QCD and Light-Front Dynamics

TL;DR

This paper explores how AdS/QCD and light-front holography provide a semiclassical framework for understanding hadron structure, spectrum, and dynamics, linking higher-dimensional theories to physical space-time wavefunctions.

Contribution

It introduces a unified approach using light-front holography to derive hadron wavefunctions and spectra from AdS/QCD, including novel insights into confinement and non-perturbative QCD effects.

Findings

Derivation of a single-variable light-front Schrödinger equation for hadrons.

Prediction of hadron spectra and wavefunctions from the soft-wall AdS/QCD model.

Insights into confinement effects on quark and gluon condensates.

Abstract

AdS/QCD, the correspondence between theories in a dilaton-modified five-dimensional anti-de Sitter space and confining field theories in physical space-time, provides a remarkable semiclassical model for hadron physics. Light-front holography allows hadronic amplitudes in the AdS fifth dimension to be mapped to frame-independent light-front wavefunctions of hadrons in physical space-time. The result is a single-variable light-front Schrodinger equation which determines the eigenspectrum and the light-front wavefunctions of hadrons for general spin and orbital angular momentum. The coordinate z in AdS space is uniquely identified with a Lorentz-invariant coordinate zeta which measures the separation of the constituents within a hadron at equal light-front time and determines the off-shell dynamics of the bound state wavefunctions as a function of the invariant mass of the constituents. The hadron eigenstates generally have components with different orbital angular momentum; e.g., the proton eigenstate in AdS/QCD with massless quarks has L=0 and L=1 light-front Fock components with equal probability. Higher Fock states with extra quark-anti quark pairs also arise. The soft-wall model also predicts the form of the non-perturbative effective coupling and its beta-function. The AdS/QCD model can be systematically improved by using its complete orthonormal solutions to diagonalize the full QCD light-front Hamiltonian or by applying the Lippmann-Schwinger method to systematically include QCD interaction terms. Some novel features of QCD are discussed, including the consequences of confinement for quark and gluon condensates. A method for computing the hadronization of quark and gluon jets at the amplitude level is outlined.