1,347 research outputs found
On the Determination of the Polarized Sea Distributions of the Nucleon
The possibilities to determine the flavor structure of the polarized sea
(antiquark) distributions of the nucleon via vector boson production at high energy polarized hadron--hadron colliders, such as the
Relativistic Heavy--Ion Collider (RHIC), are studied in detail. In particular
the perturbative stability of the expected asymmetries in two representative
models for the (un)broken flavor structure are investigated by confronting
perturbative QCD leading order predictions of the expected asymmetries with
their next--to--leading order counterparts.Comment: 28 pages, LaTe
Spin-Dependent Structure Functions of Real and Virtual Photons
The implications of the positivity constraint, , on the presently unknown spin--dependent
structure function of real and virtual photons are
studied at scales where longitudinally polarized photons dominate
physically relevant cross sections. In particular it is shown how to implement
the physical constraints of positivity and continuity at in NLO
calculations which afford a nontrivial choice of suitable (DIS) factorization
schemes related to and and appropriate boundary
conditions for the polarized parton distributions of real and virtual photons.
The predictions of two extreme `maximal' and `minimal' saturation scenarios are
presented and compared with results obtained within the framework of a simple
quark `box' calculation expected to yield reasonable estimates in the not too
small regions of and .Comment: 33 pages, LaTeX, 12 figure
The Kinetic Interpretation of the DGLAP Equation, its Kramers-Moyal Expansion and Positivity of Helicity Distributions
According to a rederivation - due to Collins and Qiu - the DGLAP equation can
be reinterpreted (in leading order) in a probabilistic way. This form of the
equation has been used indirectly to prove the bound
between polarized and unpolarized distributions, or positivity of the helicity
distributions, for any . We reanalize this issue by performing a detailed
numerical study of the positivity bounds of the helicity distributions. To
obtain the numerical solution we implement an x-space based algorithm for
polarized and unpolarized distributions to next-to-leading order in ,
which we illustrate. We also elaborate on some of the formal properties of the
Collins-Qiu form and comment on the underlying regularization, introduce a
Kramers-Moyal expansion of the equation and briefly analize its Fokker-Planck
approximation. These follow quite naturally once the master version is given.
We illustrate this expansion both for the valence quark distribution and
for the transverse spin distribution .Comment: 38 pages, 27 figures, Dedicated to Prof. Pierre Ramond for his 60th
birthda
Mesonic Parton Densities Derived From Constituent Quark Model Constraints
Using constituent quark model constraints we calculate the gluon and sea
content of pions solely in terms of their valence density and the known sea and
gluon densities of the nucleon. The resulting small-x predictions for
g^{pi}(x,Q^2) and \bar{q}^{pi}(x,Q^2) are unique and parameter free, being
entirely due to QCD dynamics. Similar ideas are applied for calculating the
gluon and sea content of kaons which, for our suggested choice of the kaon's
valence densities, turn out to be identical to the ones of the pion.Comment: 16 pages, LaTeX, 6 figures, uses epsfig and amssymb style
Pionic Parton Distributions Revisited
Using constituent quark model constraints we calculate the gluon and
sea-quark content of pions solely in terms of their valence density (fixed by
\pi N Drell-Yan data) and the known sea and gluon distributions of the nucleon,
using the most recent updated valence-like input parton densities of the
nucleon. The resulting small-x dynamical QCD predictions for g^{\pi}(x,Q^2) and
\bar{q}^{\pi}(x,Q^2) are unique and parameter free. Simple analytic
parametrizations of the resulting parton distributions of the pion are
presented in LO and NLO. These results and parametrizations will be important,
among other things, for updated formulations of the parton distributions of
real and virtual photons.Comment: 14 pages incl. 3 figures, LaTe
Bloom-Gilman duality of the nucleon structure function and the elastic peak contribution
The occurrence of the Bloom-Gilman duality in the nucleon structure function
is investigated by analyzing the Q**2-behavior of low-order moments, both
including and excluding the contribution arising from the nucleon elastic peak.
The Natchmann definition of the moments has been adopted in order to cancel out
target-mass effects. It is shown that the onset of the Bloom-Gilman duality
occurs around Q**2 ~ 2 (GeV/c)**2 if only the inelastic part of the nucleon
structure function is considered, whereas the inclusion of the nucleon elastic
peak contribution leads to remarkable violations of the Bloom-Gilman duality.Comment: in Proc. of the XVI European Conference on Few-body Problems in
Physics, Autrans (France), July 199
Testing Color Evaporation in Photon-Photon Production of J/Psi at CERN LEP II
The DELPHI Collaboration has recently reported the measurement of J/Psi
production in photon-photon collisions at LEP II. These newly available data
provide an additional proof of the importance of colored c bar{c} pairs for the
production of charmonium because these data can only be explained by
considering resolved photon processes. We show here that the inclusion of color
octet contributions to the J/Psi production in the framework of the color
evaporation model is able to reproduce this data. In particular, the
transverse-momentum distribution of the J/Psi mesons is well described by this
model.Comment: 10 pages, 5 Figures, Revtex
Interaction of small size wave packet with hadron target
We calculate in QCD the cross section for the scattering of an energetic
small-size wave packet off a hadron target. We use our results to study the
small- behaviour of , the distribution over cross
section for the pion, in the leading -order.Comment: Revised version of the report CEBAF-TH-96-0
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