75 research outputs found
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 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
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
Light-Front Holographic QCD and the Confinement Potential
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.Comment: Presented by SJB at Light-Cone 2012: Relativistic Hadronic and
Particle Physics, 10 to 15 December, 2012 at the University of Delhi, New
Delhi, Indi
Conformal Symmetry, Confinement, and Light-Front Holographic QCD
We show that (a) the conformal properties of Anti-de Sitter (AdS) space, (b)
the properties of a field theory in one dimension under the full conformal
group found by de Alfaro, Fubini and Furlan, and (c) the frame-independent
single-variable light-front Schr\"odinger equation for bound states all lead to
the same result: a relativistic nonperturbative model which successfully
incorporates salient features of hadronic physics, including confinement,
linear Regge trajectories, and results which are conventionally attributed to
spontaneous chiral symmetry breaking.Comment: Presented by SJB at the Third Workshop on the QCD Structure of the
Nucleon (QCD-N'12), Bilbao, Spain, October 22-26, 2012. This work was
supported by the Department of Energy contract DE--AC02--76SF0051
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Light-Front Holography and Gauge/Gravity Duality: The Light Meson and Baryon Spectra
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 amplitudes of the hadronic constituents at a given scale. An effective classical gravity description in a positive-sign dilaton background exp(+{kappa}{sup 2}z{sup 2}) is given for the phenomenologically successful soft-wall model which naturally encodes the internal structure of hadrons and their orbital angular momentum. Applications to the light meson and baryon spectrum are presented
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Nearly Conformal QCD and AdS/CFT
The AdS/CFT correspondence is a powerful tool to study the properties of conformal QCD at strong coupling in terms of a higher dimensional dual gravity theory. The power-law falloff of scattering amplitudes in the non-perturbative regime and calculable hadron spectra follow from holographic models dual to QCD with conformal behavior at short distances and confinement at large distances. String modes and fluctuations about the AdS background are identified with QCD degrees of freedom and orbital excitations at the AdS boundary limit. A description of form factors in space and time-like regions and the behavior of light-front wave functions can also be understood in terms of a dual gravity description in the interior of AdS
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Gauge/Gravity Duality and Hadron Physics in the Light-Front
We discuss some remarkable features of the light-front holographic mapping of classical gravity in anti-de Sitter space modified by a confining dilaton background. In particular, we show that a positive-sign dilaton solution exp(+k{sup 2}z{sup 2}) has better chances to describe the correct hadronic phenomenology than the negative solution exp (-k{sup 2}z{sup 2}) extensively studied in the literature. We also show that the use of twist-scaling dimensions, instead of canonical dimensions, is required to give a good description of the spectrum and form factors of hadrons. Another key element is the explicit connection of AdS modes of total angular momentum J with the internal structure of hadrons and the proper identification of the orbital angular momentum of the constituents
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Normal Modes of Black Hole Accretion Disks
This paper studies the hydrodynamical problem of normal modes of small adiabatic oscillations of relativistic barotropic thin accretion disks around black holes (and compact weakly magnetic neutron stars). Employing WKB techniques, we obtain the eigen frequencies and eigenfunctions of the modes for different values of the mass and angular momentum of the central black hole. We discuss the properties of the various types of modes and examine the role of viscosity, as it appears to render some of the modes unstable to rapid growth
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