4,119 research outputs found
Quark Recombination and Heavy Quark Diffusion in Hot Nuclear Matter
We discuss resonance recombination for quarks and show that it is compatible
with quark and hadron distributions in local thermal equilibrium. We then
calculate realistic heavy quark phase space distributions in heavy ion
collisions using Langevin simulations with non-perturbative T-matrix
interactions in hydrodynamic backgrounds. We hadronize the heavy quarks on the
critical hypersurface given by hydrodynamics after constructing a criterion for
the relative recombination and fragmentation contributions. We discuss the
influence of recombination and flow on the resulting heavy meson and single
electron R_AA and elliptic flow. We will also comment on the effect of
diffusion of open heavy flavor mesons in the hadronic phase.Comment: Contribution to Quark Matter 2011, submitted to J.Phys.G; 4 pages, 5
figure
Polar mesosphere summer echoes (PMSE): review of observations and current understanding
International audiencePolar mesosphere summer echoes (PMSE) are very strong radar echoes primarily studied in the VHF wavelength range from altitudes close to the polar summer mesopause. Radar waves are scattered at irregularities in the radar refractive index which at mesopause altitudes is solely determined by the electron number density. For efficient scatter, the electron number density must reveal structures at the radar half wavelength (Bragg condition; ~3 m for typical VHF radars). The question how such small scale electron number density structures are created in the mesopause region has been a longstanding open scientific question for almost 30 years. This paper reviews experimental and theoretical milestones on the way to an advanced understanding of PMSE. Based on new experimental results from in situ observations with sounding rockets, ground based observations with radars and lidars, numerical simulations with microphysical models of the life cycle of mesospheric aerosol particles, and theoretical considerations regarding the diffusivity of electrons in the ice loaded complex plasma of the mesopause region, a consistent explanation for the generation of these radar echoes has been developed. The main idea is that mesospheric neutral air turbulence in combination with a significantly reduced electron diffusivity due to the presence of heavy charged ice aerosol particles (radii ~5?50 nm) leads to the creation of structures at spatial scales significantly smaller than the inner scale of the turbulent velocity field itself. Importantly, owing to their very low diffusivity, the plasma structures acquire a very long lifetime, i.e. 10 min to hours in the presence of particles with radii between 10 and 50 nm. This leads to a temporal decoupling of active neutral air turbulence and the existence of small scale plasma structures and PMSE and thus readily explains observations proving the absence of neutral air turbulence at PMSE altitudes. With this explanation at hand, it becomes clear that PMSE are a suitable tool to permanently monitor the thermal and dynamical structure of the mesopause region allowing insights into important atmospheric key parameters like temperatures, winds, gravity wave parameters, turbulence, solar cycle effects, and long term changes
Self-consistent parametrization of the two-flavor isotropic color-superconducting ground state
Lack of Lorentz invariance of QCD at finite quark chemical potential in
general implies the need of Lorentz non-invariant condensates for the
self-consistent description of the color-superconducting ground state.
Moreover, the spontaneous breakdown of color SU(3) in this state naturally
leads to the existence of SU(3) non-invariant non-superconducting expectation
values. We illustrate these observations by analyzing the properties of an
effective 2-flavor Nambu-Jona-Lasinio type Lagrangian and discuss the
possibility of color-superconducting states with effectively gapless fermionic
excitations. It turns out that the effect of condensates so far neglected can
yield new interesting phenomena.Comment: 16 pages, 3 figure
Interpretation of Recent SPS Dilepton Data
We summarize our current theoretical understanding of in-medium properties of
the electromagnetic current correlator in view of recent dimuon data from the
NA60 experiment in In(158 AGeV)-In collisions at the CERN-SPS. We discuss the
sensitivity of the results to space-time evolution models for the hot and dense
partonic and hadronic medium created in relativistic heavy-ion collisions and
the contributions from different sources to the dilepton-excess spectra.Comment: To appear in the proceedings of the 19th International Conference on
Ultra-Relativistic Nucleus-Nucleus Collisions (Quark Matter 2006) v2:
references added, minor typos correcte
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Absolute density measurements in the middle atmosphere
In the last ten years a total of 25 sounding rockets employing ionization gauges have been launched at high latitudes ( ~ 70° N) to measure total atmospheric density and its small scale fluctuations in an altitude range between 70 and 110 km. While the determination of small scale fluctuations is unambiguous, the total density analysis has been complicated in the past by aerodynamical disturbances leading to densities inside the sensor which are enhanced compared to atmospheric values. Here, we present the results of both Monte Carlo simulations and wind tunnel measurements to quantify this aerodynamical effect. The comparison of the resulting ‘ram-factor’ profiles with empirically determined density ratios of ionization gauge measurements and falling sphere measurements provides excellent agreement. This demonstrates both the need, but also the possibility, to correct aerodynamical influences on measurements from sounding rockets. We have determined a total of 20 density profiles of the mesosphere-lower-thermosphere (MLT) region. Grouping these profiles according to season, a listing of mean density profiles is included in the paper. A comparison with density profiles taken from the reference atmospheres CIRA86 and MSIS90 results in differences of up to 40%. This reflects that current reference atmospheres are a significant potential error source for the determination of mixing ratios of, for example, trace gas constituents in the MLT region
Heavy-Quark Diffusion and Hadronization in Quark-Gluon Plasma
We calculate diffusion and hadronization of heavy quarks in high-energy
heavy-ion collisions implementing the notion of a strongly coupled quark-gluon
plasma in both micro- and macroscopic components. The diffusion process is
simulated using relativistic Fokker-Planck dynamics for elastic scattering in a
hydrodynamic background. The heavy-quark transport coefficients in the medium
are obtained from non-perturbative -matrix interactions which build up
resonant correlations close to the transition temperature. The latter also form
the basis for hadronization of heavy quarks into heavy-flavor mesons via
recombination with light quarks from the medium. The pertinent resonance
recombination satisfies energy conservation and provides an equilibrium mapping
between quark and meson distributions. The recombination probability is derived
from the resonant heavy-quark scattering rate. Consequently, recombination
dominates at low transverse momentum () and yields to fragmentation at
high . Our approach thus emphasizes the role of resonance correlations in
the diffusion and hadronization processes. We calculate the nuclear
modification factor and elliptic flow of - and -mesons for Au-Au
collisions at the Relativistic Heavy Ion Collider, and compare their
decay-electron spectra to available data. We also find that a realistic
description of the medium flow is essential for a quantitative interpretation
of the data.Comment: 16 pages, 14 figure
The Vector Probe in Heavy-Ion Reactions
We review essential elements in using the channel as a probe for
hot and dense matter as produced in (ultra-) relativistic collisions of heavy
nuclei. The uniqueness of the vector channel resides in the fact that it
directly couples to photons, both real and virtual (dileptons), enabling the
study of thermal radiation and in-medium effects on both light () and heavy () vector mesons. We emphasize the importance
of interrelations between photons and dileptons, and characterize relevant
energy/mass regimes through connections to Quark-Gluon-Plasma emission and
chiral symmetry restoration. Based on critical analysis of our current
understanding of data from fixed-target energies, we identify open key
questions to be addressed.Comment: Invited Talk at the Hot Quarks 2004 Workshop, July 18-24, 2004 (Taos
Valley, NM, USA), 15 pages latex incl 14 figs and iop style files, to appear
in the proceeding
Hadro-Chemistry and Evolution of (Anti-) Baryon Densities at RHIC
The consequences of hadro-chemical freezeout for the subsequent hadron gas
evolution in central heavy-ion collisions at RHIC and LHC energies are
discussed with special emphasis on effects due to antibaryons. Contrary to
naive expectations, their individual conservation, as implied by experimental
data, has significant impact on the chemical off-equilibrium composition of
hadronic matter at collider energies. This may reflect on a variety of
observables including source sizes and dilepton spectra.Comment: 4 pages ReVTeX incl. 3 ps-figs, submitted to PR
Enhancement of Threshold Cross Sections by In-Medium Final State Interactions
We address the problem of pion production in low energy -nucleus
collisions. For the production mechanism we assume a simple model consisting of
a coherent sum of single pion exchange and the excitation---followed by the
decay into two pions and a nucleon---of the resonance. The
production amplitude is modified by the final state interaction between the
pions calculated using the chirally improved J\"ulich meson exchange model
including the polarization of the nuclear medium by the pions. The model
reproduces well the experimentally observed cross
sections, especially the enhancement with increasing of the
mass distribution in the threshold region.Comment: 5 pages RevTeX, 3-eps figure
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