148 research outputs found
Searching for the QCD Critical Point Using Particle Ratio Fluctuations and Higher Moments of Multiplicity Distributions
Dynamical fluctuations in global conserved quantities such as baryon number,
strangeness, or charge may be observed near a QCD critical point. Results from
new measurements of dynamical , , and ratio fluctuations
are presented. The commencing of a QCD critical point search at RHIC has
extended the reach of possible measurements of dynamical , , and
ratio fluctuations from Au+Au collisions to lower energies. The STAR
experiment has performed a comprehensive study of the energy dependence of
these dynamical fluctuations in Au+Au collisions at the energies
= 7.7, 11.5, 39, 62.4, and 200 GeV. New results are compared to
previous measurements and to theoretical predictions from several models. The
measured dynamical fluctuations are found to be independent of
collision energy, while dynamical and fluctuations have a
negative value that increases toward zero at top RHIC energy. Fluctuations of
the higher moments of conserved quantities (net-proton and net-charge)
distributions, which are predicted to be sensitive to the presence of a
critical point, are also presented.Comment: 4 pages, 2 figures, Proceedings of the 21st International Conference
On Ultra-Relativistic Nucleus-Nucleus Collisions (Quark Matter 2011), Annecy,
France, May 23 - May 28, 201
Strongly Intensive Measures for Multiplicity Fluctuations
The recently proposed two families of strongly intensive measures of
fluctuations and correlations are studied within Hadron-String-Dynamics (HSD)
transport approach to nucleus-nucleus collisions. We consider the measures
and for kaon and pion multiplicities in Au+Au
collisions in a wide range of collision energies and centralities. These
strongly intensive measures appear to cancel the participant number
fluctuations. This allows to enlarge the centrality window in the analysis of
event-by-event fluctuations up to at least of 10% most central collisions. We
also present a comparison of the HSD results with the data of NA49 and STAR
collaborations. The HSD describes reasonably well. However, the
HSD results depend monotonously on collision energy and do not reproduce the
bump-deep structure of observed from the NA49 data in the
region of the center of mass energy of nucleon pair
GeV. This fact deserves further studies. The origin of this `structure' is not
connected with simple geometrical or limited acceptance effects, as these
effects are taken into account in the HSD simulations
Hadronization of Dense Partonic Matter
The parton recombination model has turned out to be a valuable tool to
describe hadronization in high energy heavy ion collisions. I review the model
and revisit recent progress in our understanding of hadron correlations. I also
discuss higher Fock states in the hadrons, possible violations of the elliptic
flow scaling and recombination effects in more dilute systems.Comment: 8 pages, 4 figures; plenary talk delivered at SQM 2006, to appear in
J. Phys.
Resonances and fluctuations of strange particle in 200 GeV Au-Au collisions
We perform an analysis of preliminary data on strange particles yields and
fluctuations within the Statistical hadronization model. We begin by describing
the theoretical disagreements between different statistical models currently on
the market. We then show how the simultaneous analysis of yields and
fluctuations can be used to differentiate between the different models, and
determine if one of them can be connected to underlying physics. We perform a
study on a RHIC 200 GeV data sample that includes stable particles, resonances,
and the event-by-event fluctuation of the ratio. We show that the
equilibrium statistical model can not describe the fluctuation, unless an
unrealistically small volume is assumed. Such small volume then makes it
impossible to describe the total particle multiplicity. The non-equilibrium
model,on the other hand, describes both the fluctuation and yields
acceptably due to the extra boost to the fluctuation provided by the high
pion chemical potential. and abundance is described
within error bars, but the is under-predicted to 1.5 standard
deviations. We suggest further measurements that have the potential to test the
non-equilibrium model, as well as gauge the effect of re-interactions between
hadronization and freeze-out.Comment: References added, equations corrected. As accepted for publication by
Journal of Physics
Fluctuations of particle ratios as a freeze-out probe
We explain how event-by-event fluctuations of particle ratios can constrain
and falsify the statistical model of particle production in heavy ion
collisions, using fluctuations as an example. We define an observable
capable of determining which statistical model, if any, governs freeze-out in
ultrarelativistic heavy ion collisions. We calculate this observable for
fluctuations, and show that it should be the same for RHIC and LHC
energies, as well as independent of centrality, if the Grand-Canonical
statistical model is an appropriate description and chemical equilibrium
applies. We describe what happens in case of deviations from this scenario,
such as light quark chemical non-equilibrium, strange quark over-saturation and
local conservation (canonical ensemble) for strange quarks. We also introduce a
similar observable capable, together with the published measurement, of
ascertaining if an interacting hadron gas phase governs the system between
thermal and chemical freeze-out, and of ascertaining its duration and impact on
hadronic chemistryComment: Proceedings for talk given at Strangeness 2007, Levoca, Slovaki
Hadronization in heavy ion collisions: recombination or fragmentation?
We show that hadron production in relativistic heavy ion collisions at
transverse momenta larger than 2 GeV/c can be explained by the competition of
two different hadronization mechanisms. Above 5 GeV/c hadron production can be
described by fragmentation of partons that are created perturbatively. Below 5
GeV/c recombination of partons from the dense and hot fireball dominates. This
can explain some of the surprising features of RHIC data like the constant
baryon-to-meson ratio of about one and the small nuclear suppression for
baryons between 2 to 4 GeV/c.Comment: Contribution to the 7th Conference on Strange Quark Matter (SQM
2003), submitted to J.Phys.G; 6 pages LaTeX, 4 eps figures, uses iopart.cl
Azimuthal anisotropy in Au+Au collisions at sqrtsNN = 200 GeV
The results from the STAR Collaboration on directed flow (v_1), elliptic flow
(v_2), and the fourth harmonic (v_4) in the anisotropic azimuthal distribution
of particles from Au+Au collisions at sqrtsNN = 200 GeV are summarized and
compared with results from other experiments and theoretical models. Results
for identified particles are presented and fit with a Blast Wave model.
Different anisotropic flow analysis methods are compared and nonflow effects
are extracted from the data. For v_2, scaling with the number of constituent
quarks and parton coalescence is discussed. For v_4, scaling with v_2^2 and
quark coalescence is discussed.Comment: 26 pages. As accepted by Phys. Rev. C. Text rearranged, figures
modified, but data the same. However, in Fig. 35 the hydro calculations are
corrected in this version. The data tables are available at
http://www.star.bnl.gov/central/publications/ by searching for "flow" and
then this pape
Centrality and transverse momentum dependence of elliptic flow of multi-strange hadrons and meson in Au+Au collisions at = 200 GeV
We present high precision measurements of elliptic flow near midrapidity
() for multi-strange hadrons and meson as a function of
centrality and transverse momentum in Au+Au collisions at center of mass energy
200 GeV. We observe that the transverse momentum dependence of
and is similar to that of and , respectively,
which may indicate that the heavier strange quark flows as strongly as the
lighter up and down quarks. This observation constitutes a clear piece of
evidence for the development of partonic collectivity in heavy-ion collisions
at the top RHIC energy. Number of constituent quark scaling is found to hold
within statistical uncertainty for both 0-30 and 30-80 collision
centrality. There is an indication of the breakdown of previously observed mass
ordering between and proton at low transverse momentum in the
0-30 centrality range, possibly indicating late hadronic interactions
affecting the proton .Comment: 7 pages and 4 figures, Accepted for publication in Physical Review
Letter
Measurements of meson production in relativistic heavy-ion collisions at RHIC
We present results for the measurement of meson production via its
charged kaon decay channel in Au+Au collisions at
, 130, and 200 GeV, and in and +Au collisions
at GeV from the STAR experiment at the BNL Relativistic
Heavy Ion Collider (RHIC). The midrapidity () meson transverse
momentum () spectra in central Au+Au collisions are found to be well
described by a single exponential distribution. On the other hand, the
spectra from , +Au and peripheral Au+Au collisions show power-law tails
at intermediate and high and are described better by Levy
distributions. The constant yield ratio vs beam species, collision
centrality and colliding energy is in contradiction with expectations from
models having kaon coalescence as the dominant mechanism for production
at RHIC. The yield ratio as a function of is consistent
with a model based on the recombination of thermal quarks up to GeV/, but disagrees at higher transverse momenta. The measured nuclear
modification factor, , for the meson increases above unity at
intermediate , similar to that for pions and protons, while is
suppressed due to the energy loss effect in central Au+Au collisions. Number of
constituent quark scaling of both and for the meson
with respect to other hadrons in Au+Au collisions at =200 GeV
at intermediate is observed. These observations support quark
coalescence as being the dominant mechanism of hadronization in the
intermediate region at RHIC.Comment: 22 pages, 21 figures, 4 table
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