5,636 research outputs found
Real-time gauge theory simulations from stochastic quantization using optimized updating
Stochastic quantisation is applied to the problem of calculating real-time
evolution on a Minkowskian space-time lattice. We employ optimized updating
using reweighting, or gauge fixing, respectively. These procedures do not
affect the underlying theory, but strongly improve the stability properties of
the stochastic dynamics.Comment: 4 pages, 3 figures, contributed talk to SEWM 2008, Amsterda
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
QCD at high Baryon Density and Temperature: Competing Condensates and the Tricritical Point
The phase diagram of strongly interacting matter is explored as a function of
temperature and baryon number density. We investigate the possible simultaneous
formation of condensates in the conventional quark--anti-quark channel
(breaking chiral symmetry) and in a quark--quark channel leading to color
superconductivity: the spontaneous breaking of color symmetry via the formation
of quark Cooper pairs. We point out that for two massless quark flavors a
tricritical point in the phase diagram separates a chiral symmetry restoring
first order transition at high densities from the second order transition at
high temperatures. Away from the chiral limit this tricritical point becomes a
second order phase transition with Ising model exponents, suggesting that a
long correlation length may develop in heavy ion collisions in which the phase
transition is traversed at the appropriate density.Comment: Talk given at the Workshop on QCD at Finite Baryon Density: A Complex
System with a Complex Action, Bielefeld, Germany, 27-30 Apr 1998; 7 pages;
references adde
Introduction to the nonequilibrium functional renormalization group
In these lectures we introduce the functional renormalization group out of
equilibrium. While in thermal equilibrium typically a Euclidean formulation is
adequate, nonequilibrium properties require real-time descriptions. For quantum
systems specified by a given density matrix at initial time, a generating
functional for real-time correlation functions can be written down using the
Schwinger-Keldysh closed time path. This can be used to construct a
nonequilibrium functional renormalization group along similar lines as for
Euclidean field theories in thermal equilibrium. Important differences include
the absence of a fluctuation-dissipation relation for general
out-of-equilibrium situations. The nonequilibrium renormalization group takes
on a particularly simple form at a fixed point, where the corresponding
scale-invariant system becomes independent of the details of the initial
density matrix. We discuss some basic examples, for which we derive a hierarchy
of fixed point solutions with increasing complexity from vacuum and thermal
equilibrium to nonequilibrium. The latter solutions are then associated to the
phenomenon of turbulence in quantum field theory.Comment: Lectures given at the 49th Schladming Winter School `Physics at all
scales: The Renormalization Group' (to appear in the proceedings); 24 pages,
3 figure
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
Isotropization far from equilibrium
Isotropization occurs on time scales much shorter than the thermal
equilibration time. This is a crucial ingredient for the understanding of
collision experiments of heavy nuclei or other nonequilibrium phenomena in
complex many body systems. We discuss in detail the limitations of estimates
based on standard ``linear'' or relaxation-time approximations, where
isotropization and thermal equilibration rates agree. For a weak-coupling
-model the relaxation-time approximation underestimates the thermal
equilibration time by orders of magnitude, in contrast to the isotropization
time. The characteristic nonequilibrium isotropization rate can be enhanced as
compared to the close-to-equilibrium value. Our results are obtained from the
two-particle irreducible effective action, which includes off-shell and memory
effects and does not involve a gradient expansion. This allows us to determine
the range of validity of a description to lowest-order in gradients, which is
typically employed in kinetic equations.Comment: 27 pages, 7 figures, NPB version, minor text change
Renormalisation of out-of-equilibrium quantum fields
We consider the initial value problem and its renormalisation in the
framework of the two-particle-irreducible (2PI) effective action. We argue that
in the case of appropriately chosen self-consistent initial conditions, the
counterterms needed to renormalise the system in equilibrium are also
sufficient to renormalise its time evolution. In this way we improve on
Gaussian initial conditions which have the disadvantage of generically not
showing a continuum limit. For a more detailed discussion see arXiv:0809.0496.Comment: To appear in the proceedings of SEWM08, Amsterdam, The Netherlands,
26-29 August 200
Ultracold atomic quantum gases far from equilibrium
We calculate the time evolution of a far-from-equilibrium initial state of a
non-relativistic ultracold Bose gas in one spatial dimension. The
non-perturbative approximation scheme is based on a systematic expansion of the
two-particle irreducible effective action in powers of the inverse number of
field components. This yields dynamic equations which contain direct
scattering, memory and off-shell effects that are not captured in mean-field
theory.Comment: 4 pages, Proc. Int. Conf. Strong and Electroweak Matter, SEWM 2006;
Nucl. Phys. A, to be publishe
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