6,258 research outputs found
A microscopic description of sound absorption in the atmosphere
The various mechanisms which contribute to sound absorption in the atmosphere are identified and a technique for computing the contribution from each is presented. The similarities between sound absorption, laser fluorescence measurements, and the opto-acoustic effect are discussed. Finally, experimental sound absorption results are compared to predictions to test the microscopic energy transfer approach
Chemical freeze-out parameters at RHIC from microscopic model calculations
The relaxation of hot nuclear matter to an equilibrated state in the central
zone of heavy-ion collisions at energies from AGS to RHIC is studied within the
microscopic UrQMD model. It is found that the system reaches the
(quasi)equilibrium stage for the period of 10-15 fm/. Within this time the
matter in the cell expands nearly isentropically with the entropy to baryon
ratio . Thermodynamic characteristics of the system at AGS and
at SPS energies at the endpoints of this stage are very close to the parameters
of chemical and thermal freeze-out extracted from the thermal fit to
experimental data. Predictions are made for the full RHIC energy AGeV. The formation of a resonance-rich state at RHIC energies is
discussed.Comment: Talk at the conference Quark Matter'2001, 4 pages, to appear in Nucl.
Phys.
Strangeness Enhancement in Heavy Ion Collisions - Evidence for Quark-Gluon-Matter ?
The centrality dependence of (multi-)strange hadron abundances is studied for
Pb(158 AGeV)Pb reactions and compared to p(158 GeV)Pb collisions. The
microscopic transport model UrQMD is used for this analysis. The predicted
Lambda/pi-, Xi-/pi- and Omega-/pi- ratios are enhanced due to rescattering in
central Pb-Pb collisions as compared to peripheral Pb-Pb or p-Pb collisions. A
reduction of the constituent quark masses to the current quark masses m_s \sim
230 MeV, m_q \sim 10 MeV, as motivated by chiral symmetry restoration, enhances
the hyperon yields to the experimentally observed high values. Similar results
are obtained by an ad hoc overall increase of the color electric field strength
(effective string tension of kappa=3 GeV/fm). The enhancement depends strongly
on the kinematical cuts. The maximum enhancement is predicted around
midrapidity. For Lambda's, strangeness suppression is predicted at
projectile/target rapidity. For Omega's, the predicted enhancement can be as
large as one order of magnitude. Comparisons of Pb-Pb data to proton induced
asymmetric (p-A) collisions are hampered due to the predicted strong asymmetry
in the various rapidity distributions of the different (strange) particle
species. In p-Pb collisions, strangeness is locally (in rapidity) not
conserved. The present comparison to the data of the WA97 and NA49
collaborations clearly supports the suggestion that conventional (free)
hadronic scenarios are unable to describe the observed high (anti-)hyperon
yields in central collisions. The doubling of the strangeness to nonstrange
suppression factor, gamma_s \approx 0.65, might be interpreted as a signal of a
phase of nearly massless particles.Comment: published version, discussion on strange mesons and new table added,
extended discussion on strange baryon yields. Latex, 20 pages, including 5
eps-figure
Atmospheric absorption of high frequency noise and application to fractional-octave bands
Pure tone sound absorption coefficients were measured at 1/12 octave intervals from 4 to 100 KHz at 5.5K temperature intervals between 255.4 and 310.9 K and at 10 percent relative humidity increments between 0 percent and saturation in a large cylindrical tube (i.d., 25.4 cm; length, 4.8 m). Special solid-dielectric capacitance transducers, one to generate bursts of sound waves and one to terminate the sound path and detect the tone bursts, were constructed to fit inside the tube. The absorption was measured by varying the transmitter receiver separation from 1 to 4 m and observing the decay of multiple reflections or change in amplitude of the first received burst. The resulting absorption was compared with that from a proposed procedure for computing sound absorption in still air. Absorption of bands of noise was numerically computed by using the pure tone results. The results depended on spectrum shape, on filter type, and nonlinearly on propagation distance. For some of the cases considered, comparison with the extrapolation of ARP-866A showed a difference as large as a factor of 2. However, for many cases, the absorption for a finite band was nearly equal to the pure tone absorption at the center frequency of the band. A recommended prediction procedure is described for 1/3 octave band absorption coefficients
Local Thermal and Chemical Equilibration and the Equation of State in Relativistic Heavy Ion Collisions
Thermodynamical variables and their time evolution are studied for central
relativistic heavy ion collisions from 10.7 to 160 AGeV in the microscopic
Ultrarelativistic Quantum Molecular Dynamics model (UrQMD). The UrQMD model
exhibits drastic deviations from equilibrium during the early high density
phase of the collision. Local thermal and chemical equilibration of the
hadronic matter seems to be established only at later stages of the quasi-
isentropic expansion in the central reaction cell with volume 125 fm.
distributions at all collision energies for with a unique
Baryon energy spectra in this cell are approximately reproduced by Boltzmann
rapidly dropping temperature. At these times the equation of state has a simple
form: . At 160 AGeV the strong deviation from
chemical equilibrium is found for mesons, especially for pions, even at the
late stage of the reaction. The final enhancement of pions is supported by
experimental data.Comment: 17 Pages, LaTex, 8 eps figures. Talk given at SQM'98 conference,
20-24 July 1998, Padova, Italy, submitted to J. Phys.
Evidence for nonhadronic degrees of freedom in the transverse mass spectra of kaons from relativistic nucleus-nucleus collisions?
We investigate transverse hadron spectra from relativistic nucleus-nucleus
collisions which reflect important aspects of the dynamics - such as the
generation of pressure - in the hot and dense zone formed in the early phase of
the reaction. Our analysis is performed within two independent transport
approaches (HSD and UrQMD) that are based on quark, diquark, string and
hadronic degrees of freedom. Both transport models show their reliability for
elementary as well as light-ion (C+C, Si+Si) reactions. However, for
central Au+Au (Pb+Pb) collisions at bombarding energies above 5
AGeV the measured transverse mass spectra have a larger
inverse slope parameter than expected from the calculation. Thus the pressure
generated by hadronic interactions in the transport models above 5
AGeV is lower than observed in the experimental data. This finding shows
that the additional pressure - as expected from lattice QCD calculations at
finite quark chemical potential and temperature - is generated by strong
partonic interactions in the early phase of central Au+Au (Pb+Pb) collisions.Comment: 4 pages, 3 figures,discussions extended, references added, to be
published in Phys. Rev. Let
Relativistic Hadron-Hadron Collisions in the Ultra-Relativistic Quantum Molecular Dynamics Model (UrQMD)
Hadron-hadron collisions at high energies are investigated in the
Ultra-relativistic-Quantum-Molecular-Dynamics approach (UrQMD). This
microscopic transport model is designed to study pp, pA and A+A collisions. It
describes the phenomenology of hadronic interactions at low and intermediate
energies ( GeV) in terms of interactions between known hadrons and
their resonances. At high energies, GeV, the excitation of color
strings and their subsequent fragmentation into hadrons dominates the multiple
production of particles in the UrQMD model. The model shows a fair overall
agreement with a large body of experimental h-h data over a wide range of h-h
center-of-mass energies. Hadronic reaction data with higher precision would be
useful to support the use of the UrQMD model for relativistic heavy ion
collisions.Comment: 66 pages, Download the UrQMD model from
http://www.th.physik.uni-frankfurt.de/~urqmd/urqmd.htm
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