4,347 research outputs found
Relativistic Langevin Dynamics in Expanding Media
We study the consequences of different realizations of diffusion processes in
relativistic Langevin simulations. We elaborate on the Ito-Stratonovich dilemma
by showing how microscopically calculated transport coefficients as obtained
from a Boltzmann/Fokker-Planck equation can be implemented to lead to an
unambiguous realization of the Langevin process. Pertinent examples within the
pre-point (Ito) and post-point (H\"anggi-Klimontovich) Langevin prescriptions
are worked out explicitly. Deviations from this implementation are shown to
generate variants of the Boltzmann distribution as the stationary (equilibrium)
solutions. Finally, we explicitly verify how the Lorentz invariance of the
Langevin process is maintained in the presence of an expanding medium,
including the case of an "elliptic flow" transmitted to a Brownian test
particle. This is particularly relevant for using heavy-flavor diffusion as a
quantitative tool to diagnose transport properties of QCD matter as created in
ultrarelativistic heavy-ion collisions.Comment: 8 pages, 4 figures; v2: Reference adde
Dilepton production from non-equilibrium hot hadronic matter
It is investigated under which conditions an adiabatic adaption of the
dynamic and spectral information of vector mesons to the changing medium in
heavy ion collisions, as assumed in schematic model calculations and
microscopic transport simulations, is a valid assumption. Therefore time
dependent medium modifications of low mass vector mesons are studied within a
non-equilibrium quantum field theoretical description. Timescales for the
adaption of the spectral properties are given and non-equilibrium dilepton
yields are calculated, leading to the result that memory effects are not
negligible for most scenarios.Comment: 6 pages, 4 figures, To appear in the proceedings of the 43rd
International Winter Meeting on Nuclear Physics, Bormio, Italy, 13 Mar - 20
Mar 200
Dileptons in High-Energy Heavy-Ion Collisions
The current status of our understanding of dilepton production in
ultrarelativistic heavy-ion collisions is discussed with special emphasis on
signals from the (approach towards) chirally restored and deconfined phases. In
particular, recent results of the CERN-SPS low-energy runs are compared to
model predictions and interpreted. Prospects for RHIC experiments are given.Comment: Invited talk at ICPAQGP, Jaipur, India, Nov. 26-30, 2001; 1 Latex and
9 eps-/ps-files Reoprt No.: SUNY-NTG-02-0
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
QCD at small non-zero quark chemical potentials
We study the effects of small chemical potentials associated with the three
light quark flavors in QCD. We use a low-energy effective field theory that
solely relies on the symmetries of the QCD partition function. We find three
different phases: a normal phase, a pion superfluid phase and a kaon superfluid
phase. The two superfluid phases are separated by a first order phase
transition, whereas the normal phase and either of the superfluid phases are
separated by a second order phase transition. We compute the quark-antiquark
condensate, the pion condensate and the kaon condensate in each phase, as well
as the isospin density, the strangeness density, and the mass spectrum.Comment: 14 pages, 5 figure
The hydrological regime of a forested tropical Andean catchment.
The hydrology of tropical mountain catchments plays a central role in ecological function, geochemical and biogeochemical cycles, erosion and sediment production, and water supply in globally important environments. There have been few studies quantifying the seasonal and annual water budgets in the montane tropics, particularly in cloud forests. We investigated the water balance and hydrologic regime of the Kosñipata catchment (basin area: 164.4 km2) over the period 2010â2011. The catchment spans over 2500 m in elevation in the eastern Peruvian Andes and is dominated by tropical montane cloud forest with some high-elevation puna grasslands. Catchment-wide rainfall was 3112 ± 414 mm yrâ1, calculated by calibrating Tropical Rainfall Measuring Mission (TRMM) 3B43 rainfall with rainfall data from nine meteorological stations in the catchment. Cloud water input to streamflow was 316 ± 116 mm yrâ1 (9.2% of total inputs), calculated from an isotopic mixing model using deuterium excess (Dxs) and ÎŽD of waters. Field streamflow was measured in 2010 by recording height and calibrating to discharge. River run-off was estimated to be 2796 ± 126 mm yrâ1. Actual evapotranspiration (AET) was 688 ± 138 mm yrâ1, determined using the Priestley and TaylorâJet Propulsion Laboratory (PT-JPL) model. The overall water budget was balanced within 1.6 ± 13.7%. Relationships between monthly rainfall and river run-off follow an anticlockwise hysteresis through the year, with a persistence of high run-off after the end of the wet season. The size of the soil and shallow groundwater reservoir is most likely insufficient to explain sustained dry-season flow. Thus, the observed hysteresis in rainfallârun-off relationships is best explained by sustained groundwater flow in the dry season, which is consistent with the water isotope results that suggest persistent wet-season sources to streamflow throughout the year. These results demonstrate the importance of transient groundwater storage in stabilising the annual hydrograph in this region of the Andes
The latitude dependence and probability distribution of polar mesospheric turbulence
International audienceWe consider in-situ observations and results from a global circulation model to study the latitude dependence and probability distribution of polar mesospheric turbulence. A comparison of summer observations at 69° N and 79° N shows that mesospheric turbulence weakens towards the summer pole. Furthermore, these data suggest that at both latitudes in about ~70% of all samples there are non-turbulent altitude bins in the considered altitude range between 70 and 95 km. The remaining 30% with detectable turbulence show an approximately log-normal distribution of dissipation rates. A low-resolution model version with a gravity wave (GW) parameterization explains the observed latitude dependence as a consequence of a downshift of the breaking levels towards the summer pole and an accompanying decay of turbulent heating per unit mass. When we do not use a GW parameterization but employ a high spatial resolution instead to simulate GW effects explicitly, the model predicts a similar latitudinal dependence with weakening turbulence towards the summer pole. In addition, the model also produces a log-normal distribution of dissipation rates. The simulated probability distribution is more narrow than in the observations since the model resolves at most mid-frequency GWs, whereas real turbulence is also excited by smaller-scale disturbances. The GW resolving simulation suggests a weaker tropospheric GW source at polar latitudes as the dominating mechanism for the latitudinal dependence
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
In-medium broadening of nucleon resonances
We analyze the effects of an in-medium broadening of nucleon resonances on
the exclusive photoproduction of mesons on nuclei as well as on the total
photoabsorption cross sections in a transport calculation. We show that the
resonance widths observed in semi-inclusive photoproduction on nuclei are
insensitive to an in-medium broadening of nucleon resonances. This is due to a
simple effect: the sizeable width of the nuclear surface and Fermi motion.Comment: 4 pages, 3 figures, minor changes in the tex
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