775 research outputs found
Light-Cone Representation of the Spin and Orbital Angular Momentum of Relativistic Composite Systems
The matrix elements of local operators such as the electromagnetic current,
the energy momentum tensor, angular momentum, and the moments of structure
functions have exact representations in terms of light-cone Fock state
wavefunctions of bound states such as hadrons. We illustrate all of these
properties by giving explicit light-cone wavefunctions for the two-particle
Fock state of the electron in QED, thus connecting the Schwinger anomalous
magnetic moment to the spin and orbital momentum carried by its Fock state
constituents. We also compute the QED one-loop radiative corrections for the
form factors for the graviton coupling to the electron and photon. Although the
underlying model is derived from elementary QED perturbative couplings, it in
fact can be used to simulate much more general bound state systems by applying
spectral integration over the constituent masses while preserving all of the
Lorentz properties, giving explicit realization of the spin sum rules and other
local matrix elements. The role of orbital angular momentum in understanding
the "spin crisis" problem for relativistic systems is clarified. We also prove
that the anomalous gravitomagnetic moment B(0) vanishes for any composite
system. This property is shown to follow directly from the Lorentz boost
properties of the light-cone Fock representation and holds separately for each
Fock state component. We show how the QED perturbative structure can be used to
model bound state systems while preserving all Lorentz properties. We thus
obtain a theoretical laboratory to test the consistency of formulae which have
been proposed to probe the spin structure of hadrons.Comment: Version to be published in Nuclear Physics B. Includes illustrations
of graviton-lepton form factors at one loop in QE
Applicability of Perturbative QCD to Pion Virtual Compton Scattering
We study explicitly the applicability of perturbative QCD (pQCD) to the pion
virtual Compton scattering. It is found that there are central-region
singularities introduced by the QCD running coupling constant, in addition to
the end-point singularities generally existed in other exclusive processes such
as the pion form factor. We introduce a simple technique to evaluate the
contributions from these singularities, so that we can arrive at a judgement
that these contributions will be unharmful to the applicability of pQCD at
certain energy scale, i.e., the ``work point'' which is defined to determine
when pQCD is applicable to exclusive processes. The applicability of pQCD for
different pion distribution amplitudes are explored in detail. We show that
pQCD begins to work at 10 . If we relax our constraint to a weak
sense, the work point may be as low as 4 .Comment: 13 Latex pages, 10 figures, to appear in PL
Factorization Approach for Inclusive Production of Doubly Heavy Baryon
We study inclusive production of doubly heavy baryon at a collider
and at hadron colliders through fragmentation. We study the production by
factorizing nonpertubative- and perturbative effects. In our approach the
production can be thought as a two-step process: A pair of heavy quarks can be
produced perturbatively and then the pair is transformed into the baryon. The
transformation is nonperturbative. Since a heavy quark moves with a small
velocity in the baryon in its rest frame, we can use NRQCD to describe the
transformation and perform a systematic expansion in the small velocity. At the
leading order we find that the baryon can be formed from two states of the
heavy-quark pair, one state is with the pair in state and in color
, another is with the pair in state and in color . Two matrix elements are defined for the transformation from the two
states, their perturbative coefficients in the contribution to the
cross-section at a collider and to the function of heavy quark
fragmentation are calculated. Our approach is different than previous
approaches where only the pair in state and in color is
taken into account. Numerical results for colliders at the two
-factories and for hadronic colliders LHC and Tevatron are given.Comment: Add results for large p_t, minor change
The interplay of soft and hard contributions in the electromagnetic pion form factor
We consider various relativistic models for the valence Fock-state wave
function of the pion. These models are obtained from simple instant-form wave
functions by applying a Melosh rotation to the spin part and by imposing
physical constraints on the parameters. We discuss how the soft and the hard
(perturbative) parts of the electromagnetic form factor are affected by the
choice of the model and by the Melosh rotation.Comment: 3 pages, 2 eps-figures, uses espcrc2.st
Factorization of Large-x Quark Distributions in a Hadron
We present a factorization formula for valence quark distributions in a
hadron in x-->1 limit. For the example of pion, we arrive at the form of
factorization by analyzing momentum flow in the leading and high-order Feynman
diagrams. The result confirms the well-known 1-x scaling rule to all orders in
perturbation theory, providing the non-perturbative matrix elements for the
infrared-divergence factors. We comment on re-summation of perturbative single
and double logarithms in 1-x.Comment: 7 pages, 2 figures include
Refactorizing NRQCD short-distance coefficients in exclusive quarkonium production
In a typical exclusive quarkonium production process, when the center-of-mass
energy, , is much greater than the heavy quark mass , large
kinematic logarithms of will unavoidably arise at each order of
perturbative expansion in the short-distance coefficients of the
nonrelativistic QCD (NRQCD) factorization formalism, which may potentially harm
the perturbative expansion. This symptom reflects that the hard regime in NRQCD
factorization is too coarse and should be further factorized. We suggest that
this regime can be further separated into "hard" and "collinear" degrees of
freedom, so that the familiar light-cone approach can be employed to reproduce
the NRQCD matching coefficients at the zeroth order of and order by
order in . Taking two simple processes, exclusive
production in annihilation and Higgs boson radiative decay into
, as examples, we illustrate how the leading logarithms of in
the NRQCD matching coefficients are identified and summed to all orders in
with the aid of Brodsky-Lepage evolution equation.Comment: v2, 17 pages, 2 figures; presentation improved, one important
reference added, and Note adde
Recalculation of Pion Compton Scattering in Perturbative QCD
We recalculated pion virtual Compton scattering in perturbative QCD in this
paper. Our calculation avoids some deficiencies in existing literatures, and
treats real Compton scattering as a limit case in which the mass of the virtual
photon equals to zero. Expressions of the hard scattering amplitudes from 10
independent diagrams are given explicitly in the text. By comparing the effects
of different distribution amplitudes on physical observables, we studied the
self-consistence of pQCD calculation of this problem.Comment: 15 pages, 8 eps figs. accepted by Phys. Lett
A Study of Gluon Propagator on Coarse Lattice
We study gluon propagator in Landau gauge with lattice QCD, where we use an
improved lattice action. The calculation of gluon propagator is performed on
lattices with the lattice spacing from 0.40 fm to 0.24 fm and with the lattice
volume from to . We try to fit our results by two
different ways, in the first one we interpret the calculated gluon propagators
as a function of the continuum momentum, while in the second we interpret the
propagators as a function of the lattice momentum. In the both we use models
which are the same in continuum limit. A qualitative agreement between two
fittings is found.Comment: Revtex 14pages, 11 figure
S-wave charmed mesons in lattice NRQCD
Heavy-light mesons can be studied using the 1/M expansion of NRQCD, provided
the heavy quark mass is sufficiently large. Calculations of the S-wave charmed
meson masses from a classically and tadpole-improved action are presented. A
comparison of O(1/M), O(1/M^2) and O(1/M^3) results allows convergence of the
expansion to be discussed. It is shown that the form of discretized heavy quark
propagation must be chosen carefully.Comment: LATTICE98(heavyqk), 3 pages including 3 figure
On Transverse-Momentum Dependent Light-Cone Wave Functions of Light Mesons
Transverse-momentum dependent (TMD) light-cone wave functions of a light
meson are important ingredients in the TMD QCD factorization of exclusive
processes. This factorization allows one conveniently resum Sudakov logarithms
appearing in collinear factorization. The TMD light-cone wave functions are not
simply related to the standard light-cone wave functions in collinear
factorization by integrating them over the transverse momentum. We explore
relations between TMD light-cone wave functions and those in the collinear
factorization. Two factorized relations can be found. One is helpful for
constructing models for TMD light-cone wave functions, and the other can be
used for resummation. These relations will be useful to establish a link
between two types of factorization.Comment: add more discussions and reference
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