151 research outputs found
Force Statistics and Correlations in Dense Granular Packings
In dense, static, polydisperse granular media under isotropic pressure, the
probability density and the correlations of particle-wall contact forces are
studied. Furthermore, the probability density functions of the populations of
pressures measured with different sized circular pressure cells is examined.
The questions answered are: (i) What is the number of contacts that has to be
considered so that the measured pressure lies within a certain error margin
from its expectation value? (ii) What is the statistics of the pressure
probability density as function of the size of the pressure cell? Astonishing
non-random correlations between contact forces are evidenced, which range at
least 10 to 15 particle diameter. Finally, an experiment is proposed to tackle
and better understand this issue.Comment: 10 pages, 12 figure
Turbulent spectra in real-time gauge field evolution
We investigate ultraviolet fixed points in the real-time evolution of
non-Abelian gauge fields. Classical-statistical lattice simulations reveal
equal-time correlation functions with a spectral index 3/2. Analytical
understanding of this result is achieved by employing a 2PI- loop expansion for
the quantum theory.Comment: 4 pages, 2 figures. Talk presented at SEWM 2008, August 26-29,
Amsterda
From Facility to Application Sensor Data: Modular, Continuous and Holistic Monitoring with DCDB
Today's HPC installations are highly-complex systems, and their complexity
will only increase as we move to exascale and beyond. At each layer, from
facilities to systems, from runtimes to applications, a wide range of tuning
decisions must be made in order to achieve efficient operation. This, however,
requires systematic and continuous monitoring of system and user data. While
many insular solutions exist, a system for holistic and facility-wide
monitoring is still lacking in the current HPC ecosystem. In this paper we
introduce DCDB, a comprehensive monitoring system capable of integrating data
from all system levels. It is designed as a modular and highly-scalable
framework based on a plugin infrastructure. All monitored data is aggregated at
a distributed noSQL data store for analysis and cross-system correlation. We
demonstrate the performance and scalability of DCDB, and describe two use cases
in the area of energy management and characterization.Comment: Accepted at the The International Conference for High Performance
Computing, Networking, Storage, and Analysis (SC) 201
Enhancing the sensitivity of the electro-optical far-field experiment for measuring CSR at KARA
At the KIT storage ring KARA (Karlsruhe Research Accelerator), a far-field electro-optical (EO) experimental setup to measure the temporal profile of the coherent synchrotron radiation (CSR) is implemented. Here, the EOSD (electro-optical spectral decoding) technique will be used to obtain single-shot measurements of the temporal CSR profile in the terahertz frequency domain. To keep the crucial high signal-to-noise ratio a setup based on balanced detection is under commission. Therefore, simulations are performed for an optimized beam path and the setup is characterized. In this contribution, the upgraded setup and first measurements are presented
Role of quantum fluctuations in a system with strong fields: Onset of hydrodynamical flow
Quantum fluctuations are believed to play an important role in the
thermalization of classical fields in inflationary cosmology but their
relevance for isotropization/thermalization of the classical fields produced in
heavy ion collisions is not completely understood. We consider a scalar
toy model coupled to a strong external source, like in the Color Glass
Condensate description of the early time dynamics of ultrarelativistic heavy
ion collisions. The leading order classical evolution of the scalar fields is
significantly modified by the rapid growth of time-dependent quantum
fluctuations, necessitating an all order resummation of such "secular" terms.
We show that the resummed expressions cause the system to evolve in accordance
with ideal hydrodynamics. We comment briefly on the thermalization of our
quantum system and the extension of our results to a gauge theory.Comment: 45 pages, 17 figure
How particles emerge from decaying classical fields in heavy ion collisions: towards a kinetic description of the Glasma
We develop the formalism discussed previously in hep-ph/0601209 and
hep-ph/0605246 to construct a kinetic theory that provides insight into the
earliest ``Glasma'' stage of a high energy heavy ion collision. Particles
produced from the decay of classical fields in the Glasma obey a Boltzmann
equation whose novel features include an inhomogeneous source term and new
contributions to the collision term. We discuss the power counting associated
with the different terms in the Boltzmann equation and outline the transition
from the field dominated regime to the particle dominated regime in high energy
heavy ion collisions.Comment: 29 pages, 16 postscript figures, some typos correcte
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
What the inflaton might tell us about RHIC/LHC
Topical phenomena in high-energy physics related to collision experiments of
heavy nuclei ("Little Bang") and early universe cosmology ("Big Bang") involve
far-from-equilibrium dynamics described by quantum field theory. One example
concerns the role of plasma instabilities for the process of thermalization in
heavy-ion collisions. The reheating of the early universe after inflation may
exhibit rather similar phenomena following a tachyonic or parametric resonance
instability. Certain universal aspects associated to nonthermal fixed points
even quantitatively agree, and considering these phenomena from a common
perspective can be fruitful.Comment: Plenary talk at SEWM08, 9 pages, 6 figure
The approach to thermalization in the classical phi^4 theory in 1+1 dimensions: energy cascades and universal scaling
We study the dynamics of thermalization and the approach to equilibrium in
the classical phi^4 theory in 1+1 spacetime dimensions. At thermal equilibrium
we exploit the equivalence between the classical canonical averages and
transfer matrix quantum traces of the anharmonic oscillator to obtain exact
results for the temperature dependence of several observables, which provide a
set of criteria for thermalization. We find that the Hartree approximation is
remarkably accurate in equilibrium. The non-equilibrium dynamics is studied by
numerically solving the equations of motion in light-cone coordinates for a
broad range of initial conditions and energy densities.The time evolution is
described by several stages with a cascade of energy towards the ultraviolet.
After a transient stage, the spatio-temporal gradient terms become larger than
the nonlinear term and a stage of universal cascade emerges.This cascade starts
at a time scale t_0 independent of the initial conditions (except for very low
energy density). Here the power spectra feature universal scaling behavior and
the front of the cascade k(t) grows as a power law k(t) sim t^alpha with alpha
lesssim 0.25. The wake behind the cascade is described as a state of Local
Thermodynamic Equilibrium (LTE) with all correlations being determined by the
equilibrium functional form with an effective time dependent temperatureTeff(t)
which slowly decreases as sim t^{-alpha}.Two well separated time scales emerge
while Teff(t) varies slowly, the wavectors in the wake with k < k(t) attain LTE
on much shorter time scales.This universal scaling stage ends when the front of
the cascade reaches the cutoff at a time t_1 sim a^{-1/alpha}. Virialization
starts to set much earlier than LTE. We find that strict thermalization is
achieved only for an infinite time scale.Comment: relevance for quantum field theory discussed providing validity
criteria. To appear in Phys. Rev.
The effect of memory on relaxation in a scalar field theory
We derive a kinetic equation with a non-Markovian collision term which
includes a memory effect, from Kadanoff-Baym equations in theory
within the three-loop level for the two-particle irreducible (2PI) effective
action. The memory effect is incorporated into the kinetic equation by a
generalized Kadanoff-Baym ansatz.Based on the kinetic equations with and
without the memory effect, we investigate an influence of this effect on decay
of a single particle excitation with zero momentum in 3+1 dimensions and the
spatially homogeneous case. Numerical results show that, while the time
evolution of the zero mode is completely unaffected by the memory effect due to
a separation of scales in the weak coupling regime, this effect leads first to
faster relaxation than the case without it and then to slower relaxation as the
coupling constant increases.Comment: 12 pages, 6 eps figure
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