39 research outputs found
Superfluid Turbulence: Nonthermal Fixed Point in an Ultracold Bose Gas
Nonthermal fixed points of far-from-equilibrium dynamics of a dilute
degenerate Bose gas are analysed in two and three spatial dimensions. For such
systems, universal power-law distributions, previously found within a
nonperturbative quantum-field theoretic approach, are shown to be related to
vortical dynamics and superfluid turbulence. The results imply an
interpretation of the momentum scaling at the nonthermal fixed points in terms
of independent vortex excitations of the superfluid. Long-wavelength acoustic
excitations on the top of these are found to follow a non-thermal power law.
The results shed light on fundamental aspects of superfluid turbulence and have
strong potential implications for related phenomena studied, e.g., in
early-universe inflation or quark-gluon plasma dynamics.Comment: 5 pages, 5 figure
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
Over-populated gauge fields on the lattice
We study nonequilibrium dynamics of SU(2) pure gauge theory starting from
initial over-population, where intense classical gauge fields are characterized
by a single momentum scale Q_s. Classical-statistical lattice simulations
indicate a quick evolution towards an approximate scaling behavior with
exponent 3/2 at intermediate times. Remarkably, the value for the scaling
exponent may be understood as arising from the leading O(g^2) contribution in
the presence of a time-dependent background field. The phenomenon is associated
to weak wave turbulence describing an energy cascade towards higher momenta.
This particular aspect is very similar to what is observed for scalar theories,
where an effective cubic interaction arises because of the presence of a
time-dependent Bose condensate.Comment: 15 pages, 4 figure
Topological susceptibility of pure gauge theory using Density of States
The topological susceptibility of the SU(3) pure gauge theory is calculated
in the deconfined phase at temperatures up to . At such large
temperatures the susceptibility is suppressed, topologically non-trivial
configurations are extremely rare. Thus, direct lattice simulations are not
feasible. The density of states (DoS) method is designed to simulate rare
events, we present an application of the DoS method to the problem of high
temperature topological susceptibility. We reconstruct the histogram of the
charge sectors that one could have obtained in a naive importance sampling. Our
findings are perfectly consistent with a free instanton gas.Comment: 10 pages, 6 figures, PLB versio
Gauge turbulence, topological defect dynamics, and condensation in Higgs models
The real-time dynamics of topological defects and turbulent configurations of
gauge fields for electric and magnetic confinement are studied numerically
within a 2+1D Abelian Higgs model. It is shown that confinement is appearing in
such systems equilibrating after a strong initial quench such as the
overpopulation of the infrared modes. While the final equilibrium state does
not support confinement, metastable vortex defect configurations appear in the
gauge field which are found to be closely related to the appearance of
physically observable confined electric and magnetic charges. These phenomena
are seen to be intimately related to the approach of a non-thermal fixed point
of the far-from-equilibrium dynamical evolution, signalled by universal scaling
in the gauge-invariant correlation function of the Higgs field. Even when the
parameters of the Higgs action do not support condensate formation in the
vacuum, during this approach, transient Higgs condensation is observed. We
discuss implications of these results for the far-from-equilibrium dynamics of
Yang-Mills fields and potential mechanisms how confinement and condensation in
non-abelian gauge fields can be understood in terms of the dynamics of Higgs
models. These suggest that there is an interesting new class of dynamics of
strong coherent turbulent gauge fields with condensates.Comment: 16 pages, 14 figure
Complex Langevin: Boundary terms and application to QCD
We employ the Complex Langevin method for simulation of complex-valued
actions. First, we show how to test for convergence of the method by
explicitely computing boundary terms and demonstrate this in a model. Then we
investigate the deconfinement phase transition of QCD with
Wilson-fermions using the Complex Langevin Method and. We give preliminary
results for the transition temperatures up to and
compute the curvature coefficient .Comment: Proceedings for The 36th Annual International Symposium on Lattice
Field Theory - LATTICE2018; update: added some acknowledgement
Bose condensation far from equilibrium
The formation of Bose condensates far from equilibrium can play an important
role in our understanding of collision experiments of heavy nuclei or for the
evolution of the early universe. In the relativistic quantum world particle
number changing processes can counteract Bose condensation, and there is a
considerable debate about the relevance of this phenomenon in this context. We
show that the involved question of Bose condensation from initial
over-population can be answered for the example of scalar field theories.
Condensate formation occurs as a consequence of an inverse particle cascade
with a universal power-law spectrum. This particle transport towards low
momenta is part of a dual cascade, in which energy is also transfered by weak
wave turbulence towards higher momenta. To highlight the importance of number
changing processes for the subsequent decay of the condensate, we also compare
to non-relativistic theories with exact number conservation. We discuss the
relevance of these results for nonabelian gauge theories.Comment: 4 pages, 5 figures, PRL version, minor change