357 research outputs found
Topological structure of the SU(3) vacuum and exceptional eigenmodes of the improved Wilson-Dirac operator
We present a study of the instanton size and spatial distributions in pure
SU(3) gauge theory using under-relaxed cooling. We also investigate the
low-lying eigenmodes of the (improved) Wilson-Dirac operator, in particular,
the appearance of zero-modes and their space-time localisation with respect to
instantons in the underlying gauge field.Comment: Contribution to Lattice97 proceedings: 3 pages, LaTeX2e, 4 postscript
figures, uses espcrc2.st
Low-Lying Eigenvalues of the Wilson-Dirac Operator
An exploratory study of the low-lying eigenvalues of the Wilson-Dirac
operator and their corresonding eigenvectors is presented. Results for the
eigenvalues from quenched and unquenched simulations are discussed. The
eigenvectors are studied with respect to their localization properties in the
quenched approximation for the cases of SU(2) and SU(3).Comment: Poster presented at LATTICE96(poster). 4 pages, LaTeX, fully coloured
versions of Figs. 4 and 5 are included as separate gzipped PostScript files
or can be obtained from
http://www.desy.de/library/cgi-bin/showprep.pl?desy-rep%2F199615
CP Violation and Strong Phases from Penguins in Decays
We calculate direct CP-violating observables in charged decays
arising from the interference of amplitudes with different strong and CKM
phases. The perturbative strong phases develop at order from
absorptive parts of one-loop matrix elements of the next-to-leading logarithm
corrected effective Hamiltonian. CPT constraints are maintained. Based on this
model, we find that partial rate asymmetries between charge conjugate
decays can be as high as 15-30\% for certain channels with branching ratios in
the range. The small values of the coefficients of angular
correlations, which we calculated previously to be of order , are not
significantly degraded by the strong phases. The charge asymmetries of rates
and angular distributions would provide unambiguous evidence for direct CP
violation.Comment: 24 pages, 3 figures (upon request), LaTeX, preprint DESY 93-19
Dynamical Simulations of Lattice QCD
Lattice calculations of Quantum Chromodynamics (QCD) are continuously becoming more realistic. Where Ukawa famously concluded only fourteen years ago that simulations including two physically light sea quarks are basically impossible even with today’s computers, algorithmic developments over the last years have changed this situation drastically. Nowadays up and down quark masses light enough to control the chiral extrapolation reliably are standard and also the sea quark effects of strange (and charm) quark are included.Modern lattice simulations are an intricate interplay between a large variety of numerical methods on one side and the computer hardware on the other side. The main areas of progress have been the solvers used for the Dirac equation, fermion determinant factorisations and better integrators for the molecular dynamics which is at the heart of most algorithms used for QCD simulations.In lattice QCD simulations the path integral is computed via a Markov Chain Monte Carlo method. In virtually all projects with dynamical fermions a variant of the Hybrid Monte Carlo algorithm is employed to generate the Markov chain, where the fields are updated using molecular dynamics. But there is considerable freedom in how to include the fermion determinant into the simulation. Factorisations of this determinant have been essential in the progress of recent years, being successful in particular together with improved integrators of the molecular dynamics.The solution of the Dirac equation constitutes the most computer time consuming element of simulations with fermions. The dramatic speedup for small fermion mass due to locally deflated solvers5, 6 has therefore had a significant impact on what is possible in the simulations. These algorithms have practically eliminated the increase in cost of the solution as the quark mass is lowered
The -parameter in 3-flavour QCD and by the ALPHA collaboration
We present results by the ALPHA collaboration for the -parameter in
3-flavour QCD and the strong coupling constant at the electroweak scale,
, in terms of hadronic quantities computed on the CLS gauge
configurations. The first part of this proceedings contribution contains a
review of published material \cite{Brida:2016flw,DallaBrida:2016kgh} and yields
the -parameter in units of a low energy scale, . We
then discuss how to determine this scale in physical units from experimental
data for the pion and kaon decay constants. We obtain MeV which translates to
using perturbation theory to match between 3-, 4- and 5-flavour QCD.Comment: 21 pages. Collects contributions of A. Ramos, S. Sint and R. Sommer
to the 34th annual International Symposium on Lattice Field Theory; LaTeX
input encoding problem fixe
A new simulation algorithm for lattice QCD with dynamical quarks
A previously introduced multi-boson technique for the simulation of QCD with
dynamical quarks is described and some results of first test runs on a
lattice with Wilson quarks and gauge group SU(2) are reported.Comment: 7 pages, postscript file (166 KB
B-physics with Wilson fermions
We report the final results of the ALPHA collaboration for some B-physics
observables: , and . We employ CLS configurations with 2
flavors of improved Wilson fermions in the sea and pion masses ranging
down to 190 MeV. The b-quark is treated in HQET to order . The
renormalization, the matching and the improvement were performed
non-perturbatively, and three lattice spacings reaching fm are used
in the continuum extrapolation
Decay constants of B-mesons from non-perturbative HQET with two light dynamical quarks
We present a computation of B-meson decay constants from lattice QCD
simulations within the framework of Heavy Quark Effective Theory for the
b-quark. The next-to-leading order corrections in the HQET expansion are
included non-perturbatively. Based on Nf=2 gauge field ensembles, covering
three lattice spacings a (0.08-0.05)fm and pion masses down to 190MeV, a
variational method for extracting hadronic matrix elements is used to keep
systematic errors under control. In addition we perform a careful
autocorrelation analysis in the extrapolation to the continuum and to the
physical pion mass limits. Our final results read fB=186(13)MeV, fBs=224(14)MeV
and fBs/fB=1.203(65). A comparison with other results in the literature does
not reveal a dependence on the number of dynamical quarks, and effects from
truncating HQET appear to be negligible.Comment: 16 pages including figures and table
The b-quark mass from non-perturbative Heavy Quark Effective Theory at
We report our final estimate of the b-quark mass from lattice QCD
simulations using Heavy Quark Effective Theory non-perturbatively matched to
QCD at . Treating systematic and statistical errors in a conservative
manner, we obtain GeV after an extrapolation to the physical point.Comment: 15 pages including figures and tables; as published in Phys.Lett.B /
typo in table 4 corrected / footnote 1 expande
Preparing for N(f) = 2 simulations at small lattice spacings
We discuss some large effects of dynamical fermions. One is a cutoff effect, others concern the contribution of multi-pion states to correlation functions and are expected to survive the continuum limit. We then turn to the preparation for simulations at small lattice spacings which we are planning down to around a=0.04fm in order to understand the size of O(a^2)-effects of the standard O(a)-improved theory. The dependence of the lattice spacing on the bare coupling is determined through the Schr'odinger functional renormalized coupling
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