157 research outputs found
Numerical Methods for the QCD Overlap Operator:III. Nested Iterations
The numerical and computational aspects of chiral fermions in lattice quantum
chromodynamics are extremely demanding. In the overlap framework, the
computation of the fermion propagator leads to a nested iteration where the
matrix vector multiplications in each step of an outer iteration have to be
accomplished by an inner iteration; the latter approximates the product of the
sign function of the hermitian Wilson fermion matrix with a vector. In this
paper we investigate aspects of this nested paradigm. We examine several Krylov
subspace methods to be used as an outer iteration for both propagator
computations and the Hybrid Monte-Carlo scheme. We establish criteria on the
accuracy of the inner iteration which allow to preserve an a priori given
precision for the overall computation. It will turn out that the accuracy of
the sign function can be relaxed as the outer iteration proceeds. Furthermore,
we consider preconditioning strategies, where the preconditioner is built upon
an inaccurate approximation to the sign function. Relaxation combined with
preconditioning allows for considerable savings in computational efforts up to
a factor of 4 as our numerical experiments illustrate. We also discuss the
possibility of projecting the squared overlap operator into one chiral sector.Comment: 33 Pages; citations adde
Real-Time Detection of Polymerase Activity Using Supercritical Angle Fluorescence
We investigated the incorporation efficiencies of different fluorescently labelled dNTPs with polymerases by complementary strand synthesis. For this reason single stranded DNA was immobilized on a coverslip and the increase of fluorescence due to the synthesis of the corresponding strand with tagged dNTPs was detected with a supercritical angle fluorescence biosensor in real-time. By comparison of the observed signal intensities it was possible to conclude that the system Cy5-dCTP—Klenow (exonuclease free) fragment gives the best incorporation yield of the investigated enzymes and dNTP
Determination of SU(2) ChPT LECs from 2+1 flavor staggered lattice simulations
By fitting pion masses and decay constants from 2+1 flavor staggered lattice
simulations to the predictions of NLO and NNLO SU(2) chiral perturbation theory
we determine the low-energy constants l_3 and l_4. The lattice ensembles were
generated by the Wuppertal-Budapest collaboration and cover pion masses in the
range of 135 to 435 MeV and lattice scales between 0.7 and 2.0 GeV. By choosing
a suitable scaling trajectory, we were able to demonstrate that precise and
stable results for the LECs can be obtained from continuum ChPT to NLO. The
pion masses available in this work also allow us to study the applicability of
using ChPT to extrapolate from higher masses to the physical pion mass.Comment: 8 pages, 8 figures, 1 table, talk presented at Xth Quark Confinement
and the Hadron Spectrum, Munich, October 201
SU(2) chiral perturbation theory low-energy constants from 2+1 flavor staggered lattice simulations
We extract the next-to-leading-order low-energy constants \bar\ell_3 and
\bar\ell_4 of SU(2) chiral perturbation theory, based on precise lattice data
for the pion mass and decay constant on ensembles generated by the
Wuppertal-Budapest Collaboration for QCD thermodynamics. These ensembles
feature 2+1 flavors of two-fold stout-smeared dynamical staggered fermions
combined with Symanzik glue, with pion masses varying from 135 to 435 MeV,
lattice scales between 0.7 and 2.0 GeV, while m_s is kept fixed at its physical
value. Moderate taste splittings and the scale being set through the pion decay
constant allow us to restrict ourselves to the taste pseudoscalar state and to
use formulas from continuum chiral perturbation theory. Finally, by dropping
the data points near 135 MeV from the fits, we can explore the range of pion
masses that is needed in SU(2) chiral perturbation theory to reliably
extrapolate to the physical point.Comment: 40 pages, 22 figures, 3 tables; v2: expanded discussion, matches
published versio
QCD thermodynamics with continuum extrapolated Wilson fermions II
We continue our investigation of 2+1 flavor QCD thermodynamics using
dynamical Wilson fermions in the fixed scale approach. Two additional pion
masses, approximately 440 MeV and 285 MeV, are added to our previous work at
545 MeV. The simulations were performed at 3 or 4 lattice spacings at each pion
mass. The renormalized chiral condensate, strange quark number susceptibility
and Polyakov loop is obtained as a function of the temperature and we observe a
decrease in the light chiral pseudo-critical temperature as the pion mass is
lowered while the pseudo-critical temperature associated with the strange quark
number susceptibility or the Polyakov loop is only mildly sensitive to the pion
mass. These findings are in agreement with previous continuum results obtained
in the staggered formulation.Comment: 19 pages, 13 figures, published versio
Scaling study for 2 HEX smeared fermions: hadron and quark masses
The goal of this study is to investigate the scaling behaviour of our 2 HEX
action. For this purpose, we compute the spectrum and compare the
results to our 6 EXP action. We find a large scaling window up to along with small scaling corrections at the 2%-level and
full compatibility with our previous study. As a second important observable to
be tested for scaling, we chose the non-perturbatively renormalized quenched
strange quark mass. Here we find a fairly flat scaling with a broad scaling
range up to and perfect agreement with the
literature.Comment: PoS for the XXVIII International Symposium on Lattice Field Theory,
Lattice2010, 7 pages, 4 figure
Leptonic decay-constant ratio from lattice QCD using 2+1 clover-improved fermion flavors with 2-HEX smearing
We present a calculation of the leptonic decay-constant ratio in
2+1 flavor QCD. Our data set includes five lattice spacings and pion masses
reaching down below the physical one. Special emphasis is placed on a careful
study of all systematic uncertainties, especially the continuum extrapolation.
Our result is perfectly compatible with the first-row unitarity constraint of
the Standard Model.Comment: 19 pages, 6 figures, 3 tables; v2: added supplementary analysis,
version published in Phys. Rev.
Systematic errors in partially-quenched QCD plus QED lattice simulations
At the precision reached in current lattice QCD calculations, electromagnetic
effects are becoming numerically relevant. Here, electromagnetic effects are
included by superimposing degrees of freedom on QCD
configurations from the Budapest-Marseille-Wuppertal Collaboration. We present
preliminary results for the electromagnetic corrections to light pseudoscalars
mesons masses and discuss some of the associated systematic errors.Comment: 7 pages, 2 figures, The XXIX International Symposium on Lattice Field
Theory, July 10-16, 2011, Squaw Valley, Lake Tahoe, California, US
High-precision scale setting in lattice QCD
Scale setting is of central importance in lattice QCD. It is required to
predict dimensional quantities in physical units. Moreover, it determines the
relative lattice spacings of computations performed at different values of the
bare coupling, and this is needed for extrapolating results into the continuum.
Thus, we calculate a new quantity, , for setting the scale in lattice QCD,
which is based on the Wilson flow like the scale (M. Luscher, JHEP 1008
(2010) 071). It is cheap and straightforward to implement and compute. In
particular, it does not involve the delicate fitting of correlation functions
at asymptotic times. It typically can be determined on the few per-mil level.
We compute its continuum extrapolated value in 2+1-flavor QCD for physical and
non-physical pion and kaon masses, to allow for mass-independent scale setting
even away from the physical mass point. We demonstrate its robustness by
computing it with two very different actions (one of them with staggered, the
other with Wilson fermions) and by showing that the results agree for physical
quark masses in the continuum limit.Comment: 15 pages, 7 figures, 2 tables; Version published in JHE
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