4,263 research outputs found
Beta-function, Renormalons and the Mass Term from Perturbative Wilson Loops
Several Wilson loops on several lattice sizes are computed in Perturbation
Theory via a stochastic method. Applications include: Renormalons, the Mass
Term in Heavy Quark Effective Theory and (possibly) the beta-function.Comment: 3 pages, 1 eps figure. Contributed to 17th International Symposium on
Lattice Field Theory (LATTICE 99), Pisa, Italy, 29 Jun - 3 Jul 199
The Dirac operator spectrum: a perturbative approach
By computing the Dirac operator spectrum by means of Numerical Stochastic
Perturbation Theory, we aim at throwing some light on the widely accepted
picture for the mechanism which is behind the Bank-Casher relation. The latter
relates the chiral condensate to an accumulation of eigenvalues in the low end
of the spectrum. This can be in turn ascribed to the usual mechanism of
repulsion among eigenvalues which is typical of quantum interactions. First
results appear to confirm that NSPT can indeed enable us to inspect a huge
reshuffling of eigenvalues due to quantum repulsion.Comment: 8 pages, 6 figures; talk presented at the 27th International
Symposium on Lattice Field Theory (Lattice 2009), Beijing, China, 26-31 Jul
200
High-loop perturbative renormalization constants for Lattice QCD (III): three-loop quark currents for Iwasaki gauge action and n_f=4 Wilson fermions
This is the third of a series of papers on three-loop computation of
renormalization constants for Lattice QCD. Our main point of interest are
results for the regularization defined by Iwasaki gauge action and n_f=4 Wilson
fermions. Our results for quark bilinears renormalized according to the RI'-MOM
scheme can be compared to non-perturbative results. The latter are available
for Twisted Mass QCD: being defined in the chiral limit, renormalization
constants must be the same. We also address more general problems. In
particular, we discuss a few methodological issues connected to summing the
perturbative series such as the effectiveness of Boosted Perturbation Theory
and the disentanglement of irrelevant and finite volume contributions.
Discussing these issues we consider ont only the new results of this paper, but
also those for the regularization defined by tree-level Symanzik improved gauge
action and n_f=2 Wilson fermions, which we presented in a recent paper of ours.
We finally comment to which extent the techniques we put at work in the NSPT
context can provide a fresher look into the lattice version of the RI'-MOM
scheme.Comment: 20 pages, 4 figures, pdflatex The Section on different ways of
summing the series has been updated: a few extra informations have been
provided and a clearer notation has been introduce
High loop renormalization constants for Wilson fermions/Symanzik improved gauge action
We present the current status of our computation of quark bilinear
renormalization constants for Wilson fermions and Symanzik improved gauge
action. Computations are performed in Numerical Stochastic Perturbation Theory.
Volumes range from 10^4 to 32^4. Renormalization conditions are those of the
RI'-MOM scheme, imposed at different values of the physical scale. Having
measurements available at several momenta, irrelevant effects are taken into
account by means of hypercubic symmetric Taylor expansions. Finite volumes
effects are assessed repeating the computations at different lattice sizes. In
this way we can extrapolate our results to the continuum limit, in infinite
volume.Comment: 8 pages, 3 figures, talk presented at the 27th International
Symposium on Lattice Field Theory (Lattice 2009), Beijing, China, 26-31 Jul
200
Developments and new applications of numerical stochastic perturbation theory
A review of new developments in numerical stochastic perturbation theory
(NSPT) is presented. In particular, the status of the extension of the method
to gauge fixed lattice QCD is reviewed and a first application to compact
(scalar) QED is presented. Lacking still a general proof of the convergence of
the underlying stochastic processes, a self-consistent method for testing the
results is discussed.Comment: 3 pages, 1 figure. Poster presented at the Lattice97 conference,
Edinburgh, U
Making Racing Fun Through Player Modeling and Track Evolution
This paper addresses the problem of automatically constructing tracks tailor-made to maximize the enjoyment of individual players in a simple car racing game. To this end, some approaches to player modeling are investigated, and a method of using evolutionary algorithms to construct racing tracks is presented. A simple player-dependent metric of entertainment is proposed and used as the fitness function when evolving tracks. We conclude that accurate player modeling poses some significant challenges, but track evolution works well given the right track representation
Four-loop lattice-regularized vacuum energy density of the three-dimensional SU(3) + adjoint Higgs theory
The pressure of QCD admits at high temperatures a factorization into purely
perturbative contributions from "hard" thermal momenta, and slowly convergent
as well as non-perturbative contributions from "soft" thermal momenta. The
latter can be related to various effective gluon condensates in a dimensionally
reduced effective field theory, and measured there through lattice simulations.
Practical measurements of one of the relevant condensates have suffered,
however, from difficulties in extrapolating convincingly to the continuum
limit. In order to gain insight on this problem, we employ Numerical Stochastic
Perturbation Theory to estimate the problematic condensate up to 4-loop order
in lattice perturbation theory. Our results seem to confirm the presence of
"large" discretization effects, going like , where is the
lattice spacing. For definite conclusions, however, it would be helpful to
repeat the corresponding part of our study with standard lattice perturbation
theory techniques.Comment: 35 pages. v2: minor corrections, published versio
Quantum field theories on the Lefschetz thimble
In these proceedings, we summarize the Lefschetz thimble approach to the sign
problem of Quantum Field Theories. In particular, we review its motivations,
and we summarize the results of the application of two different algorithms to
two test models.Comment: contributions to 31st International Symposium on Lattice Field Theory
- LATTICE 2013, July 29 - August 3, 2013, Mainz, Germany and QCD-TNT-III, 2-6
September, 2013, European Centre for Theoretical Studies in Nuclear Physics
and Related Areas (ECT*), Villazzano, Trento (Italy
An efficient flamelet progress-variable method for modeling non-premixed flames in weak electric fields
Combustion stabilization and enhancement of the flammability limits are
mandatory objectives to improve nowadays combustion chambers. At this purpose,
the use of an electric field in the flame region provides a solution which is,
at the same time, easy to implement and effective to modify the flame
structure. The present work describes an efficient flamelet progress-variable
approach developed to model the fluid dynamics of flames immersed in an
electric field. The main feature of this model is that it can use complex
ionization mechanisms without increasing the computational cost of the
simulation. The model is based on the assumption that the combustion process is
not directly influenced by the electric field and has been tested using two
chemi-ionization mechanisms of different complexity in order to examine its
behavior with and without the presence of heavy anions in the mixture. Using a
one- and two-dimensional numerical test cases, the present approach has been
able to reproduce all the major aspects encountered when a flame is subject to
an imposed electric field and the main effects of the different chemical
mechanisms. Moreover, the proposed model is shown to produce a large reduction
in the computational cost, being able to shorten the time needed to perform a
simulation up to 40 times.Comment: 26 pages, 13 figures, paper accepted for publication on Computers and
Fluid
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