636 research outputs found
Latest results from the PHOBOS experiment
Over the past years PHOBOS has continued to analyze the large datasets
obtained from the first five runs of the Relativistic Heavy Ion Collider (RHIC)
at Brookhaven National Laboratory. The two main analysis streams have been
pursued. The first one aims to obtain a broad and systematic survey of global
properties of particle production in heavy ion collisions. The second class
includes the study of fluctuations and correlations in particle production.
Both type of studies have been performed for a variety of the collision
systems, covering a wide range in collision energy and centrality. The uniquely
large angular coverage of the PHOBOS detector and its ability to measure
charged particles down to very low transverse momentum is exploited. The latest
physics results from PHOBOS, as presented at Quark Matter 2008 Conference, are
contained in this report.Comment: 9 pages, 9 figures, presented at the 20th International Conference on
Ultra-Relativistic Nucleus-Nucleus Collisions, "Quark Matter 2008", Jaipur,
India, Feb.4-10, 200
Temporal evolution of tubular initial conditions and their influence on two-particle correlations in relativistic nuclear collisions
Relativistic nuclear collisions data on two-particle correlations exhibit
structures as function of relative azimuthal angle and rapidity. A unified
description of these near-side and away-side structures is proposed for low to
moderate transverse momentum. It is based on the combined effect of tubular
initial conditions and hydrodynamical expansion. Contrary to expectations, the
hydrodynamics solution shows that the high energy density tubes (leftover from
the initial particle interactions) give rise to particle emission in two
directions and this is what leads to the various structures. This description
is sensitive to some of the initial tube parameters and may provide a probe of
the strong interaction. This explanation is compared with an alternative one
where some triangularity in the initial conditions is assumed. A possible
experimental test is suggested.Comment: 6 pages, 6 figure
Trends in multiparticle production and some "predictions" for pp and PbPb collisions at LHC
Based on trends seen at lower energies we "predict" the multiplicities and
pseudorapidity distributions of particle density and elliptic flow that will be
seen in PbPb and pp collisions at the LHC. We argue that, if these predictions
turn out to be correct, either these quantities are insensitive to the state of
matter created in high energy heavy ion collisions or the observed simplicity
and universality of the data must be telling us something profound about the
mechanism of particle production, which to this date is not well understood.Comment: Invited Talk at SQM2007 Conferenc
The Rise and Fall of the Ridge in Heavy Ion Collisions
Recent data from heavy ion collisions at RHIC show unexpectedly large
near-angle correlations that broaden longitudinally with increasing centrality.
The amplitude of this ridge-like correlation rises rapidly, reaches a maximum,
and then falls in the most central collisions. In this letter we explain how
this behavior can be explained as initial-state coordinate-space anisotropies
converted into final-state momentum-space correlations. We propose
as a useful way to study length scales
and provide a prediction for the ridge in Pb+Pb collisions at
2.76 TeV.Comment: 1 Figure and text adde
Study on initial geometry fluctuations via participant plane correlations in heavy ion collisions: part II
Further investigation of the participant plane correlations within a Glauber
model framework is presented, focusing on correlations between three or four
participant planes of different order. A strong correlation is observed for
which is a reflection of the
elliptic shape of the overlap region. The correlation between the corresponding
experimental reaction plane angles can be easily measured. Strong correlations
of similar geometric origin are also observed for
,
,
,
,
, and
, which are also measurable.
Experimental measurements of the corresponding reaction plane correlators in
heavy ion collisions at RHIC and the LHC may improve our understanding of the
physics underlying the measured higher order flow harmonics.Comment: 5 pages, 5 figure
Hydrodynamics: Fluctuating Initial Conditions and Two-particle Correlations
Event-by-event hydrodynamics (or hydrodynamics with fluctuating initial
conditions) has been developed in the past few years. Here we discuss how it
may help to understand the various structures observed in two-particle
correlations.Comment: 7 pages, 9 figures, presented at the Workshop on Saturation, the
Color Glass Condensate and Glasma: What Have we Learned from RHIC
Anomalous NMR Magnetic Shifts in CeCoIn_5
We report ^{115}In and ^{59}Co Nuclear Magnetic Resonance (NMR) measurements
in the heavy fermion superconductor CeCoIn_5 above and below T_c. The hyperfine
couplings of the In and Co are anisotropic and exhibit dramatic changes below
50K due to changes in the crystal field level populations of the Ce ions. Below
T_c the spin susceptibility is suppressed, indicating singlet pairing.Comment: 4 pages, 4 figure
Wounded nucleon model with realistic nucleon-nucleon collision profile and observables in relativistic heavy-ion collisions
We investigate the influence of the nucleon-nucleon collision profile
(probability of interaction as a function of the nucleon-nucleon impact
parameter) in the wounded nucleon model and its extensions on several
observables measured in relativistic heavy-ion collisions. We find that the
participant eccentricity coefficient, , as well as the higher
harmonic coefficients, , are reduced by 10-20% for
mid-peripheral collisions when the realistic (Gaussian) profile is used, as
compared to the case with the commonly-used hard-sphere profile. Similarly, the
multiplicity fluctuations, treated as the function of the number of wounded
nucleons in one of the colliding nuclei, are reduced by 10-20%. This
demonstrates that the Glauber Monte Carlo codes should necessarily use the
realistic nucleon-nucleon collision profile in precision studies of these
observables. The Gaussian collision profile is built-in in {\tt GLISSANDO}.Comment: 8 pages, 7 figure
Quark-Gluon Plasma at RHIC and the LHC: Perfect Fluid too Perfect?
Relativistic heavy ion collisions have reached energies that enable the
creation of a novel state of matter termed the quark-gluon plasma. Many
observables point to a picture of the medium as rapidly equilibrating and
expanding as a nearly inviscid fluid. In this article, we explore the evolution
of experimental flow observables as a function of collision energy and attempt
to reconcile the observed similarities across a broad energy regime in terms of
the initial conditions and viscous hydrodynamics. If the initial spatial
anisotropies are very similar for all collision energies from 39 GeV to 2.76
TeV, we find that viscous hydrodynamics might be consistent with the level of
agreement for v2 of unidentified hadrons as a function of pT . However, we
predict a strong collision energy dependence for the proton v2(pT). The results
presented in this paper highlight the need for more systematic studies and a
re-evaluation of previously stated sensitivities to the early time dynamics and
properties of the medium.Comment: 11 pages, 9 figures, submitted to the New Journal of Physics focus
issue "Strongly Correlated Quantum Fluids: From Ultracold Quantum Gases to
QCD Plasmas
Azimuthal anisotropy of long-range correlations at LHC energy in Monte Carlo model with string fusion
Long-range multiplicity correlations in intervals separated in pseudorapidity and azimuth are studied in the framework of string fusion approach. We applied a Monte Carlo model, in which the string configurations in the transverse plane and rapidity are simulating event-by-event. The string interaction is realized in the lattice string fusion approach with introduction of a grid in the transverse plane. We assumed that the azimuthal anisotropy of particle production is caused by parton energy loss traveling trough the media formed by clusters of fused strings : Îpt/Îx = âα(pt âη)2/3, where η is a string density. In the cellular approach the Bresenhamâs line algorithm has been applied. It is obtained that in AA collisions, the parton energy loss seems to play considerable role, in particular, by providing large contribution to the correlation of mean transverse momentum with multiplicity. The developed approach provides non-zero values flows in p-Pb collisions at LHC energies and produces the pattern similar to the one of the experimental di-hadron analysis
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