112 research outputs found
Theoretical Analysis of Acceptance Rates in Multigrid Monte Carlo
We analyze the kinematics of multigrid Monte Carlo algorithms by
investigating acceptance rates for nonlocal Metropolis updates. With the help
of a simple criterion we can decide whether or not a multigrid algorithm will
have a chance to overcome critial slowing down for a given model. Our method is
introduced in the context of spin models. A multigrid Monte Carlo procedure for
nonabelian lattice gauge theory is described, and its kinematics is analyzed in
detail.Comment: 7 pages, no figures, (talk at LATTICE 92 in Amsterdam
Block Spin Effective Action for 4d SU(2) Finite Temperature Lattice Gauge Theory
The Svetitsky-Yaffe conjecture for finite temperature 4d SU(2) lattice gauge
theory is confirmed by observing matching of block spin effective actions of
the gauge model with those of the 3d Ising model. The effective action for the
gauge model is defined by blocking the signs of the Polyakov loops with the
majority rule. To compute it numerically, we apply a variant of the IMCRG
method of Gupta and Cordery.Comment: LaTeX2e, 22 pages, 8 Figure
Kinematics of Multigrid Monte Carlo
We study the kinematics of multigrid Monte Carlo algorithms by means of
acceptance rates for nonlocal Metropolis update proposals. An approximation
formula for acceptance rates is derived. We present a comparison of different
coarse-to-fine interpolation schemes in free field theory, where the formula is
exact. The predictions of the approximation formula for several interacting
models are well confirmed by Monte Carlo simulations. The following rule is
found: For a critical model with fundamental Hamiltonian H(phi), absence of
critical slowing down can only be expected if the expansion of
in terms of the shift psi contains no relevant (mass) term. We also introduce a
multigrid update procedure for nonabelian lattice gauge theory and study the
acceptance rates for gauge group SU(2) in four dimensions.Comment: 28 pages, 8 ps-figures, DESY 92-09
Effective Field Theories
Effective field theories encode the predictions of a quantum field theory at
low energy. The effective theory has a fairly low ultraviolet cutoff. As a
result, loop corrections are small, at least if the effective action contains a
term which is quadratic in the fields, and physical predictions can be read
straight from the effective Lagrangean.
Methods will be discussed how to compute an effective low energy action from
a given fundamental action, either analytically or numerically, or by a
combination of both methods. Basically,the idea is to integrate out the high
frequency components of fields. This requires the choice of a "blockspin",i.e.
the specification of a low frequency field as a function of the fundamental
fields. These blockspins will be the fields of the effective field theory. The
blockspin need not be a field of the same type as one of the fundamental
fields, and it may be composite. Special features of blockspins in nonabelian
gauge theories will be discussed in some detail.
In analytical work and in multigrid updating schemes one needs interpolation
kernels \A from coarse to fine grid in addition to the averaging kernels
which determines the blockspin. A neural net strategy for finding optimal
kernels is presented.
Numerical methods are applicable to obtain actions of effective theories on
lattices of finite volume. The constraint effective potential) is of particular
interest. In a Higgs model it yields the free energy, considered as a function
of a gauge covariant magnetization. Its shape determines the phase structure of
the theory. Its loop expansion with and without gauge fields can be used to
determine finite size corrections to numerical data.Comment: 45 pages, 9 figs., preprint DESY 92-070 (figs. 3-9 added in ps
format
Computing the Roughening Transition of Ising and Solid-On-Solid Models by BCSOS Model Matching
We study the roughening transition of the dual of the 2D XY model, of the
Discrete Gaussian model, of the Absolute Value Solid-On-Solid model and of the
interface in an Ising model on a 3D simple cubic lattice. The investigation
relies on a renormalization group finite size scaling method that was proposed
and successfully tested a few years ago. The basic idea is to match the
renormalization group flow of the interface observables with that of the
exactly solvable BCSOS model. Our estimates for the critical couplings are
, and for
the XY-model, the Discrete Gaussian model and the Absolute Value Solid-On-Solid
model, respectively. For the inverse roughening temperature of the Ising
interface we find . To the best of our knowledge,
these are the most precise estimates for these parameters published so far.Comment: 25 pages, LaTeX file, no figure
Critical Exponents of the 3D Ising Universality Class From Finite Size Scaling With Standard and Improved Actions
We propose a method to obtain an improved Hamiltonian (action) for the Ising
universality class in three dimensions. The improved Hamiltonian has suppressed
leading corrections to scaling. It is obtained by tuning models with two
coupling constants. We studied three different models: the +1,-1 Ising model
with nearest neighbour and body diagonal interaction, the spin-1 model with
states 0,+1,-1, and nearest neighbour interaction, and phi**4-theory on the
lattice (Landau-Ginzburg Hamiltonian). The remarkable finite size scaling
properties of the suitably tuned spin-1 model are compared in detail with those
of the standard Ising model. Great care is taken to estimate the systematic
errors from residual corrections to scaling. Our best estimates for the
critical exponents are nu= 0.6298(5) and eta= 0.0366(8), where the given error
estimates take into account the statistical and systematic uncertainties.Comment: 55 pages, 12 figure
Computing Masses from Effective Transfer Matrices
We study the use of effective transfer matrices for the numerical computation
of masses (or correlation lengths) in lattice spin models. The effective
transfer matrix has a strongly reduced number of components. Its definition is
motivated by a renormalization group transformation of the full model onto a
1-dimensional spin model. The matrix elements of the effective transfer matrix
can be determined by Monte Carlo simulation. We show that the mass gap can be
recovered exactly from the spectrum of the effective transfer matrix. As a
first step towards application we performed a Monte Carlo study for the
2-dimensional Ising model. For the simulations in the broken phase we employed
a multimagnetical demon algorithm. The results for the tunnelling correlation
length are particularly encouraging.Comment: (revised version: a few references added) LaTeX file, 25 pages, 6
PostScript figures, (revised version: a few references added
Quantum broadening of k-strings in gauge theories
We study the thickness of the confining flux tube generated by a pair of
sources in higher representations of the gauge group. Using a simple geometric
picture we argue that the area of the cross-section of the flux tube, as
measured by a Wilson loop probe, grows logarithmically with source separation,
as a consequence of the quantum fluctuations of the underlying k-string. The
slope of the logarithm turns out to be universal, i.e. it is the same for all
the representations and all the gauge theories. We check these predictions in a
3D Z_4 lattice gauge model by comparing the broadening of the 1-string and the
2-string.Comment: 15 pages, 5 figure
Comparison of the ‘Denver regimen’ against acute tuberculosis in the mouse and guinea pig
Objectives: In this study, we sought to compare the sterilizing activity of human-equivalent doses of the ‘Denver regimen ’ against acute tuberculosis (TB) infection in the standard mouse model and in the guinea pig. Methods: Pharmacokinetic studies in guinea pigs were used to establish human-equivalent doses for rifampicin, isoniazid and pyrazinamide. Guinea pigs and mice were aerosol-infected with Mycobacterium tuberculosis CDC1551 and treatment was started 2 weeks later with rifampicin/isoniazid/pyrazinamide for up to 6 months. For the first 2 weeks of therapy, the dosing frequency was 5 days/week, and for the remaining period, twice weekly. Treatment was discontinued in groups of 30 mice and 10 guinea pigs at 5 months and at 6 months, and these animals were held for a further 3 months in order to assess relapse rates. Results: Guinea pig lungs became culture-negative after 3 months of predominantly twice-weekly treatment and relapse rates were 0 % (0/10) both after 5 months and after 6 months of treatment. In contrast, all mice remained culture-positive despite 6 months of the same treatment, and 93 % (28/30) and 69 % (20/29) of mice relapsed after treatment for 5 and 6 months, respectively. Conclusions: Treatment with rifampicin/isoniazid/pyrazinamide administered at human-equivalent doses is much more potent against acute TB infection in guinea pigs than in mice. Our findings have importan
Efficient gene-driven germ-line point mutagenesis of C57BL/6J mice
BACKGROUND: Analysis of an allelic series of point mutations in a gene, generated by N-ethyl-N-nitrosourea (ENU) mutagenesis, is a valuable method for discovering the full scope of its biological function. Here we present an efficient gene-driven approach for identifying ENU-induced point mutations in any gene in C57BL/6J mice. The advantage of such an approach is that it allows one to select any gene of interest in the mouse genome and to go directly from DNA sequence to mutant mice. RESULTS: We produced the Cryopreserved Mutant Mouse Bank (CMMB), which is an archive of DNA, cDNA, tissues, and sperm from 4,000 G(1 )male offspring of ENU-treated C57BL/6J males mated to untreated C57BL/6J females. Each mouse in the CMMB carries a large number of random heterozygous point mutations throughout the genome. High-throughput Temperature Gradient Capillary Electrophoresis (TGCE) was employed to perform a 32-Mbp sequence-driven screen for mutations in 38 PCR amplicons from 11 genes in DNA and/or cDNA from the CMMB mice. DNA sequence analysis of heteroduplex-forming amplicons identified by TGCE revealed 22 mutations in 10 genes for an overall mutation frequency of 1 in 1.45 Mbp. All 22 mutations are single base pair substitutions, and nine of them (41%) result in nonconservative amino acid substitutions. Intracytoplasmic sperm injection (ICSI) of cryopreserved spermatozoa into B6D2F1 or C57BL/6J ova was used to recover mutant mice for nine of the mutations to date. CONCLUSIONS: The inbred C57BL/6J CMMB, together with TGCE mutation screening and ICSI for the recovery of mutant mice, represents a valuable gene-driven approach for the functional annotation of the mammalian genome and for the generation of mouse models of human genetic diseases. The ability of ENU to induce mutations that cause various types of changes in proteins will provide additional insights into the functions of mammalian proteins that may not be detectable by knockout mutations
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