  Light-Front Hamiltonian theory, derived from the quantization of the QCD
Lagrangian at fixed light-front time \tau = t+z/c, provides a rigorous
frame-independent framework for solving nonperturbative QCD. The eigenvalues of
the light-front QCD Hamiltonian predict the hadronic mass spectrum, and the
eigensolutions provide the light-front wavefunctions describing hadron
structure. The valence Fock-state wavefunctions of the light-front QCD
Hamiltonian satisfy a single-variable relativistic equation of motion,
analogous to the nonrelativistic radial Schrodinger equation, with an effective
confining potential U which systematically incorporates the effects of higher
quark and gluon Fock states. The potential U has a unique form if one requires
that the action for zero quark mass remains conformally invariant. The
holographic mapping of gravity in AdS space to QCD with a specific soft-wall
dilaton yields the same light-front Schrodinger equation. It also gives a
precise relation between the bound-state amplitudes in the fifth dimension z of
AdS space and the boost-invariant light-front wavefunctions describing the
internal structure of hadrons in physical space-time. The elastic and
transition form factors of the pion and the nucleons are found to be well
described in this framework. The predictions include a zero-mass pion in the
chiral limit, and linear Regge trajectories with the same slope in the radial
quantum number n and orbital angular momentum L. The light-front AdS/QCD
holographic approach thus gives a frame-independent representation of
color-confining dynamics and the excitation spectra of light-quark hadrons in
terms of a single mass parameter. We also discuss the implications of the
underlying conformal template of QCD for renormalization scale-setting and the
implications of light-front quantization for the value of the cosmological
constant.
