21,897 research outputs found
QCD and Light-Front Holography
The soft-wall AdS/QCD model, modified by a positive-sign dilaton metric,
leads to a remarkable one-parameter description of nonperturbative hadron
dynamics. The model predicts a zero-mass pion for zero-mass quarks and a Regge
spectrum of linear trajectories with the same slope in the leading orbital
angular momentum of hadrons and the radial quantum number . Light-Front
Holography maps the amplitudes which are functions of the fifth dimension
variable of anti-de Sitter space to a corresponding hadron theory quantized
on the light front. The resulting Lorentz-invariant relativistic light-front
wave equations are functions of an invariant impact variable which
measures the separation of the quark and gluonic constituents within the hadron
at equal light-front time. The result is a semi-classical frame-independent
first approximation to the spectra and light-front wavefunctions of meson and
baryon light-quark bound states, which in turn predict the behavior of the pion
and nucleon form factors. The effects of chiral symmetry breaking increase as
one goes toward large interquark separation, consistent with spectroscopic
data, and 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. The
soft-wall model also predicts the form of the non-perturbative effective
coupling which agrees with the effective coupling extracted
from the Bjorken sum rule. 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 in order
to systematically include the QCD interaction terms. A new perspective on quark
and gluon condensates is also reviewed.Comment: Invited talk, presented by SJB at the 50th Crakow School, Zakopane,
Poland; final version to appear in proceeding
AdS/QCD, Light-Front Holography, and Sublimated Gluons
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.Comment: Invited talk, presented by SJB at the International Workshop on QCD
Green's Functions, Confinement and Phenomenology, 5-9 September 2011, Trento,
Ital
AdS/QCD, Light-Front Holography, and Color Confinement
A remarkable holographic feature of dynamics in AdS space in five dimensions
is that it is dual to Hamiltonian theory in physical space-time, quantized at
fixed light-front time {\tau} = t+z/c. This light-front holographic principle
provides a precise relation between the bound-state amplitudes in AdS space and
the boost-invariant light-front wavefunctions describing the internal structure
of hadrons in physical space-time. The fifth dimension coordinate z is dual to
the light front variable {\zeta} describing the invariant separation of the
quark constituents. The resulting valence Fock-state wavefunction
eigensolutions of the light-front QCD Hamiltonian satisfy a single-variable
relativistic equation of motion, analogous to the nonrelativistic radial
Schr\"odinger equation. The soft-wall dilaton profile exp ({\kappa}^2
{\zeta}^2) provides a model for the light-front potential which is
color-confining and reproduces well the linear Regge behavior of the
light-quark hadron spectrum in both L, the orbital angular momentum, and n, the
radial node number. The pion mass vanishes in the chiral limit and other
features of chiral symmetry are satisfied. The resulting running QCD coupling
displays an infrared fixed point. The elastic and transition form factors of
the pion and the nucleons are also found to be well described in this
framework. The light-front AdS/QCD holographic approach thus gives a
frame-independent analytic first approximation of the color-confining dynamics,
spectroscopy, and excitation spectra of relativistic light-quark bound states
in QCD.Comment: Contribution to the Proceedings of the conference, Xth Quark
Confinement and the Hadron Spectrum, October 8-12, 2012, TUM Campus Garching,
Munich, German
QCD on the Light-Front -- A Systematic Approach to Hadron Physics
Light-Front Hamiltonian theory provides a rigorous frame-independent
framework for solving nonperturbative QCD. The valence Fock-state wavefunctions
of the light-front QCD Hamiltonian satisfy a single-variable relativistic
equation of motion, analogous to the nonrelativistic radial Schr\"odinger
equation, with an effective confining potential U which systematically
incorporates the effects of higher quark and gluon Fock states. Remarkably, the
potential U has a unique form of a harmonic oscillator potential if one
requires that the chiral QCD action remains conformally invariant. A mass gap
arises when one extends the formalism of de Alfaro, Fubini and Furlan to
light-front Hamiltonian theory. The valence LF meson wavefunctions for zero
quark mass satisfy a single-variable relativistic equation of motion in the
invariant variable , which is conjugate to the
invariant mass squared. The result is a nonperturbative relativistic
light-front quantum mechanical wave equation which incorporates color
confinement and other essential spectroscopic and dynamical features of hadron
physics, including a massless pion for zero quark mass and linear Regge
trajectories with the same slope in the radial quantum number n and orbital
angular momentum L. The corresponding light-front Dirac equation provides a
model of nucleons. The same light-front equations arise from the holographic
mapping of the soft-wall model modification of AdS_5 space with a unique
dilaton profile to QCD (3+1) at fixed light-front time. Light-front holography
thus provides a precise relation between amplitudes in the fifth dimension of
AdS space and light-front wavefunctions. 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.Comment: Invited talk, presented by SJB at LightCone 2013+, May 20- May 24,
2013, Skiathos, Greece. arXiv admin note: text overlap with arXiv:1309.4856,
arXiv:1308.5251, arXiv:1302.539
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