679 research outputs found
Spin filter in deeply virtual Compton scattering amplitudes
Whether the kinematics includes the hard transverse photon momenta or not makes a dramatic difference in computing deeply virtual Compton scattering in terms of the widely used reduced operators that define generalized parton distributions. Our tree-level complete deeply virtual Compton scattering amplitude including the lepton current plays the role of spin filter to analyze such kinematic dependence on the contribution of longitudinally polarized virtual photon as well as the conservation of angular momentum. © 2011 American Physical Society
The box diagram in Yukawa theory
We present a light-front calculation of the box diagram in Yukawa theory. The
covariant box diagram is finite for the case of spin-1/2 constituents
exchanging spin-0 particles. In light-front dynamics, however, individual
time-ordered diagrams are divergent. We analyze the corresponding light-front
singularities and show the equivalence between the light-front and covariant
results by taming the singularities.Comment: 21 pages, 17 figures. submittes to Phys. Rev.
Ideas of Four-Fermion Operators in Electromagnetic Form Factor Calculations
Four-fermion operators have been utilized in the past to link the
quark-exchange processes in the interaction of hadrons with the effective
meson-exchange amplitudes. In this paper, we apply the similar idea of a Fierz
rearrangement to the electromagnetic processes and focus on the electromagnetic
form factors of the nucleon and the electron. We explain the motivation of
using four-fermion operators and discuss the advantage of this method in
computing electromagnetic processes.Comment: 8 pages, 3figure
Beam Spin Asymmetry in Electroproduction of Pseudoscalar or Scalar Meson Production off the Scalar Target
We discuss the electroproduction of pseudoscalar () or scalar
() meson production off the scalar target. The most general formulation
of the differential cross section for the or meson production
process involves only one or two hadronic form factors, respectively, on a
scalar target. The Rosenbluth-type separation of the differential cross section
provides the explicit relation between the hadronic form factors and the
different parts of the differential cross section in a completely
model-independent manner. The absence of the beam spin asymmetry for the
pseudoscalar meson production provides a benchmark for the experimental data
analysis. The measurement of the beam spin asymmetry for the scalar meson
production may also provide a unique opportunity not only to explore the
imaginary part of the hadronic amplitude in the general formulation but also to
examine the significance of the chiral-odd generalized parton distribution
(GPD) contribution in the leading-twist GPD formulation.Comment: 8 pages, 1 figur
The Vector Meson Form Factor Analysis in Light-Front Dynamics
We study the form factors of vector mesons using a covariant fermion field
theory model in dimensions. Performing a light-front calculation in the
frame in parallel with a manifestly covariant calculation, we note the
existence of a nonvanishing zero-mode contribution to the light-front current
and find a way of avoiding the zero-mode in the form factor calculations.
Upon choosing the light-front gauge (\ep^+_{h=\pm}=0) with circular
polarization and with spin projection , only the
helicity zero to zero matrix element of the plus current receives zero-mode
contributions. Therefore, one can obtain the exact light-front solution of the
form factors using only the valence contribution if only the helicity
components, , and , are used. We also compare our
results obtained from the light-front gauge in the light-front helicity basis
(i.e. ) with those obtained from the non-LF gauge in the instant form
linear polarization basis (i.e. ) where the zero-mode contributions to
the form factors are unavoidable.Comment: 33 pages; typo in Eq.(15) is corrected; comment on Ref.[9] is
corrected; version to appear in Phys. Rev.
Transition Form Factors between Pseudoscalar and Vector Mesons in Light-Front Dynamics
We study the transition form factors between pseudoscalar and vector mesons
using a covariant fermion field theory model in dimensions. Performing
the light-front calculation in the frame in parallel with the
manifestly covariant calculation, we note that the suspected nonvanishing
zero-mode contribution to the light-front current does not exist in our
analysis of transition form factors. We also perform the light-front
calculation in a purely longitudinal frame and confirm that the form
factors obtained directly from the timelike region are identical to the ones
obtained by the analytic continuation from the spacelike region. Our results
for the decay process satisfy the constraints on the
heavy-to-heavy semileptonic decays imposed by the flavor independence in the
heavy quark limit.Comment: 20 pages, 14 figure
Frame Dependence of Spin-One Angular Conditions in Light Front Dynamics
We elaborate the frame dependence of the angular conditions for spin-1 form
factors. An extra angular condition is found in addition to the usual angular
condition relating the four helicity amplitudes. Investigating the
frame-dependence of the angular conditions, we find that the extra angular
condition is in general as complicated as the usual one, although it becomes
very simple in the frame involving only two helicity amplitudes. It
is confirmed that the angular conditions are identical in frames that are
connected by kinematical transformations. The high behaviors of the
physical form factors and the limiting behaviors in special reference frames
are also discussed.Comment: 29 pages RevTeX. submitted to Phys. Rev.
Light-Front Singularities
Calculations in light-front quantization are sometimes found to lead to singularities that are not present in the corresponding manifestly covariant treatment. We give some examples that were found in the framework of perturbation theory, but must also occur in nonperturbative calculations. In the case of anomalies, regularization-scheme dependences were found that not only occur between the light-front approach and manifestly covariant calculations, but also among the latter ones. © 2010 The Author(s)
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