1,278 research outputs found
The RHMC algorithm for theories with unknown spectral bounds
The Rational Hybrid Monte Carlo (RHMC) algorithm extends the Hybrid Monte
Carlo algorithm for lattice QCD simulations to situations involving fractional
powers of the determinant of the quadratic Dirac operator. This avoids the
updating increment () dependence of observables which plagues the Hybrid
Molecular-dynamics (HMD) method. The RHMC algorithm uses rational
approximations to fractional powers of the quadratic Dirac operator. Such
approximations are only available when positive upper and lower bounds to the
operator's spectrum are known. We apply the RHMC algorithm to simulations of 2
theories for which a positive lower spectral bound is unknown: lattice QCD with
staggered quarks at finite isospin chemical potential and lattice QCD with
massless staggered quarks and chiral 4-fermion interactions (QCD). A
choice of lower bound is made in each case, and the properties of the RHMC
simulations these define are studied. Justification of our choices of lower
bounds is made by comparing measurements with those from HMD simulations, and
by comparing different choices of lower bounds.Comment: Latex(Revtex 4) 25 pages, 8 postscript figure
The Quantum Effective Action, Wave Functions and Yang-Mills (2+1)
We explore the relationship between the quantum effective action and the
ground state (and excited state) wave functions of a field theory. Applied to
the Yang-Mills theory in 2+1 dimensions, we find the leading terms of the
effective action from the ground state wave function previously obtained in the
Hamiltonian formalism by solving the Schrodinger equation.Comment: 16 pages, expanded discussion section, added references, version
accepted for Phys. Rev.
The spectrum of the three-dimensional adjoint Higgs model and hot SU(2) gauge theory
We compute the mass spectrum of the SU(2) adjoint Higgs model in 2+1
dimensions at several points located in the (metastable) confinement region of
its phase diagram. We find a dense spectrum consisting of an almost unaltered
repetition of the glueball spectrum of the pure gauge theory, and additional
bound states of adjoint scalars. For the parameters chosen, the model
represents the effective finite temperature theory for pure SU(2) gauge theory
in four dimensions, obtained after perturbative dimensional reduction.
Comparing with the spectrum of screening masses obtained in recent simulations
of four-dimensional pure gauge theory at finite temperature, for the low lying
states we find quantitative agreement between the full and the effective theory
for temperatures as low as T = 2 Tc. This establishes the model under study as
the correct effective theory, and dimensional reduction as a viable tool for
the description of thermodynamic properties. We furthermore compare the
perturbative contribution O(g.T) with the non-perturbative contributions
O(g^2.T) and O(g^3.T) to the Debye mass. The latter turns out to be dominated
by the scale g^2.T, whereas higher order contributions are small corrections.Comment: LaTeX. Typos corrected and references adde
On the Phase Diagram of the SU(2) Adjoint Higgs Model in 2+1 Dimensions
The phase diagram is investigated for SU(2) lattice gauge theory in d=3,
coupled to adjoint scalars. For small values of the quartic scalar coupling,
lambda, the transition separating Higgs and confinement phases is found to be
first-order, in agreement with earlier work by Nadkarni. The surface of
second-order transitions conjectured by Nadkarni, however, is shown instead to
correspond to crossover behaviour. This conclusion is based on a finite size
analysis of the scalar mass and susceptibility. The nature of the phase
transition at the termination of first-order behaviour is investigated and we
find evidence for a critical point at which the scalar mass vanishes. The
photon mass and confining string tension are measured and are found to be
negligibly small in the Higgs phase. This is correlated with the very small
density of magnetic monopoles in the Higgs phase. The string tension and photon
mass rise rapidly as the crossover is traversed towards the symmetric phase.Comment: LaTeX. Replaced with version to be published in Physics Letters B.
Minor changes onl
The Friedberg-Lee model at finite temperature and density
The Friedberg-Lee model is studied at finite temperature and density. By
using the finite temperature field theory, the effective potential of the
Friedberg-Lee model and the bag constant and have been
calculated at different temperatures and densities. It is shown that there is a
critical temperature when
and a critical chemical potential for fixing
the temperature at . We also calculate the soliton solutions
of the Friedberg-Lee model at finite temperature and density. It turns out that
when (or ), there is a bag constant (or
) and the soliton solutions are stable. However, when (or
) the bag constant (or ) and there is no soliton solution anymore, therefore, the
confinement of quarks disappears quickly.Comment: 12 pages, 11 figures; version accepted for publication in Phys. Rev.
Use of recurrence quantification analysis to examine associations between changes in text structure across an expressive writing intervention and reductions in distress symptoms in women wth breast cancer
The current study presents an exploratory analysis of using Recurrence Quantification Analysis (RQA) to analyze text data from an Expressive Writing Intervention (EWI) for Danish women treated for Breast Cancer. The analyses are based on the analysis of essays from a subsample with the average age 54.6 years (SD = 9.0), who completed questionnaires for cancer-related distress (IES) and depression symptoms (BDI-SF). The results show a significant association between an increase in recurrent patterns of text structure from first to last writing session and a decrease in cancer-related distress at 3 months post-intervention. Furthermore, the change in structure from first to last essay displayed a moderate, but significant correlation with change in cancer-related distress from baseline to 9 months post-intervention. The results suggest that changes in recurrence patterns of text structure might be an indicator of cognitive restructuring that leads to amelioration of cancer-specific distress
String Breaking in Non-Abelian Gauge Theories with Fundamental Matter Fields
We present clear numerical evidence for string breaking in three-dimensional
SU(2) gauge theory with fundamental bosonic matter through a mixing analysis
between Wilson loops and meson operators representing bound states of a static
source and a dynamical scalar. The breaking scale is calculated in the
continuum limit. In units of the lightest glueball we find . The implications of our results for QCD are discussed.Comment: 4 pages, 2 figures; equations (4)-(6) corrected, numerical results
and conclusions unchange
Overlap of the Wilson loop with the broken-string state
Numerical experiments on most gauge theories coupled with matter failed to
observe string-breaking effects while measuring Wilson loops only. We show
that, under rather mild assumptions, the overlap of the Wilson loop operator
with the broken-string state obeys a natural upper bound implying that the
signal of string-breaking is in general too weak to be detected by the
conventional updating algorithms.
In order to reduce the variance of the Wilson loops in 3-D Z_2 gauge Higgs
model we use a new algorithm based on the L\"uscher-Weisz method combined with
a non-local cluster algorithm which allows to follow the decay of rectangular
Wilson loops up to values of the order of 10^{-24}. In this way a sharp signal
of string breaking is found.Comment: 12 pages, 3 figure
Hot electroweak matter near to the endpoint of the phase transition
The electroweak phase transition is investigated near to its endpoint in the
framework of an effective three-dimensional model. We measure the very weak
interface tension with the tunneling correlation length method. First results
for the mass spectrum and the corresponding wave functions in the symmetric
phase are presented.Comment: 3 pages, 5 figures, uses espcrc2.sty, contribution to LATTICE9
The Polyakov Loop and its Relation to Static Quark Potentials and Free Energies
It appears well accepted in the literature that the correlator of Polyakov
loops in a finite temperature system decays with the "average" free energy of
the static quark-antiquark system, and can be decomposed into singlet and
adjoint (or octet for QCD) contributions. By fixing a gauge respecting the
transfer matrix, attempts have been made to extract those contributions
separately. In this paper we point out that the "average" and "adjoint"
channels of Polyakov loop correlators are misconceptions. We show analytically
that all channels receive contributions from singlet states only, and give a
corrected definition of the singlet free energy. We verify this finding by
simulations of the 3d SU(2) pure gauge theory in the zero temperature limit,
which allows to cleanly extract the ground state exponents and the non-trivial
matrix elements. The latter account for the difference between the channels
observed in previous simulations.Comment: 14 pages, 3 figures, 1 table; note and reference adde
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