615 research outputs found
Telecommunications and data acquisition support for the Pioneer Venus Project: Pioneers 12 and 13, prelaunch through March 1984
The support provided by the Telecommunications and Data Acquisition organization of the Jet Propulsion Laboratory (JPL) to the Pioneer Venus missions is described. The missions were the responsibility of the Ames Research Center (ARC). The Pioneer 13 mission and its spacecraft design presented one of the greatest challenges to the Deep Space Network (DSN) in the implementation and operation of new capabilities. The four probes that were to enter the atmosphere of Venus were turned on shortly before arrival at Venus, and the DSN had to acquire each of these probes in order to recover the telemetry being transmitted. Furthermore, a science experiment involving these probes descending through the atmosphere required a completed new data type to be generated at the ground stations. This new data type is known as the differential very long baseline interferometry. Discussions between ARC and JPL of the implementation requirements involved trade-offs in spacecraft design and led to a very successful return of science data. Specific implementation and operational techniques are discussed, not only for the prime mission, but also for the extended support to the Pioneer 12 spacecraft (in orbit around Venus) with its science instruments including that for radar observations of the planet
In-medium Yang-Mills equations: a derivation and canonical quantization
The equations for Yang-Mills field in a medium are derived in a linear
approximation with respect to the gauge coupling parameter and the external
field. The obtained equations closely resemble the macroscopic Maxwell
equations. A canonical quantization is performed for a family of Fermi-like
gauges in the case of constant and diagonal (in the group indices) tensors of
electric permittivity and magnetic permeability. The physical subspace is
defined and the gauge field propagator is evaluated for a particular choice of
the gauge. The propagator is applied for evaluation of the cross-section of
ellastic quark scattering in the Born approximation. Possible applications to
Cherenkov-type gluon radiation are commented briefly.Comment: 27 pages, references added, version extended with emphasis on
non-Abelian gauge group impact on medium characteristics. To appear in J.
Phys.
Hadronization of a Quark-Gluon Plasma in the Chromodielectric Model
We have carried out simulations of the hadronization of a hot, ideal but
effectively massive quark-gluon gas into color neutral clusters in the
framework of the semi-classical SU(3) chromodielectric model. We have studied
the possible quark-gluon compositions of clusters as well as the final mass
distribution and spectra, aiming to obtain an insight into relations between
hadronic spectral properties and the confinement mechanism in this model.Comment: 34 pages, 37 figure
Bremsstrahlung from an Equilibrating Quark-Gluon Plasma
The photon production rate from a chemically equilibrating quark-gluon plasma
likely to be produced at RHIC (BNL) and LHC (CERN) energies is estimated taking
into account bremsstrahlung. The plasma is assumed to be in local thermal
equilibrium, but with a phase space distribution that deviates from the Fermi
or Bose distribution by space-time dependent factors (fugacities). The photon
spectrum is obtained by integrating the photon rate over the space-time history
of the plasma, adopting a boost invariant cylindrically symmetric transverse
expansion of the system with different nuclear profile functions. Initial
conditions obtained from a self-screened parton cascade calculation and, for
comparison, from the HIJING model are used. Compared to an equilibrated plasma
at the same initial energy density, taken from the self-screened parton
cascade, a moderate suppression of the photon yield by a factor of one to five
depending on the collision energy and the photon momentum is observed. The
individual contributions to the photon production, however, are completely
different in the both scenarios.Comment: 14 pages, 4 figures, shortened version to be published in Phys. Rev.
Photon Emission from a Parton Gas at Chemical Non-Equilibrium
We compute the hard photon production rate of a chemically non-equilibrated
quark-gluon plasma. We assume that the plasma is already thermally
equilibrated, i.~e. describable by a temperature, but with a phase-space
distribution that deviates from the Fermi/Bose distribution by a time dependent
factor (fugacity). The photon spectrum is obtained by integrating the photon
rate over the space-time evolution of the quark-gluon plasma. Some consequences
for ultrarelativistic heavy ion collisions are discussed.Comment: 17 pages, uuencoded Postscript file, figures include
Chemical equilibration and thermal dilepton production from the quark gluon plasma at finite baryon density
The chemical equilibration of a highly unsaturated quark-gluon plasma has
been studied at finite baryon density. It is found that in the presence of
small amount of baryon density, the chemical equilibration for gluon becomes
slower and the temperature decreases less steeply as compared to the baryon
free plasma. As a result, the space time integrated yield of dilepton is
enhanced if the initial temperature of the plasma is held fixed. Even at a
fixed initial energy density, the suppression of the dilepton yields at higher
baryo-chemical potential is compensated, to a large extent, by the slow cooling
of the plasma.Comment: Latex, 19 pages, 8 postscript figures. To appear in Phys. Rev.
Hydrodynamical Description of 200 A GeV/c S+Au Collisions: Hadron and Electromagnetic Spectra
We study relativistic S+Au collisions at 200 A GeV/c using a hydrodynamical
approach. We test various equations of state (EOSs), which are used to describe
the strongly interacting matter at densities attainable in the CERN-SPS heavy
ion experiments. For each EOS, suitable initial conditions can be determined to
reproduce the experimental hadron spectra; this emphasizes the ambiguity
between the initial conditions and the EOS in such an approach. Simultaneously,
we calculate the resulting thermal photon and dielectron spectra, and compare
with experiments. If one allows the excitation of resonance states with
increasing temperature, the electro-magnetic signals from scenarios with and
without phase transition are very similar and are not resolvable within the
current experimental resolution. With regard to the CERES dilepton data, none
of the EOSs considered, in conjunction with the standard leading order dilepton
rates, succeed in reproducing the observed excess of dileptons below the rho
peak. Our work, however, suggests that an improved measurement of the photon
and dilepton spectra has the potential to strongly constrain the EOS.Comment: Uses REVTeX, 48 pages, 13 Postscript figure
Robust Detection of Hierarchical Communities from Escherichia coli Gene Expression Data
Determining the functional structure of biological networks is a central goal
of systems biology. One approach is to analyze gene expression data to infer a
network of gene interactions on the basis of their correlated responses to
environmental and genetic perturbations. The inferred network can then be
analyzed to identify functional communities. However, commonly used algorithms
can yield unreliable results due to experimental noise, algorithmic
stochasticity, and the influence of arbitrarily chosen parameter values.
Furthermore, the results obtained typically provide only a simplistic view of
the network partitioned into disjoint communities and provide no information of
the relationship between communities. Here, we present methods to robustly
detect coregulated and functionally enriched gene communities and demonstrate
their application and validity for Escherichia coli gene expression data.
Applying a recently developed community detection algorithm to the network of
interactions identified with the context likelihood of relatedness (CLR)
method, we show that a hierarchy of network communities can be identified.
These communities significantly enrich for gene ontology (GO) terms, consistent
with them representing biologically meaningful groups. Further, analysis of the
most significantly enriched communities identified several candidate new
regulatory interactions. The robustness of our methods is demonstrated by
showing that a core set of functional communities is reliably found when
artificial noise, modeling experimental noise, is added to the data. We find
that noise mainly acts conservatively, increasing the relatedness required for
a network link to be reliably assigned and decreasing the size of the core
communities, rather than causing association of genes into new communities.Comment: Due to appear in PLoS Computational Biology. Supplementary Figure S1
was not uploaded but is available by contacting the author. 27 pages, 5
figures, 15 supplementary file
Dynamics of Hot Bulk QCD Matter: from the Quark-Gluon Plasma to Hadronic Freeze-Out
We introduce a combined macroscopic/microscopic transport approach employing
relativistic hydrodynamics for the early, dense, deconfined stage of the
reaction and a microscopic non-equilibrium model for the later hadronic stage
where the equilibrium assumptions are not valid anymore. Within this approach
we study the dynamics of hot, bulk QCD matter, which is expected to be created
in ultra-relativistic heavy ion collisions at the SPS, the RHIC and the LHC.
Our approach is capable of self-consistently calculating the freeze-out of the
hadronic system, while accounting for the collective flow on the hadronization
hypersurface generated by the QGP expansion. In particular, we perform a
detailed analysis of the reaction dynamics, hadronic freeze-out, and transverse
flow.Comment: 55 pages, 15 figure
Feasibility studies of the time-like proton electromagnetic form factor measurements with PANDA at FAIR
The possibility of measuring the proton electromagnetic form factors in the
time-like region at FAIR with the \PANDA detector is discussed. Detailed
simulations on signal efficiency for the annihilation of into a
lepton pair as well as for the most important background channels have been
performed. It is shown that precision measurements of the differential cross
section of the reaction can be obtained in a wide
angular and kinematical range. The individual determination of the moduli of
the electric and magnetic proton form factors will be possible up to a value of
momentum transfer squared of (GeV/c). The total cross section will be measured up to (GeV/c).
The results obtained from simulated events are compared to the existing data.
Sensitivity to the two photons exchange mechanism is also investigated.Comment: 12 pages, 4 tables, 8 figures Revised, added details on simulations,
4 tables, 9 figure
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