371 research outputs found
New effective treatment of the light-front nonvalence contribution in timelike exclusive processes
We discuss a necessary nonvalence contribution in timelike exclusive
processes. Following a Schwinger-Dyson type of approach, we relate the
nonvalence contribution to an ordinary light-front wave function that has been
extensively tested in the spacelike exclusive processes. A complicate four-body
energy denominator is exactly cancelled in summing the light-front time-ordered
amplitudes. Applying our method to and
where a rather substantial nonvalence contribution is expected, we find not
only an improvement in comparing with the experimental data but also a
covariance(i.e. frame-independence) of existing light-front constituent quark
model.Comment: 10 pages including 5 figures; Changes: 1-added some sentences;
2-enlarged the figures; 3-added some reference
A QCD Calculation of the Interaction of Quarkonium with Nuclei
The interaction of quarkonium with nuclei is studied in the limit of QCD, where the binding energy is found to be exactly
computable. The dominant contribution to the interaction is from two-gluon
operators. The forward matrix elements of these two-gluon operators can be
determined from the QCD scale anomaly, and from deep inelastic scattering. We
apply our results to the and , treating the \qqbar
interaction as purely Coulombic. We find the binds in nuclear matter
with a binding energy of a few \mev, while for the binding is of
order 10 \mev. For the in particular we expect confinement effects
to produce large corrections to this result.Comment: (10 pages
End-point singularities of Feynman graphs on the light cone
We show that some Lorentz components of the Feynman integrals calculated in
terms of the light-cone variables may contain end-point singularities which
originate from the contribution of the big-circle integral in the complex k_
plane. These singularities appear in various types of diagrams (two-point
functions, three-point functions, etc) and provide the covariance of the
Feynman integrals on the light-cone. We propose a procedure for calculating
Feynman integrals which guarantees that the end-point singularities do not
appear in the light-cone representations of the invariant amplitudes.Comment: final version to appear in PLB; few references adde
Light-Front-Quantized QCD in Covariant Gauge
The light-front (LF) canonical quantization of quantum chromodynamics in
covariant gauge is discussed. The Dirac procedure is used to eliminate the
constraints in the gauge-fixed front form theory quantum action and to
construct the LF Hamiltonian formulation. The physical degrees of freedom
emerge naturally. The propagator of the dynamical part of the free
fermionic propagator in the LF quantized field theory is shown to be causal and
not to contain instantaneous terms. Since the relevant propagators in the
covariant gauge formulation are causal, rotational invariance---including the
Coulomb potential in the static limit---can be recovered, avoiding the
difficulties encountered in light-cone gauge. The Wick rotation may also be
performed allowing the conversion of momentum space integrals into Euclidean
space forms. Some explicit computations are done in quantum electrodynamics to
illustrate the equivalence of front form theory with the conventional covariant
formulation. LF quantization thus provides a consistent formulation of gauge
theory, despite the fact that the hyperplanes used to impose
boundary conditions constitute characteristic surfaces of a hyperbolic partial
differential equation.Comment: LaTex, 16 page
Light Front Nuclear Physics: Toy Models, Static Sources and Tilted Light Front Coordinates
The principles behind the detailed results of a light-front mean field theory
of finite nuclei are elucidated by deriving the nucleon mode equation using a
simple general argument, based on the idea that a static source in equal time
coordinates corresponds to a moving source in light front coordinates. This
idea also allows us to solve several simple toy model examples: scalar field in
a box, 1+1 dimensional bag model, three-dimensional harmonic oscillator and the
Hulth\'en potential. The latter provide simplified versions of momentum
distributions and form factors of relevance to experiments. In particular, the
relativistic correction to the mean square radius of a nucleus is shown to be
very small. Solving these simple examples suggests another more general
approach-- the use of tilted light front coordinates. The simple examples are
made even simpler.Comment: 19 pages, references adde
Productions of X(1835) as baryonium with sizable gluon content
The X(1835) has been treated as a baryonium with sizable gluon content, and
to be almost flavor singlet. This picture allows us to rationally understand
X(1835) production in radiative decays, and its large couplings with
, . The processes and have been examined. It has been found
that , which is compatible with CLEO's recently
experimental result (Phys.Rev. (2006) 032001;hep-ex/0510015). The
branching fractions of , with and
have been estimated by the quark-pair
creation model. We show that they are heavily suppressed, so the signal of
X(1835) is very difficult, if not impossible, to be observed in these
processes. The experimental checks for these estimations are expected. The
existence of the baryonium nonet is conjectured, and a model independent
derivation of their production branching fractions is presented.Comment: 21 pages, 4 figure
Boost-Invariant Running Couplings in Effective Hamiltonians
We apply a boost-invariant similarity renormalization group procedure to a
light-front Hamiltonian of a scalar field phi of bare mass mu and interaction
term g phi^3 in 6 dimensions using 3rd order perturbative expansion in powers
of the coupling constant g. The initial Hamiltonian is regulated using momentum
dependent factors that approach 1 when a cutoff parameter Delta tends to
infinity. The similarity flow of corresponding effective Hamiltonians is
integrated analytically and two counterterms depending on Delta are obtained in
the initial Hamiltonian: a change in mu and a change of g. In addition, the
interaction vertex requires a Delta-independent counterterm that contains a
boost invariant function of momenta of particles participating in the
interaction. The resulting effective Hamiltonians contain a running coupling
constant that exhibits asymptotic freedom. The evolution of the coupling with
changing width of effective Hamiltonians agrees with results obtained using
Feynman diagrams and dimensional regularization when one identifies the
renormalization scale with the width. The effective light-front Schroedinger
equation is equally valid in a whole class of moving frames of reference
including the infinite momentum frame. Therefore, the calculation described
here provides an interesting pattern one can attempt to follow in the case of
Hamiltonians applicable in particle physics.Comment: 24 pages, LaTeX, included discussion of finite x-dependent
counterterm
Puzzles in charmonium decays
The open charm effects via intermediate hadron loop transitions seem to play
a crucial role in the understanding of several existing "puzzles" in charmonium
exclusive decays, such as the non- decays, and
" puzzle" etc. In the charmonium energy region, non-perturbative
mechanisms could be still sizeable, and as a consequence the intermediate
hadron loop transitions also provide a mechanism for the
helicity-selection-rule (HSR) violation. We report our recent progress on those
existing puzzles.Comment: Invited talk on the 15th International Conference in QCD, June 28-
July 3, 2010, Montpellier, Franc
Predictive AdS/QCD Model for Mass Spectra of Mesons with Three Flavors
The predictive soft-wall AdS/QCD model with a modified 5D metric at the
infrared region is constructed to obtain a non-trivial dilaton solution for
three flavor quarks , and . Such a model is shown to incorporate both
the chiral symmetry breaking and linear confinement. After considering some
high-order terms including the chiral symmetry breaking
term, we find that the resulting predictions for the SU(3) octet and singlet
resonance states of pseudoscalar, scalar, vector and axial-vector mesons agree
well with the experimentally confirmed resonance states. Contributions from the
instanton effects given by the determinant term are also discussed. It is
observed that the chiral symmetry breaking phenomena of
and can be well described in this model, while the SU(3)
flavor symmetry breaking effect due to quark mass difference in the source term
is not enough to explain all of the current experimental data.Comment: 20 pages, 1 figure, minor corrections, reference added, to appear in
Phys. Rev.
Light Front Formalism for Composite Systems and Some of Its Applications in Particle and Relativistic Nuclear Physics
Light front formalism for composite systems is presented. Derivation of
equations for bound state and scattering problems are given. Methods of
constructing of elastic form factors and scattering amplitudes of composite
particles are reviewed. Elastic form factors in the impulse approximation are
calculated. Scattering amplitudes for relativistic bound states are
constructed. Some model cases for transition amplitudes are considered. Deep
inelastic form factors (structure functions) are expressed through light front
wave functions. It is shown that taking into account of transverse motion of
partons leads to the violation of Bjorken scaling and structure functions
become square of transverse momentum dependent. Possible explanation of the
EMC-effect is given. Problem of light front relativization of wave functions of
lightest nuclei is considered. Scaling properties of deuteron, and
light front wave functions are checked in a rather wide energy range.Comment: Review paper, Submitted to Phys. Rep., 89 pages, 23 figure
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