238 research outputs found
Density Fluctuations in the Quark-Gluon Plasma
Using the kinetic theory we discuss how the particle and energy densities of
the quark-gluon plasma fluctuate in a space-time cell. The fluctuations in the
equilibrium plasma and in that one from the early stage of ultrarelativistic
heavy-ion collisions are estimated. Within the physically interesting values of
the parameters involved the fluctuations appear sizeable in both cases.Comment: 8 pages, no macro
p_T-fluctuations in high-energy p-p and A-A collisions
The event-by-event p_T-fluctuations in proton-proton and central Pb-Pb
collisions, which have been experimentally studied by means of the so-called
Phi-measure, are analyzed. The contribution due to the correlation which
couples the average p_T to the event multiplicity is computed. The correlation
appears to be far too weak to explain the preliminary experimental value of Phi
(p_T) in p-p interactions. The significance of the result is discussed.Comment: 5 pages, 2 figures, minor improvement
Thermalization vs. Isotropization & Azimuthal Fluctuations
Hydrodynamic description requires a local thermodynamic equilibrium of the
system under study but an approximate hydrodynamic behaviour is already
manifested when a momentum distribution of liquid components is not of
equilibrium form but merely isotropic. While the process of equilibration is
relatively slow, the parton system becomes isotropic rather fast due to the
plasma instabilities. Azimuthal fluctuations observed in relativistic heavy-ion
collisions are argued to distinguish between a fully equilibrated and only
isotropic parton system produced in the collision early stage.Comment: 12 pages, presented at `Correlations and Fluctuations in Relativistic
Nuclear Collisions', MIT, April 05, minor correction
Fluctuations of the azimuthal particle distribution in NA49 at the CERN SPS
Event-by-event fluctuations and correlations in azimuthal angle are currently
widely investigated in various experiments. In this paper the measure
(earlier used in experiments to evaluate fluctuations in transverse momentum)
is now applied to azimuthal angle . Properties of this
function are investigated through fast generators and with complex models such
as Pythia, Hijing, and UrQMD. Preliminary results of NA49 on are
also presented. The system size dependence (p+p, C+C, Si+Si and 6 centralities
of Pb+Pb) at the highest SPS energy (158 GeV) is shown, as well as the
energy dependence (20 - 158 GeV) for the 7.2\% most central Pb+Pb
interactions.Comment: Proceedings from Hot Quarks 2010,June 21-2
Energy Loss of a Heavy Fermion in an Anisotropic QED Plasma
We compute the leading-order collisional energy loss of a heavy fermion
propagating in a QED plasma with an electron distribution function which is
anisotropic in momentum space. We show that in the presence of such
anisotropies there can be a significant directional dependence of the heavy
fermion energy loss with the effect being quite large for highly-relativistic
velocities. We also repeat the analysis of the isotropic case more carefully
and show that the final result depends on the intermediate scale used to
separate hard and soft contributions to the energy loss. We then show that the
canonical isotropic result is obtained in the weak-coupling limit. For
intermediate-coupling we use the residual scale dependence as a measure of our
theoretical uncertainty. We also discuss complications which could arise due to
the presence of unstable soft photonic modes and demonstrate that the
calculation of the energy loss is safe.Comment: 19 pages, 18 figures. v2 - Correction to normalization of numerical
results; some figures modified as a result; discussion of role of unstable
modes added along with two new figure
Fluctuating initial conditions in heavy-ion collisions from the Glauber approach
In the framework of the Glauber approach we analyze the shape parameters of
the early-formed system and their event-by-event fluctuations. We test a
variety of models: the conventional wounded nucleon model, a model admixing
binary collisions to the wounded nucleons, a model with hot spots, as well as
the hot-spot model where the deposition of energy occurs with a superimposed
probability distribution. We look in detail at the so-called participant
multipole moments, obtained by an averaging procedure where in each event the
system is translated to its center of mass and aligned with the major principal
axis of the ellipse of inertia. Quantitative comparisons indicate substantial
relative effects for eccentricity in variants of Glauber models. On the other
hand, the dependence of the scaled standard deviation of the participant
eccentricity on the chosen model is weak. For all models the values range from
about 0.5 for the central collisions to about 0.3-0.4 for peripheral
collisions, both for the gold-gold and copper-copper collisions. They are
dominated by statistics and change only by 10-15% from model to model. We
provide an approximate analytic expansion for the multipole moments and their
fluctuations given in terms of the fixed-axes moments. For central collisions
and in the absence of correlations it gives the simple formula for the scaled
standard deviation of the participant eccentricity: sqrt(4/pi-1). Similarly, we
obtain expansions for the radial profiles of the multipole distributions. We
investigate the relevance of the shape-fluctuation effects for jet quenching
and find them important only for very central events. Finally, we argue how
smooth hydro leads to the known result v_4 ~ v_2^2, and further to the
prediction Delta v_4/v_4 = 2 Delta v_2/v_2.Comment: 20 pages, 15 figures, additions include comparison to the CGC result
New results from fluctuation analysis in NA49 at the CERN SPS
The exploration of the phase diagram of strongly interacting matter,
particularly the study of the phase transition from hadronic to partonic matter
and the search for a hypothetical critical endpoint of the first order
transition line, is one of the most challenging tasks in present heavy ion
physics.
In this talk new results on chemical (particle ratio), transverse momentum,
multiplicity and azimuthal angle fluctuations will be presented. We also
discuss their connection to the onset of deconfinement and to the critical
endpoint.Comment: The Proceedings of the International Conference "Critical Point and
Onset of Deconfinement - CPOD 2011", Wuhan, November 7-11, 201
Perturbative and Nonperturbative Kolmogorov Turbulence in a Gluon Plasma
In numerical simulations of nonabelian plasma instabilities in the hard-loop
approximation, a turbulent spectrum has been observed that is characterized by
a phase-space density of particles with exponent , which is larger than expected from relativistic
scatterings. Using the approach of Zakharov, L'vov and Falkovich, we analyse
possible Kolmogorov coefficients for relativistic -particle
processes, which give at most perturbatively for an energy cascade.
We discuss nonperturbative scenarios which lead to larger values. As an extreme
limit we find the result generically in an inherently nonperturbative
effective field theory situation, which coincides with results obtained by
Berges et al.\ in large- scalar field theory. If we instead assume that
scaling behavior is determined by Schwinger-Dyson resummations such that the
different scaling of bare and dressed vertices matters, we find that
intermediate values are possible. We present one simple scenario which would
single out .Comment: published versio
Contribution of -terms to the total interaction cross sections of relativistic elementary atoms with atoms of matter
It is shown that the corrections of order to the total cross
sections for interaction of elementary hydrogen-like atoms with target atoms,
reported in the previously published paper [S.Mrowczynski, Phys.Rev. D36, 1520
(1987)], do not include some terms of the same order of magnitude. That results
in a significant contribution of these corrections in particular cases. The
full -corrections have been derived and it is shown that they are
really small and could be omitted for most practical applications.Comment: 5 page
Measuring charge fluctuations in high-energy nuclear collisions
Various measures of charge fluctuations in heavy-ion collisions are
discussed. Advantages of the Phi-measure are demonstrated and its relation to
other fluctuation measures is established. To get the relation, Phi is
expressed through the moments of multiplicity distribution. We study how the
measures act in the case of a `background' model which represents the classical
hadron gas in equilibrium. The model assumes statistical particle production
constrained by charge conservation. It also takes into account both the effect
of incomplete experimental apparatus acceptance and that of tracking
inefficiency. The model is shown to approximately agree with the PHENIX and
preliminary STAR data on the electric charge fluctuations. Finally,
`background-free' measures are discussed.Comment: 12 pages, 6 figures, numerous but minor changes, Phys. Rev. C in
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