503 research outputs found
Soil bacterial communities are shaped by temporal and environmental filtering: evidence from a longâterm chronosequence
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/113766/1/emi12762.pd
Nucleus-nucleus collisions at high baryon densities
We study central collision of Pb+Pb at 20, 40, 80 and 160 AGeV within the
UrQMD transport approach and compare rapidity distributions of pi-, K+, K- and
Lambda with the recent measurements from the NA49 Collaboration at 40, 80 and
160 AGeV. It is found that the UrQMD model reasonably describes the data,
however, systematically overpredicts the pi- yield by about 20%, whereas the K+
yield is underestimated by about 15%. The K- yields are in a good agreement
with the experimental data, the Lambda yields are also in a reasonable
correspondence with the data for all energies. We find that hadronic flavour
exchange reactions largely distort the information about the initial
strangeness production mechanism at all energies considered.Comment: 9 pages, including 3 eps figures, submitted to Phys. Lett.
Evidence for non-hadronic interactions of charm degrees of freedom in heavy-ion collisions at relativistic energies
Within the Hadron-String Dynamics (HSD) transport approach we study the
suppression pattern of charmonia at RHIC with respect to centrality and
rapidity employing various model concepts such as variants of the 'comover
absorption' model or the 'charmonium melting' scenario. We find that especially
the ratio of the forward to mid-rapidity nuclear modification factors of J/Psi
(R_AA (forward) / R_AA (mid)) cannot be explained by the interactions with
'formed' comoving mesons or by the 'color screening mechanism' alone. Only when
incorporating interactions of the c or c-bar quark with a pre-hadronic medium
satisfactory results are obtained. A detailed comparison to the PHENIX data
demonstrates that non-hadronic interactions are mandatory to describe the
narrowing of the J/Psi rapidity distribution from p+p to central Au+Au
collisions. The Psi' to J/Psi ratio is found to be crucial in disentangling the
different charmonium absorption scenarios especially in the RHIC energy range.
Furthermore, a comparison of the transport calculations to the statistical
model of Gorenstein and Gazdzicki as well as the statistical hadronization
model of Andronic et al. shows differences in the energy dependence as well as
centrality dependence of the J/Psi to pion ratio which may be exploited
experimentally to disentangle different concepts. We find additionally that the
collective flow of charm in the HSD transport appears compatible with the data
at SPS energies but substantially underestimates the data at top RHIC energies
such that the large elliptic flow v_2 of charm seen experimentally has to be
attributed to early interactions of non-hadronic degrees of freedom.Comment: 35 pages, 16 Figs, v2: additional figure and corresponding changes to
the tex
Probing hadronic formation times with antiprotons in p+A reactions at AGS energies
The production of antiprotons in reactions is calculated in a
microscopic transport approach employing hadronic and string degrees of freedom
(HSD). It is found that the abundancies of antiprotons as observed by the E910
Collaboration in reactions at 12.3 GeV/c as well as 17.5 GeV/c can
approximately be described on the basis of primary proton-nucleon and secondary
meson-baryon production channels for all targets. The transport calculations
demonstrate that the antiproton rapidity distributions for heavy targets are
sensitive to the (or hadron) formation time in the nuclear medium.
Within our analysis the data from the E910 Collaboration are reasonably
described with a formation time of fm/c in the hadron rest frame.Comment: 18 pages, LaTeX, 8 postscript figures; submitted to Nucl. Phys.
Aspects of thermal and chemical equilibration of hadronic matter
We study thermal and chemical equilibration in 'infinite' hadron matter as
well as in finite size relativistic nucleus-nucleus collisions using a BUU
cascade transport model that contains resonance and string degrees-of-freedom.
The 'infinite' hadron matter is simulated within a cubic box with periodic
boundary conditions. The various equilibration times depend on baryon density
and energy density and are much shorter for particles consisting of light
quarks then for particles including strangeness. For kaons and antikaons the
chemical equilibration time is found to be larger than 40 fm/c for all
baryon and energy densities considered. The inclusion of continuum excitations,
i.e. hadron 'strings', leads to a limiting temperature of 150 MeV.
We, furthermore, study the expansion of a hadronic fireball after
equilibration. The slope parameters of the particles after expansion increase
with their mass; the pions leave the fireball much faster then nucleons and
accelerate subsequently heavier hadrons by rescattering ('pion wind'). If the
system before expansion is close to the limiting temperature , the slope
parameters for all particles after expansion practically do not depend on
(initial) energy and baryon density. Finally, the equilibration in relativistic
nucleus-nucleus collision is considered. Since the reaction time here is much
shorter than the equilibration time for strangeness, a chemical equilibrium of
strange particles in heavy-ion collisions is not supported by our transport
calculations. However, the various particle spectra can approximately be
described within the blast model.Comment: 39 pages, LaTeX, including 18 postscript figures, Nucl. Phys. A, in
pres
Universal behavior of baryons and mesons' transverse momentum distributions in the framework of percolation of strings
In the framework of percolation of strings, we present predictions for the
and for mesons and baryons and for ratios
at LHC energies.Comment: Presented at "Heavy Ion Collisions at the LHC: last call for
predictions", Geneva Switzerland, May 14th-June 8t
Influence of Impact Parameter on Thermal Description of Relativistic Heavy Ion Collisions at GSI/SIS
Attention is drawn to the role played by the size of the system in the
thermodynamic analysis of particle yields in relativistic heavy ion collisions
at SIS energies. This manifests itself in the non-linear dependence of K+ and
K- yields in collisions at 1 -- 2 A.GeV on the number of participants. It
is shown that this dependence can be quantitatively well described in terms of
a thermal model with a canonical strangeness conservation. The measured
particle multiplicity ratios (pi+/p, pi-/pi+, d/p, K+/pi+ and K+/K- but not
eta/pi0) in central Au-Au and Ni-Ni collisions at 0.8 -- 2.0 A.GeV are also
explained in the context of a thermal model with a common freeze-out
temperature and chemical potential. Including the concept of collective flow a
consistent picture of particle energy distributions is derived with the flow
velocity being strongly impact-parameter dependent.Comment: revtex, 20 figure
Open charm production in relativistic nucleus-nucleus collisions
We calculate excitation functions for open charm mesons in reactions
from AGS to RHIC energies within the HSD transport approach which is based on
string, quark, diquark () and hadronic degrees
of freedom. The open charm cross sections from and reactions are
fitted to results from PYTHIA and scaled in magnitude to the available
experimental data. From our dynamical calculations we find an approximate
-scaling for pions, kaons, -mesons and -- when discarding
final state elastic scattering of kaons and -mesons with pions -- in
central collisions of at 160 AGeV (with an apparent slope of
176 MeV) without employing the assumption of a Quark-Gluon Plasma (QGP). We
demonstrate that this result is essentially due to a relative -scaling in
collisions at 17.3 GeV. At lower bombarding energies of
25 AGeV a suppression of -mesons by a factor of 10 relative to
a global -scaling with slope 143 MeV is expected. However, when
incorporating attractive -meson self energies as suggested by QCD sum rules,
an approximate -scaling is regained even at 25 AGeV. The effects of
-meson rescattering and charmonium absorption are discussed, furthermore,
with respect to rapidity and transverse mass distributions in central
collisions of at 25, 160 AGeV and 21.3 ATeV.Comment: 46 pages, LaTeX, including 19 postscript figures, to be published in
Nucl. Phys.
Charmonium dynamics in nucleus-nucleus collisions at SPS and FAIR energies
Charmonium production and suppression in In+In and Pb+Pb reactions at SPS
energies is investigated with the HSD transport approach within the 'hadronic
comover model' as well as the 'QGP threshold scenario'. The results of the
transport calculations for J/Psi suppression and the Psi prime to J/Psi ratio
are compared with the recent data of the NA50 and NA60 Collaborations. We find
that the comover absorption model - with a single parameter |M_0|^2 for the
matrix element squared for charmonium-meson dissociation - performs best with
respect to all data sets. The 'threshold scenario' - within different
assumptions for the melting energy densities - yields a reasonable suppression
for J/Psi but fails in reproducing the Psi prime to J/Psi ratio for Pb+Pb at
158 A GeV. Predictions for Au+Au reactions are presented for a bombarding
energy of 25 A GeV in the different scenarios which will allow for a clear
distinction between the models from the experimental side at the future FAIR
facility.Comment: 22 pages, 10 figures. v2: comments added according to referee
suggestions; references updated; Nucl. Phys. A, in pres
Charmonium from Statistical Hadronization of Heavy Quarks -- a Probe for Deconfinement in the Quark-Gluon Plasma
We review the statistical hadronization picture for charmonium production in
ultra-relativistic nuclear collisions. Our starting point is a brief reminder
of the status of the thermal model description of hadron production at high
energy. Within this framework an excellent account is achieved of all data for
hadrons built of (u,d,s) valence quarks using temperature, baryo-chemical
potential and volume as thermal parameters. The large charm quark mass brings
in a new (non-thermal) scale which is explicitely taken into account by fixing
the total number of charm quarks produced in the collision. Emphasis is placed
on the description of the physical basis for the resulting statistical
hadronization model. We discuss the evidence for statistical hadronization of
charmonia by analysis of recent data from the SPS and RHIC accelerators.
Furthermore we discuss an extension of this model towards lower beam energies
and develop arguments about the prospects to observe medium modifications of
open and hidden charm hadrons. With the imminent start of the LHC accelerator
at CERN, exciting prospects for charmonium production studies at the very high
energy frontier come into reach. We present arguments that, at such energies,
charmonium production becomes a fingerprint of deconfinement: even if no
charmonia survive in the quark-gluon plasma, statistical hadronization at the
QCD phase boundary of the many tens of charm quarks expected in a single
central Pb-Pb collision could lead to an enhanced, rather than suppressed
production probability when compared to results for nucleon-nucleon reactions
scaled by the number of hard collisions in the Pb-Pb system.Comment: review article, 27 pages, Landoldt review volume "Relativistic Heavy
Ion Physics", Reinhard Stock, edito
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