4,706 research outputs found
The scale dependent nuclear effects in parton distributions for practical applications
The scale dependence of the ratios of parton distributions in a proton of a
nucleus and in the free proton, ,
is studied within the framework of the lowest order leading-twist DGLAP
evolution. By evolving the initial nuclear distributions obtained with the
GRV-LO and CTEQ4L sets at a scale , we show that the ratios
are only moderately sensitive to the choice of a specific modern
set of free parton distributions. We propose that to a good first
approximation, this parton distribution set-dependence of the nuclear ratios
can be neglected in practical applications. With this result, we
offer a numerical parametrization of for all parton flavours
in any , and at any and any GeV for
computing cross sections of hard processes in nuclear collisions.Comment: 14 pages, including 4 eps-figure
Update on Counting Valence Quarks at RHIC
We update our former analysis of the Nuclear Modification Factors (NMF) for
different hadron species at RHIC and LHC. This update is motivated by the new
experimental data from STAR which presents differences with the preliminary
data used to fix some of the parameters in our model. The main change is the
use of AKK fragmentation functions for the hard part of the spectrum and minor
adjustments of the coalescence (soft) contribution. We confirm that observation
of the NMF for the f_0 meson can shed light on its quark composition.Comment: 4 pages, 4 figure
Nuclear parton distributions in the DGLAP approach
Determination of the nuclear parton distributions within the framework of
perturbative QCD, the DGLAP equations in particular, is discussed. Scale and
flavour dependent nuclear effects in the parton distributions are compared with
the scale and flavour independent parametrizations of HIJING and of the Hard
Probe Collaboration. A comparison with the data from deep inelastic
lepton-nucleus scattering and the Drell-Yan process in proton-nucleus
collisions is shown.Comment: 19 pages, 6 eps-figures, to appear in the Proceedings of the Hard
Probe Collaboratio
Constraints for nuclear gluon shadowing from DIS data
The dependence of the ratios of the cross sections of deep inelastic
lepton--nucleus scattering is studied in the framework of leading twist, lowest
order perturbative QCD. The slope of the ratio is computed by using the DGLAP evolution equations, and shown
to be sensitive to the nuclear gluon distribution functions. Four different
parametrizations for the nuclear effects of parton distributions are studied.
We show that the NMC data on the dependence of
rule out the case where nuclear shadowing (suppression) of gluons at is much larger than the shadowing observed in the ratio . We also show that the possible nonlinear correction terms due to gluon
fusion in the evolution equations do not change this conclusion. Some
consequences for computation of RHIC multiplicities, which probe the region
x\gsim0.01, are also discussed.Comment: 11 pages, 3 eps figure
Constraints for nuclear gluon densities from DIS data
The dependence of the ratios of nuclear structure functions is
studied by performing QCD evolution of nuclear parton distribution functions.
The log slope of these ratios is very sensitive to the nuclear gluon
distribution function. Taking different parametrizations, we show that the NMC
data on the dependence of rule out the case
where nuclear shadowing (suppression) of gluons at is much larger
than the shadowing observed in the ratio . We also take into
account modifications to the DGLAP evolution by including gluon fusion terms
and see that the effect is small at present energies, and, in any case, a
strong gluon shadowing is not favored. The region studied () is
the most relevant for RHIC multiplicities.Comment: 4 pages, 3 postscript figures. Contributed to 37th Rencontres de
Moriond on QCD and Hadronic Interactions, Les Arcs, France, 16-23 Mar 200
Scalar-tensor theory with EGB term from Einstein Chern-Simons gravity
It is shown that the compactification a la Randall Sundrum of the so called,
five dimensional Einstein Chern Simons action gravity leads to an action for a
four dimensional scalar tensor gravity that includes a Gauss Bonnet term, which
belongs to a particular case of the action of the Horndeski theory. The five
dimensional action includes new gravitational degrees of freedom that were
introduced requiring that the action be invariant under symmetries greater than
the usual Poincare or (A)dS symmetries, namely the so called generalized
Poincare algebras B5.Comment: To be published in Nucl. Phys.
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