756 research outputs found
AdS/QCD, Light-Front Holography, and the Nonperturbative Running Coupling
The combination of Anti-de Sitter space (AdS) methods with light-front (LF)
holography provides a remarkably accurate first approximation for the spectra
and wavefunctions of meson and baryon light-quark bound states. The resulting
bound-state Hamiltonian equation of motion in QCD leads to relativistic
light-front wave equations in terms of an invariant impact variable
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- modes in anti--de Sitter (AdS) space. The eigenvalues
give the hadronic spectrum, and the eigenmodes represent the probability
distributions of the hadronic constituents at a given scale. A positive-sign
confining dilaton background modifying AdS space gives a very good account of
meson and baryon spectroscopy and form factors. The light-front holographic
mapping of this model also leads to a non-perturbative effective coupling
which agrees with the effective charge defined by the
Bjorken sum rule and lattice simulations. It displays a transition from
perturbative to nonperturbative conformal regimes at a momentum scale
GeV. The resulting -function appears to capture the essential
characteristics of the full -function of QCD, thus giving further
support to the application of the gauge/gravity duality to the confining
dynamics of strongly coupled QCD.Comment: Invited talk, presented by SJB at SCGT09, 2009 International Workshop
on Strong Coupling Gauge Theories in the LHC Era, Nagoya, December 8-11,
2009, updated figur
The QCD Running Coupling
We review the present knowledge for , the fundamental coupling
underlying the interactions of quarks and gluons in QCD. The dependence of
on momentum transfer encodes the underlying dynamics of
hadron physics -from color confinement in the infrared domain to asymptotic
freedom at short distances. We review constraints on at high
, as predicted by perturbative QCD, and its analytic behavior at small
, based on models of nonperturbative dynamics. In the introductory part of
this review, we explain the phenomenological meaning of , the reason
for its running, and the challenges facing a complete understanding of its
analytic behavior in the infrared domain. In the second, more technical, part
of the review, we discuss the behavior of in the high
domain of QCD. We review how is defined, including its
renormalization scheme dependence, the definition of its renormalization scale,
the utility of effective charges, as well as Commensurate Scale Relations which
connect the various definitions of without renormalization-scale
ambiguity. We also report recent measurements and theoretical analyses which
have led to precise QCD predictions at high energy. In the last part of the
review, we discuss the challenge of understanding the analytic behavior
in the infrared domain. We also review important methods for
computing , including lattice QCD, the Schwinger-Dyson equations, the
Gribov-Zwanziger analysis and light-front holographic QCD. After describing
these approaches and enumerating their conflicting predictions, we discuss the
origin of these discrepancies and how to remedy them. Our aim is not only to
review the advances in this difficult area, but also to suggest what could be
an optimal definition of in order to bring better unity to the
subject.Comment: Invited review article for Progress in Particle and Nuclear Physics.
195 pages, 18 figures. V3: Minor corrections and addenda compared to V1 and
V2. V4: typo fixed in Eq. (3.21
Connecting the Hadron Mass Scale to the Fundamental Mass Scale of Quantum Chromodynamics
Establishing an explicit connection between the long distance physics of
confinement and the dynamical interactions of quarks and gluons at short
distances has been a long-sought goal of quantum chromodynamics. Using
holographic QCD, we derive a direct analytic relation between the scale
which determines the masses of hadrons and the scale
which controls the predictions of perturbative QCD at very short distances. The
resulting prediction GeV in the
scheme agrees well with the experimental average GeV. We also
derive a relation between and the QCD string tension .
This connection between the fundamental hadronic scale underlying the physics
of quark confinement and the perturbative QCD scale controlling hard collisions
can be carried out in any renormalization scheme.Comment: 11 pages, 4 figures. Final version published in Phys. Lett.
Effect of Orbital Angular Momentum on Valence-Quark Helicity Distributions
We study the quark helicity distributions at large x in perturbative QCD,
taking into account contributions from the valence Fock states of the nucleon
which have nonzero orbital angular momentum. These states are necessary to have
a nonzero anomalous magnetic moment. We find that the quark orbital angular
momentum contributes a large logarithm to the negative helicity quark
distributions in addition to its power behavior, scaling as (1-x)^5\log^2(1-x)
in the limit of x\to 1. Our analysis shows that the ratio of the polarized over
unpolarized down quark distributions, \Delta d/d, will still approach 1 in this
limit. By comparing with the experimental data, we find that this ratio should
cross zero at x\approx 0.75.Comment: 10 pages, 3 eps figure
Applied analytical combustion/emissions research at the NASA Lewis Research Center
Emissions of pollutants from future commercial transports are a significant concern. As a result, the Lewis Research Center (LeRC) is investigating various low emission combustor technologies. As part of this effort, a combustor analysis code development program was pursued to guide the combustor design process, to identify concepts having the greatest promise, and to optimize them at the lowest cost in the minimum time
Hadron and Nuclear Physics on the Light Front
The QCD light-front (LF) Hamiltonian equation H_{LF}|\Psi> = M^2 |\Psi>
derived from quantization at fixed LF time \tau = t+z/c provides a causal,
frame-independent, method for computing hadron spectroscopy as well as
dynamical observables such as structure functions, transverse momentum
distributions, and distribution amplitudes. The LF formalism also leads to
novel nuclear phenomena, such as "hidden color", "color transparency",
"nuclear-bound quarkonium" and the shadowing and antishadowing of nuclear
structure functions. For example, there are five distinct color-singlet Fock
state representations of the six color-triplet quarks of the deuteron. The
hidden color Fock states become manifest when the deuteron is probed when it
has small transverse size, as in measurements of the deuteron form factor at
large momentum transfer. The QCD Lagrangian with zero quark mass has no
explicit mass scale. However, as shown by de Alfaro, Fubini, and Furlan (dAFF),
a mass scale can appear in the equations of motion without affecting the
conformal invariance of the Action. When one applies the dAFF procedure to the
QCD LF Hamiltonian, it leads to a color confining potential \kappa^4 \zeta^2
for mesons, where \zeta^2 is the LF radial variable conjugate to the qbar-q
invariant mass squared. The same result, including spin terms, is obtained
using LF holography (AdS/QCD) -- the duality between LF dynamics and AdS_5 --
if one modifies the AdS_5 Action by the dilaton exp{+\kappa^2 z^2} in the fifth
dimension z. If this procedure is generalized using superconformal algebra, the
resulting LF eigensolutions provide unified Regge spectroscopy of mesons,
baryons, and tetraquarks, including remarkable supersymmetric relations between
the masses of mesons, baryons and tetraquarks with a universal Regge slope...Comment: 8 pages, 3 figures. Contribution to the proceedings of 13th
International Conference on Nucleus-Nucleus Collisions (NN2018) in Omiya,
Japan. Plenary talk given by S. J. Brodsky. arXiv admin note: text overlap
with arXiv:1802.08552, arXiv:1709.01191, arXiv:1611.071
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