767 research outputs found
Dynamics of strange, charm and high momentum hadrons in relativistic nucleus-nucleus collisions
We investigate hadron production and attenuation of hadrons with strange and
charm quarks (or antiquarks) as well as high transverse momentum hadrons in
relativistic nucleus-nucleus collisions from 2 AGeV to 21.3 ATeV within two
independent transport approaches (UrQMD and HSD). Both transport models are
based on quark, diquark, string and hadronic degrees of freedom, but do not
include any explicit phase transition to a quark-gluon plasma. From our
dynamical calculations we find that both models do not describe the maximum in
the ratio at 20 - 30 AGeV in central Au+Au collisions found
experimentally, though the excitation functions of strange mesons are
reproduced well in HSD and UrQMD. Furthermore, the transport calculations show
that the charmonium recreation by reactions is
comparable to the dissociation by 'comoving' mesons at RHIC energies contrary
to SPS energies. This leads to the final result that the total
suppression as a function of centrality at RHIC should be less than the
suppression seen at SPS energies where the 'comover' dissociation is
substantial and the backward channels play no role. Furthermore, our transport
calculations -- in comparison to experimental data on transverse momentum
spectra from , d+Au and Au+Au reactions -- show that pre-hadronic effects
are responsible for both the hardening of the hadron spectra for low transverse
momenta (Cronin effect) as well as the suppression of high hadrons. The
mutual interactions of formed hadrons are found to be negligible in central
Au+Au collisions at = 200 GeV for 6 GeV/c and the
sizeable suppression seen experimentally is attributed to a large extent to the
interactions of 'leading' pre-hadrons with the dense environment.Comment: 10 pages, incl. 7 postscript figures, invited talk presented at the
25th Course "Heavy Ion Reactions from Nuclear to Quark Matter", Erice, Sept.
16-24, 2003, to be publ. in Prog. Part. Nucl. Phy
Production of -pairs at HERA-
The production of -pairs as a possible measure of the polarized gluon
distribution is studied for proton--nucleon collisions at
\sqrt{s} =40\;\mbox{GeV}^2 (HERA-). Possibilities of
reconstructing the helicity state of at least one of the 's are
critically reviewed. The observation of production asymmetries in the single
polarized mode of HERA- is found to be not feasible.Comment: 8 pages, LATeX, 3 figures availabe as .uu-fil
Natural Inflation From Fermion Loops
``Natural'' inflationary theories are a class of models in which inflation is
driven by a pseudo-Nambu-Goldstone boson. In this paper we consider two models,
one old and one new, in which the potential for inflation is generated by loop
effects from a fermion sector which explicitly breaks a global symmetry.
In both models, we retrieve the ``standard'' natural inflation potential,
, as a limiting case of the exact one-loop potential, but we
carry out a general analysis of the models including the limiting case.
Constraints from the COBE DMR observation and from theoretical consistency are
used to limit the parameters of the models, and successful inflation occurs
without the necessity of fine-tuning the parameters.Comment: (Revised) 15 pages, LaTeX (revTeX), 8 figures in uuencoded PostScript
format. Version accepted for publication in Phys. Rev. D 15. Corrected
definition of power spectrum and added three reference
Constructing a CGE database using GEMPACK for an African country
This paper describes how we transformed the 2002 Ugandan Supply Use
Table (SUT) into the required structure of a database for the static UgandanCGEmodel
Dixon et al. (ORANI: A multisectoral model of the Australian economy, 1982). We
describe the unique features captured in the Ugandan SUT as well as that of the CGE
database. We highlight the structural differences of the published data and that of the
CGE database. In describing the SUT we identify data issues, such as negative capital
rentals and omitted data entries that had to be addressed before the database could be
constructed. The ideas put forward in this paper describe, in a pragmatic manner, not
only how to transform published data into a CGE database, but also how to create an
additional sector in the CGE database. For the Ugandan CGE database, we created an
additional Oil sector.http://link.springer.com/journal/106142016-12-31hb2016Economic
Anomalous acoustic reflection on a sliding interface or a shear band
We study the reflection of an acoustic plane wave from a steadily sliding
planar interface with velocity strengthening friction or a shear band in a
confined granular medium. The corresponding acoustic impedance is utterly
different from that of the static interface. In particular, the system being
open, the energy of an in-plane polarized wave is no longer conserved, the work
of the external pulling force being partitioned between frictional dissipation
and gain (of either sign) of coherent acoustic energy. Large values of the
friction coefficient favor energy gain, while velocity strengthening tends to
suppress it. An interface with infinite elastic contrast (one rigid medium) and
V-independent (Coulomb) friction exhibits spontaneous acoustic emission, as
already shown by M. Nosonovsky and G.G. Adams (Int. J. Ing. Sci., {\bf 39},
1257 (2001)). But this pathology is cured by any finite elastic contrast, or by
a moderately large V-strengthening of friction.
We show that (i) positive gain should be observable for rough-on-flat
multicontact interfaces (ii) a sliding shear band in a granular medium should
give rise to sizeable reflection, which opens a promising possibility for the
detection of shear localization.Comment: 13 pages, 10 figure
Strangeness production time and the K+/pi+ horn
We construct a hadronic kinetic model which describes production of strange
particles in ultrarelativistic nuclear collisions in the energy domain of SPS.
We test this model on description of the sharp peak in the excitation function
of multiplicity ratio K+/pi+ and demonstrate that hadronic model reproduces
these data rather well. The model thus must be tested on other types of data in
order to verify the hypothesis that deconfinement sets in at lowest SPS
energies.Comment: proceedings of Hot Quarks 0
Nuclear shadowing at low Q^2
We re-examine the role of vector meson dominance in nuclear shadowing at low
Q^2. We find that models which incorporate both vector meson and partonic
mechanisms are consistent with both the magnitude and the Q^2 slope of the
shadowing data.Comment: 7 pages, 2 figures; to appear in Phys. Rev.
Investigating the origins of transverse spin asymmetries at RHIC
We discuss possible origins of transverse spin asymmetries in hadron-hadron
collisions and propose an explanation in terms of a chiral-odd T-odd
distribution function with intrinsic transverse momentum dependence, which
would signal a correlation between the transverse spin and the transverse
momentum of quarks inside an unpolarized hadron. We will argue that despite its
conceptual problems, it can account for single spin asymmetries, for example in
p + p(transversely polarized) -> pion + X, and at the same time for the large
cos(2 phi) asymmetry in the unpolarized Drell-Yan cross section, which still
lacks understanding. We use the latter asymmetry to arrive at a crude model for
this function and show explicitly how it relates unpolarized and polarized
observables in the Drell-Yan process, as could be measured with the
proton-proton collisions at RHIC. Moreover, it would provide an alternative
method of accessing the transversity distribution function h_1. For future
reference we also list the complete set of azimuthal asymmetries in the
unpolarized and polarized Drell-Yan process at leading order involving T-odd
distribution functions with intrinsic transverse momentum dependence.Comment: 14 pages, Revtex, 4 Postscript figures, uses aps.sty, epsf.sty, Minor
mistakes in cross sections corrected, Conclusions are unaffecte
Azimuthal asymmetries in lepton-pair production at a fixed-target experiment using the LHC beams (AFTER)
A multi-purpose fixed-target experiment using the proton and lead-ion beams
of the LHC was recently proposed by Brodsky, Fleuret, Hadjidakis and Lansberg,
and here we concentrate our study on some issues related to the spin physics
part of this project (referred to as AFTER). We study the nucleon spin
structure through and processes with a fixed-target experiment using
the LHC proton beams, for the kinematical region with 7 TeV proton beams at the
energy in center-of-mass frame of two nucleons GeV. We calculate
and estimate the azimuthal asymmetries of unpolarized and
dilepton production processes in the Drell--Yan continuum region and at the
-pole. We also calculate the , and
azimuthal asymmetries of and dilepton production
processes with the target proton and deuteron longitudinally or transversally
polarized in the Drell--Yan continuum region and around resonances region.
We conclude that it is feasible to measure these azimuthal asymmetries,
consequently the three-dimensional or transverse momentum dependent parton
distribution functions (3dPDFs or TMDs), at this new AFTER facility.Comment: 15 pages, 40 figures. Version accepted for publication in EPJ
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