5,492 research outputs found
Variational techniques in non-perturbative QCD
We review attempts to apply the variational principle to understand the
vacuum of non-abelian gauge theories. In particular, we focus on the method
explored by Ian Kogan and collaborators, which imposes exact gauge invariance
on the trial Gaussian wave functional prior to the minimization of energy. We
describe the application of the method to a toy model -- confining compact QED
in 2+1 dimensions -- where it works wonderfully and reproduces all known
non-trivial results. We then follow its applications to pure Yang-Mills theory
in 3+1 dimensions at zero and finite temperature. Among the results of the
variational calculation are dynamical mass generation and the analytic
description of the deconfinement phase transition.Comment: 71 pages, 1 figure. To be published in the memorial volume "From
Fields to Strings: Cirvumnavigating Theoretical Physics", World Scientific,
2004. Dedicated to the memory of Ian Koga
Asymmetric dynamics and critical behavior in the Bak-Sneppen model
We investigate, using mean-field theory and simulation, the effect of
asymmetry on the critical behavior and probability density of Bak-Sneppen
models. Two kinds of anisotropy are investigated: (i) different numbers of
sites to the left and right of the central (minimum) site are updated and (ii)
sites to the left and right of the central site are renewed in different ways.
Of particular interest is the crossover from symmetric to asymmetric scaling
for weakly asymmetric dynamics, and the collapse of data with different numbers
of updated sites but the same degree of asymmetry. All non-symmetric rules
studied fall, independent of the degree of asymmetry, in the same universality
class. Conversely, symmetric variants reproduce the exponents of the original
model. Our results confirm the existence of two symmetry-based universality
classes for extremal dynamics.Comment: 14 pages, 8 figures, 1 tabl
Forward-Backward rapidity correlations at all rapidities
We discuss forward-bacward rapidity correlations in the general situation of
asymmetrical collisions, asymmetric rapidity windows, higher rapidities and
higher energy. We give predictions for RHIC and LHC.Comment: 6 pages, 3 figure
Variational analysis of the deconfinement phase transition
We study the deconfining phase transition in 3+1 dimensional pure SU(N)
Yang-Mills theory using a gauge invariant variational calculation. We
generalize the variational ansatz of Phys. Rev. D52, 3719 (1995) to mixed
states (density matrices) and minimize the free energy. For N > 3 we find a
first order phase transition with the transition temperature of T_C = 450 Mev.
Below the critical temperature the Polyakov loop has vanishing expectation
value, while above T_C, its average value is nonzero. According to the standard
lore this corresponds to the deconfining transition. Within the accuracy of our
approximation the entropy of the system in the low temperature phase vanishes.
The latent heat is not small but, rather, is of the order of the
nonperturbative vacuum energy.Comment: 15 pages, correction of minor typos only, submitted to JHE
On the cosmological effects of the Weyssenhoff spinning fluid in the Einstein-Cartan framework
The effects of non-Riemannian structures in Cosmology have been studied long
ago and are still a relevant subject of investigation. In the seventies, it was
discovered that singularity avoidance and early accelerated expansion can be
induced by torsion in the Einstein-Cartan theory. In this framework, torsion is
not dynamical and is completely expressed by means of the spin sources. Thus,
in order to study the effects of torsion in the Einstein-Cartan theory, one has
to introduce matter with spin. In principle, this can be done in several ways.
In this work we consider the cosmological evolution of the universe in the
presence of a constant isotropic and homogeneous axial current and the
Weyssenhoff spinning fluid. We analyse possible solutions of this model, with
and without the spinning fluid.Comment: Work presented at the 7th Alexander Friedmann International Seminar
on Gravitation and Cosmology, Joao Pessoa, Brazil, 29 Jun - 5 Jul 2008. No
figures, 5 pages. New version with dynamical equation corrected, new
reference and a brief comparison with its experimental bound
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