78 research outputs found
NLO Corrections to Deeply-Virtual Compton Scattering
We have calculated the NLO corrections to the twist-2 part of the
deeply-virtual Compton scattering amplitude. Our results for the transverse and
antisymmetric parts agree with recent calculations by Ji and Osborne and by
Belitsky and M\"uller. In addition we present NLO results for the longitudinal
part of the amplitude.Comment: 8 pages, Latex. Error in polarised gluonic coefficient in Eq.(8)
correcte
Polarization as a Probe to the Production Mechanisms of Charmonium in Collisions
Measurements of the polarization of \jp produced in pion-nucleus collisions
are in disagreement with leading twist QCD prediction where \jp is observed
to have negligible polarization whereas theory predicts substantial
polarization. We argue that this discrepancy cannot be due to poorly known
structure functions nor the relative production rates of \jp and .
The disagreement between theory and experiment suggests important higher twist
corrections, as has earlier been surmised from the anomalous non-factorized
nuclear -dependence of the \jp cross section.Comment: 8 page
production in annihilation
Inclusive production of in annihilation is an excellent
probe of the role of higher Fock states in the production of heavy quarkonia.
Within the non-relativistic QCD approach, contributions from the short-distance
production of colour-octet pairs are significantly larger than those
from colour-singlet production. At the same time, production rates
are significantly smaller than expected in the colour evaporation approach.
Measurements of production at CLEO and future B-factories will
thus constitute a major test of theoretical approaches to the production of
heavy quarkonia.Comment: 11 pages, Revtex, 2 figure
Nuclear Shadowing and the Optics of Hadronic Fluctuations
A coordinate space description of shadowing in deep-inelastic lepton-nucleus
scattering is presented. The picture in the laboratory frame is that of
quark-gluon fluctuations of the high-energy virtual photon, propagating
coherently over large light-cone distances in the nuclear medium. We discuss
the detailed dependence of the coherence effects on the invariant mass of the
fluctuation. We comment on the issue of possible saturation in the shadowing
effects at very small Bjorken-.Comment: 11 pages, 5 figure
Optimal computation of brightness integrals parametrized on the unit sphere
We compare various approaches to find the most efficient method for the
practical computation of the lightcurves (integrated brightnesses) of
irregularly shaped bodies such as asteroids at arbitrary viewing and
illumination geometries. For convex models, this reduces to the problem of the
numerical computation of an integral over a simply defined part of the unit
sphere. We introduce a fast method, based on Lebedev quadratures, which is
optimal for both lightcurve simulation and inversion in the sense that it is
the simplest and fastest widely applicable procedure for accuracy levels
corresponding to typical data noise. The method requires no tessellation of the
surface into a polyhedral approximation. At the accuracy level of 0.01 mag, it
is up to an order of magnitude faster than polyhedral sums that are usually
applied to this problem, and even faster at higher accuracies. This approach
can also be used in other similar cases that can be modelled on the unit
sphere. The method is easily implemented in lightcurve inversion by a simple
alteration of the standard algorithm/software.Comment: Astronomy and Astrophysics, in pres
Nuclear Quark and Gluon Distributions in Coordinate Space
In coordinate space, quark and gluon distributions of the nucleon are defined as correlation functions involving two field operators separated by a light-cone distance . We study the nuclear modifications of these distributions. The largest effect is a strong depletion of parton distributions (shadowing) at large longitudinal distances, which starts for all parton species at fm, i.e. at the average nucleon-nucleon separation in nuclei. On the other hand, the nuclear radius does not play a significant role. At $l intrinsic structure of individual nucleons is evidently not very much affected by nuclear binding. In particular, there is no evidence for a significant increase of the quark or gluon correlation length in bound nucleons
Hard exclusive leptoproduction on polarized targets
We consider the exclusive production of mesons in polarized
virtual-photon--proton collisions. We derive helicity amplitudes for the
dominant subprocess and show that polarization
asymmetries vanish in the case of collinear scattering. Thus, contrary to what
has been suggested earlier, this process is not a good probe of the polarized
gluon distribution of the proton.Comment: Latex, 12 pages, 1 ps figure; typos corrected, some points clarified
in the tex
Color-Octet Contributions to Photoproduction
We have calculated the leading color-octet contributions to the production of
particles in photon-proton collisions. Using the values for the
color-octet matrix elements extracted from fits to prompt data at the
Tevatron, we demonstrate that distinctive color-octet signatures should be
visible in photoproduction. However, these predictions appear at
variance with recent experimental data obtained at HERA, indicating that the
phenomenological importance of the color-octet contributions is smaller than
expected from theoretical considerations and suggested by the Tevatron fits.Comment: 10 pages, LaTeX, epsfig, 4 figure
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