10,055 research outputs found
Two-loop QCD gauge coupling at high temperatures
We determine the 2-loop effective gauge coupling of QCD at high temperatures,
defined as a matching coefficient appearing in the dimensionally reduced
effective field theory. The result allows to improve on one of the classic
non-perturbative probes for the convergence of the weak-coupling expansion at
high temperatures, the comparison of full and effective theory determinations
of an observable called the spatial string tension. We find surprisingly good
agreement almost down to the critical temperature of the deconfinement phase
transition. We also determine one new contribution of order O(g^6T^4) to the
pressure of hot QCD.Comment: 17 page
Spatial string tension revisited
The spatial string tension, a classic non-perturbative probe for the
convergence of the weak-coupling expansion at high temperatures, can be
determined in full QCD as well as in a dimensionally reduced effective theory.
Comparing both approaches, we find surprisingly good agreement almost down to
the critical temperature of the deconfinement phase transition.Comment: 6 pages, 2 figures, talk given at Lattice 2005 (nonzero temperature
and density
Constraints on the NMSSM from the oblique parameters
Electroweak precision measurements, encoded in the oblique parameters, give
strong constraints on physics beyond the Standard Model. The oblique parameters
S, T, U (V, W, X) are calculated in the next-to-minimal supersymmetric model
(NMSSM). We outline the calculation of the oblique parameters in terms of
one-loop gauge-boson selfenergies and find sensitive restrictions for the NMSSM
parameter space.Comment: 11 pages, 7 figures, 1 tabl
Engineering test procedure for the patient assist device
The test procedure completely tests the functioning of the patient assist device. All modes of operation are included in the test. Patient activated switches may be chosen for use with this device as desired. These switches, however, must be single-pole, single-throw, momentarily-on switches
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
Non-perturbative plaquette in 3d pure SU(3)
We present a determination of the elementary plaquette and, after the
subsequent ultraviolet subtractions, of the finite part of the gluon
condensate, in lattice regularization in three-dimensional pure SU(3) gauge
theory. Through a change of regularization scheme to MSbar and a matching back
to full four-dimensional QCD, this result determines the first non-perturbative
contribution in the weak-coupling expansion of hot QCD pressure.Comment: 6 pages, 4 figures, talk presented at Lattice 2005 (Non-zero
temperature and density
Four-loop pressure of massless O(N) scalar field theory
Inspired by the corresponding problem in QCD, we determine the pressure of
massless O(N) scalar field theory up to order g^6 in the weak-coupling
expansion, where g^2 denotes the quartic coupling constant. This necessitates
the computation of all 4-loop vacuum graphs at a finite temperature: by making
use of methods developed by Arnold and Zhai at 3-loop level, we demonstrate
that this task is manageable at least if one restricts to computing the
logarithmic terms analytically, while handling the ``constant'' 4-loop
contributions numerically. We also inspect the numerical convergence of the
weak-coupling expansion after the inclusion of the new terms. Finally, we point
out that while the present computation introduces strategies that should be
helpful for the full 4-loop computation on the QCD-side, it also highlights the
need to develop novel computational techniques, in order to be able to complete
this formidable task in a systematic fashion.Comment: 34 page
Renormalization of infrared contributions to the QCD pressure
Thanks to dimensional reduction, the infrared contributions to the QCD
pressure can be obtained from two different three-dimensional effective field
theories, called the Electrostatic QCD (Yang-Mills plus adjoint Higgs) and the
Magnetostatic QCD (pure Yang-Mills theory). Lattice measurements have been
carried out within these theories, but a proper interpretation of the results
requires renormalization, and in some cases also improvement, i.e. the removal
of terms of O(a) or O(a^2). We discuss how these computations can be
implemented and carried out up to 4-loop level with the help of Numerical
Stochastic Perturbation Theory.Comment: 7 pages, 4 figures, talk presented at Lattice 2006 (High temperature
and density
The leading non-perturbative coefficient in the weak-coupling expansion of hot QCD pressure
Using Numerical Stochastic Perturbation Theory within three-dimensional pure
SU(3) gauge theory, we estimate the last unknown renormalization constant that
is needed for converting the vacuum energy density of this model from lattice
regularization to the MSbar scheme. Making use of a previous non-perturbative
lattice measurement of the plaquette expectation value in three dimensions,
this allows us to approximate the first non-perturbative coefficient that
appears in the weak-coupling expansion of hot QCD pressure.Comment: 16 pages. v2: published versio
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