge trajectories of light-quark mesons and baryons with a single parameter?
• RQ2 Does the holographically derived effective coupling αₛ^(AdS)(Q²) match the effective coupling α_g₁(Q²) extracted from the Bjorken sum rule below Q² < 1 GeV²?
• RQ3 How does the sublimation of soft gluons (Q² < 1 GeV²) into a confining potential explain anomalies in hadron scattering and decays?
• RQ4 Can the light-front wavefunctions derived via holography accurately describe hadronic form factors and spin-dependent distributions?
• RQ5 What is the role of the dilaton profile in generating confinement and matching QCD’s β-function behavior?

Key findings

• The soft-wall AdS/QCD model with a dilaton profile exp(κ²z²) successfully predicts linear Regge trajectories for both mesons and baryons with the same slope in orbital angular momentum L and radial quantum number n.
• The holographically derived effective coupling αₛ^(AdS)(Q²) exhibits an infrared fixed point and agrees quantitatively with α_g₁(Q²) extracted from the Bjorken sum rule for Q² < 1 GeV².
• The model predicts a transition from nonperturbative to perturbative regimes in the β-function at Q ~ 1 GeV, matching the expected QCD behavior.
• The light-front wavefunctions derived via holography encode the spin and orbital angular momentum structure of valence quarks, explaining the small quark spin contribution to nucleon spin.
• The framework accounts for the absence of the Landshoff mechanism in large-angle hadron scattering and resolves the J/ψ→ρπ decay puzzle through the sublimation of soft gluons into a confining potential.
• The model provides a quantitative description of factorization-breaking effects such as Sivers and Boer-Mulders functions, linking them to the nonperturbative structure of the gluon propagator.