696 research outputs found
Frame-Independence of Exclusive Amplitudes in the Light-Front Quantization
While the particle-number-conserving convolution formalism established in the
Drell-Yan-West reference frame is frequently used to compute exclusive
amplitudes in the light-front quantization, this formalism is limited to only
those frames where the light-front helicities are not changed and the good
(plus) component of the current remains unmixed. For an explicit demonstration
of such criteria, we present the relations between the current matrix elements
in the two typical reference frames used for calculations of the exclusive
amplitudes, i.e. the Drell-Yan-West and Breit frames and investigate both
pseudoscalar and vector electromagnetic currents in detail. We find that the
light-front helicities are unchanged and the good component of the current does
not mix with the other components of the current under the transformation
between these two frames. Thus, the pseudoscalar and vector form factors
obtained by the diagonal convolution formalism in both frames must indeed be
identical. However, such coincidence between the Drell-Yan-West and Breit
frames does not hold in general. We give an explicit example in which the
light-front helicities are changed and the plus component of the current is
mixed with other components under the change of reference frame. In such a
case, the relationship between the frames should be carefully analyzed before
the established convolution formalism in the Drell-Yan-West frame is used.Comment: 14 pages, 4 figure
Poincare Invariant Algebra From Instant to Light-Front Quantization
We present the Poincare algebra interpolating between instant and light-front
time quantizations. The angular momentum operators satisfying SU(2) algebra are
constructed in an arbitrary interpolation angle and shown to be identical to
the ordinary angular momentum and Leutwyler-Stern angular momentum in the
instant and light-front quantization limits, respectively. The exchange of the
dynamical role between the transverse angular mometum and the boost operators
is manifest in our newly constructed algebra.Comment: 21 pages, 3 figures, 1 tabl
Angular Conditions,Relations between Breit and Light-Front Frames, and Subleading Power Corrections
We analyze the current matrix elements in the general collinear (Breit)
frames and find the relation between the ordinary (or canonical) helicity
amplitudes and the light-front helicity amplitudes. Using the conservation of
angular momentum, we derive a general angular condition which should be
satisfied by the light-front helicity amplitudes for any spin system. In
addition, we obtain the light-front parity and time-reversal relations for the
light-front helicity amplitudes. Applying these relations to the spin-1 form
factor analysis, we note that the general angular condition relating the five
helicity amplitudes is reduced to the usual angular condition relating the four
helicity amplitudes due to the light-front time-reversal condition. We make
some comments on the consequences of the angular condition for the analysis of
the high- deuteron electromagnetic form factors, and we further apply the
general angular condition to the electromagnetic transition between spin-1/2
and spin-3/2 systems and find a relation useful for the analysis of the
N- transition form factors. We also discuss the scaling law and the
subleading power corrections in the Breit and light-front frames.Comment: 24 pages,2 figure
Burkhardt-Cottingham sum rule and forward spin polarizabilities in Heavy Baryon Chiral Perturbation Theory
We study spin-dependent sum rules for forward virtual Compton
scattering(VVCS) off the nucleon in heavy baryon chiral perturbation theory at
order . We show how these sum rules can be evaluated from low energy
expansions (in the virtual photon energy) of the forward VVCS amplitudes. We
study in particular the Burkhardt -Cottingham sum rule in HBChPT and higher
terms in the low energy expansion, which can be related to the generalized
forward spin polarizabilities of the nucleon. The dependence of these
observables on the photon virtuality can be accessed, at small and
intermediate values, from existing and forthcoming data at Jefferson Lab.Comment: 16 pages,4 fig
Generalized Parton Distributions at x->1
Generalized parton distributions at large are studied in perturbative QCD
approach. As and at finite , there is no dependence for the
GPDs which means that the active quark is at the center of the transverse
space. We also obtain the power behavior: for pion; and
for nucleon, where
represents the additional dependence on .Comment: 7 pages, 2 figure
Quark Imaging in the Proton Via Quantum Phase-Space Distributions
We develop the concept of quantum phase-space (Wigner) distributions for
quarks and gluons in the proton. To appreciate their physical content, we
analyze the contraints from special relativity on the interpretation of elastic
form factors, and examine the physics of the Feynman parton distributions in
the proton's rest frame. We relate the quark Wigner functions to the
transverse-momentum dependent parton distributions and generalized parton
distributions, emphasizing the physical role of the skewness parameter. We show
that the Wigner functions allow to visualize quantum quarks and gluons using
the language of the classical phase space. We present two examples of the quark
Wigner distributions and point out some model-independent features.Comment: 20 pages with 3 fiture
Accessing the Longitudinally Polarized Photon Content of the Proton
We investigate the QED Compton process (QEDCS) in longitudinally polarized
lepton-proton scattering both in the elastic and inelastic channels and show
that the cross section can be expressed in terms of the polarized equivalent
photon distribution of the proton. We provide the necessary kinematical
constraints to extract the polarized photon content of the proton using this
process at HERMES, COMPASS and eRHIC. We also discuss the suppression of the
major background process coming from virtual Compton scattering. We point out
that such an experiment can give valuable information on in the
small , broad region at the future polarized collider eRHIC and
especially in the lower , medium region in fixed target experiments.Comment: Version to appear in PR
A QCD Analysis of the Mass Structure of the Nucleon
{}From the deep-inelastic momentum sum rule and the trace anomaly of the
energy-momentum tensor, I derive a separation of the nucleon mass into the
contributions of the quark and gluon kinetic and potential energies, the quark
masses, and the trace anomaly.Comment: 9 pages, MIT-CTP #2368, revtex with 1 tabl
Deeply virtual electroproduction of photons and mesons on the nucleon : leading order amplitudes and power corrections
We estimate the leading order amplitudes for exclusive photon and meson
electroproduction reactions at large Q^2 in the valence region in terms of
skewed quark distributions. As experimental investigations can currently only
be envisaged at moderate values of Q^2, we estimate power corrections due to
the intrinsic transverse momentum of the partons in the meson wavefunction and
in the nucleon. To this aim the skewed parton distribution formalism is
generalized so as to include the parton intrinsic transverse momentum
dependence. Furthermore, for the meson electroproduction reactions, we
calculate the soft overlap type contributions and compare with the leading
order amplitudes. We give first estimates for these different power corrections
in kinematics which are relevant for experiments in the near future.Comment: 59 pages, 21 figure
Current in the light-front Bethe-Salpeter formalism II: Applications
We pursue applications of the light-front reduction of current matrix
elements in the Bethe-Salpeter formalism. The normalization of the reduced wave
function is derived from the covariant framework and related to non-valence
probabilities using familiar Fock space projection operators. Using a simple
model, we obtain expressions for generalized parton distributions that are
continuous. The non-vanishing of these distributions at the crossover between
kinematic regimes (where the plus component of the struck quark's momentum is
equal to the plus component of the momentum transfer) is tied to higher Fock
components. Moreover continuity holds due to relations between Fock components
at vanishing plus momentum. Lastly we apply the light-front reduction to
time-like form factors and derive expressions for the generalized distribution
amplitudes in this model.Comment: 12 pages, 6 figures, RevTex
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