AdS/QCD and light front holography: A new approximation to QCD

The combination of Anti-de Sitter space (AdS) methods with light-front holography leads to a semi-classical (list approximation to the spectrum and wavefunctions of meson and baryon light-quark bound states Starting from the bound-state Hamiltonian equation of motion in QCD; we derive relativistic light-front wave equations in terms of an invariant impact variable zeta which measures the separation of the quark and gluonic constituents within the hadron at equal light-front time These equations of motion in physical space-time are equivalent to the equations of motion which describe the propagation of spin-J modes in anti-de Sitter (AdS) space Its eigenvalues give the hadronic spectrum, and its eigenmodes represent the probability distributions of the hadronic constituents at a given scale Applications to the light meson and baryon spectra are presented The predicted meson spectrum has a string-theory Regge form M-2 = 4 kappa(2)(n+L+S/2), i e the square of the eigenmass is linear in both L and n, where n counts the number of nodes of the wavefunction in the radial variable zeta The space-like pion and nucleon form factors are also well reproduced One thus obtains a remarkable connection between the description of hadronic modes in AdS space and the Hamiltonian formulation of QCD in physical space-tune quantized on the light-front at fixed light-front time T The 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 in order to systematically include the QCD interaction terms