415 research outputs found
Event anisotropy in 4.2A GeV/c C+C collisions
The directed and elliptic flow of protons and negative pions in 4.2A GeV/c
C+C collisions is studied using the Fourier analysis of azimuthal
distributions. It is found that the protons exhibit pronounced directed flow,
while the flow of pions is either non existent or too weak to be detected
experimentally. Also, it is found that in the entire rapidity interval the
elliptic flow is very small if not zero. These results are confirmed by the
Quark-Gluon-String Model (QGSM) and the relativistic transport model (ART 1.0),
except that these models predict very weak antiflow of pions. The more detailed
comparison with the QGSM suggests that the decay of resonances and rescattering
of secondaries dominantly determine the proton and negative pion flow at this
energy.Comment: 7 pages, 3 figures, TeX file changed from double to single-spacin
Cross-sections for nuclide production in 56Fe target irradiated by 300, 500,750, 1000, 1500, and 2600 MeV protons compared with data on hydrogen target irradiation by 300, 500, 750, 1000, and 1500 MeV/nucleon 56Fe ions
Cross-sections for radioactive nuclide production in 56Fe(p,x) reactions at
300, 500, 750, 1000, 1500, and 2600 MeV were measured using the ITEP U-10
proton accelerator. In total, 221 independent and cumulative yields of products
of half-lives from 6.6 min to 312 days have been obtained via the
direct-spectrometry method. The measured data have been compared with the
experimental data obtained elsewhere by the direct and inverse kinematics
methods and with calculations by 15 codes, namely: MCNPX (INCL, CEM2k, BERTINI,
ISABEL), LAHET (BERTINI, ISABEL), CEM03 (.01, .G1, .S1), LAQGSM03 (.01, .G1,
>.S1), CASCADE-2004, LAHETO, and BRIEFF. Most of our data are in a good
agreement with the inverse kinematics results and disprove the results of some
earlier activation measurements that were quite different from the inverse
kinematics measurements. The most significant calculation-to-experiment
differences are observed in the yields of the A<30 light nuclei, indicating
that further improvements in nuclear reaction models are needed, and pointing
out as well to a necessity of more complete measurements of such reactions.Comment: 53 pages, 9 figures, 6 tables, only pdf file, submitted to Phys. Rev.
The initial state of ultra-relativistic heavy ion collision
A model for energy, pressure and flow velocity distributions at the beginning
of ultra-relativistic heavy ion collisions is presented, which can be used as
an initial condition for hydrodynamic calculations. Our model takes into
account baryon recoil for both target and projectile, arising from the
acceleration of partons in an effective field, produced in the collision. The
typical field strength (string tension) for RHIC energies is about 5-12 GeV/fm,
what allows us to talk about "string ropes". The results show that a QGP forms
a tilted disk, such that the direction of the largest pressure gradient stays
in the reaction plane, but deviates from both the beam and the usual transverse
flow directions. Such initial conditions may lead to the creation of "antiflow"
or "third flow component".Comment: 28 pages, 9 figures. The presentation has been changed considerably.
Some parts of the model have been reformulated, what led to modifications in
several equations: (20-38), Apps. A, B. All the figures have been changed
from 100 GeV/nucl initial energy to the achieved RHIC energy of 65 GeV/nucl.
The last subplots in the Figs. 3, 4, 5, 6 present E=T^{00} in the laboratory
frame now, instead of the energy density in the local rest frame, e, shown in
the initial version. We also added the App. C to clarify the transformation
from space-time to lightcone coordinates and bac
Production of spectator hypermatter in relativistic heavy-ion collisions
We study the formation of large hyper-fragments in relativistic heavy-ion
collisions within two transport models, DCM and UrQMD. Our goal is to explore a
new mechanism for the formation of strange nuclear systems via capture of
hyperons by relatively cold spectator matter produced in semi-peripheral
collisions. We investigate basic characteristics of the produced
hyper-spectators and evaluate the production probabilities of multi-strange
systems. Advantages of the proposed mechanisms over an alternative coalescence
mechanism are analysed. We also discuss how such systems can be detected taking
into account the background of free hyperons. This investigation is important
for the development of new experimental methods for producing hyper-nuclei in
peripheral relativistic nucleus-nucleus collisions, which are now underway at
GSI and are planned for the future FAIR and NICA facilities.Comment: 28 pages, including 10 figures. Submitted to Phys. Rev.
Anisotropic flow of charged and identified hadrons in the quark-gluon string model for Au+Au collisions at sqrt(s) = 200 GeV
The pseudorapidity behaviour of the azimuthal anisotropy parameters v_1 and
v_2 of inclusive charged hadrons and their dependence on the centrality has
been studied in Au+Au collisions at full RHIC energy of sqrt(s) = 200 GeV
within the microscopic quark-gluon string model. The QGSM simulation results
for the directed flow v_1 show antiflow alignment within the pseudorapidity
range |eta| < 2 in a fair agreement with the experimental v_1(eta) data, but
cannot reproduce the further development of the antiflow up to |eta| around
3.5. The eta dependence of the elliptic flow v_2 extracted from the simulations
agrees well with the experimental data in the whole pseudorapidity range for
different centrality classes. The centrality dependence of the integrated
elliptic flow of charged hadrons in the QGSM almost coincides with the PHOBOS
experimental distribution. The transverse momentum dependence of the elliptic
flow of identified and inclusive charged hadrons is studied also. The model
reproduces quantitatively the low p_T part of the distributions rather good,
but underestimates the measured elliptic flow for transverse momenta p_T > 1
GeV/c. Qualitatively, however, the model is able to reproduce the saturation of
the v_2(p_T) spectra with rising p_T as well as the crossing of the elliptic
flow for mesons and baryons.Comment: REVTeX, 10 pages, 10 figures, v2: extended discussion of the model
results, accepted for publication in Phys. Rev.
Microscopic study of freeze-out in relativistic heavy ion collisions at SPS energies
The freeze-out conditions in the light (S+S) and heavy (Pb+Pb) colliding
systems of heavy nuclei at 160 AGeV/ are analyzed within the microscopic
Quark Gluon String Model (QGSM). We found that even for the most heavy systems
particle emission takes place from the whole space-time domain available for
the system evolution, but not from the thin ''freeze-out hypersurface", adopted
in fluid dynamical models. Pions are continuously emitted from the whole volume
of the reaction and reflect the main trends of the system evolution. Nucleons
in Pb+Pb collisions initially come from the surface region. For both systems
there is a separation of the elastic and inelastic freeze-out. The mesons with
large transverse momenta, , are predominantly produced at the early stages
of the reaction. The low -component is populated by mesons coming mainly
from the decay of resonances. This explains naturally the decreasing source
sizes with increasing , observed in HBT interferometry. Comparison with
S+S and Au+Au systems at 11.6 AGeV/ is also presented.Comment: REVTEX, 26 pages incl. 9 figures and 2 tables, to be published in the
Physical Review
Transverse momentum distributions and their forward- backward correlations in the percolating colour string approach
The forward-backward correlations in the distributions, which present a
clear signature of non-linear effects in particle production, are studied in
the model of percolating colour strings. Quantitative predictions are given for
these correlations at SPS, RHIC and LHC energies. Interaction of strings also
naturally explains the flattening of distributions and increase of
with energy and atomic number for nuclear collisionsComment: 6 pages in LaTex, 3 figures in Postscrip
Study of correlations at LHC and RHIC energies in collisions within the quark-gluon string model
The Quark Gluon String Model (QGSM) reproduces well the global
characteristics of the collisions at RHIC and LHC, e.g., the
pseudorapidity and transverse momenta distributions at different centralities.
The main goal of this work is to employ the Monte Carlo QGSM for description of
femtoscopic characteristics in collisions at RHIC and LHC. The study is
concentrated on the low multiplicity and multiplicity averaged events, where no
collective effects are expected. The different procedures for fitting the
one-dimensional correlation functions of pions are studied and compared with
the space-time distributions extracted directly from the model. Particularly,
it is shown that the double Gaussian fit reveals the contributions coming
separately from resonances and from directly produced particles. The comparison
of model results with the experimental data favors decrease of particle
formation time with rising collision energy.Comment: 9 pages, 14 figures, 2 table
Transverse momentum fluctuations and percolation of strings
The behaviour of the transverse momentum fluctuations with the centrality of
the collision shown by the Relativistic Heavy Ion Collider data is naturally
explained by the clustering of color sources. In this framework, elementary
color sources --strings-- overlap forming clusters, so the number of effective
sources is modified. These clusters decay into particles with mean transverse
momentum that depends on the number of elementary sources that conform each
cluster, and the area occupied by the cluster. The transverse momentum
fluctuations in this approach correspond to the fluctuations of the transverse
momentum of these clusters, and they behave essentially as the number of
effective sources.Comment: 16 pages, RevTex, 4 postscript figures. Enhanced version. New figure
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