19 research outputs found
High-precision calculation of the strange nucleon electromagnetic form factors
We report a direct lattice QCD calculation of the strange nucleon
electromagnetic form factors and in the kinematic range . For the first time, both and
are shown to be nonzero with high significance. This work uses
closer-to-physical lattice parameters than previous calculations, and achieves
an unprecedented statistical precision by implementing a recently proposed
variance reduction technique called hierarchical probing. We perform
model-independent fits of the form factor shapes using the -expansion and
determine the strange electric and magnetic radii and magnetic moment. We
compare our results to parity-violating electron-proton scattering data and to
other theoretical studies.Comment: 6 pages, 5 figures. v2: references adde
The leading hadronic contribution to the running of the Weinberg angle using covariant coordinate-space methods
We present a preliminary study of the leading hadronic contribution to the
running of the Weinberg angle . The running is extracted
from the correlation function of the electromagnetic current with the vector
part of the weak neutral current using both the standard time-momentum
representation method and the Lorentz-covariant coordinate-space method
recently introduced by Meyer. Both connected and disconnected contributions
have been computed on non-perturbatively -improved
Wilson fermions configurations from the CLS initiative. Similar covariant
coordinate-space methods can be used to compute the leading hadronic
contribution to the anomalous magnetic moment of the muon and to
the running of the QED coupling .Comment: 7 pages, 2 figures, talk presented at The 36th Annual International
Symposium on Lattice Field Theory, July 22-28, 2018, East Lansing, MI, US
Frequency-splitting estimators of single-propagator traces
Single-propagator traces are the most elementary fermion Wick contractions
which occur in numerical lattice QCD, and are usually computed by introducing
random-noise estimators to profit from volume averaging. The additional
contribution to the variance induced by the random noise is typically orders of
magnitude larger than the one due to the gauge field. We propose a new family
of stochastic estimators of single-propagator traces built upon a frequency
splitting combined with a hopping expansion of the quark propagator, and test
their efficiency in two-flavour QCD with pions as light as 190 MeV. Depending
on the fermion bilinear considered, the cost of computing these diagrams is
reduced by one to two orders of magnitude or more with respect to standard
random-noise estimators. As two concrete examples of physics applications, we
compute the disconnected contributions to correlation functions of two vector
currents in the isosinglet omega channel and to the hadronic vacuum
polarization relevant for the muon anomalous magnetic moment. In both cases,
estimators with variances dominated by the gauge noise are computed with a
modest numerical effort. Theory suggests large gains for disconnected three and
higher point correlation functions as well. The frequency-splitting estimators
and their split-even components are directly applicable to the newly proposed
multi-level integration in the presence of fermions.Comment: 26 pages, 8 figures, LaTe
Interpolating the Trace of the Inverse of Matrix
We develop heuristic interpolation methods for the function , where the
matrices and are symmetric and positive definite and
is a real variable. This function is featured in many applications in
statistics, machine learning, and computational physics. The presented
interpolation functions are based on the modification of a sharp upper bound
that we derive for this function, which is a new trace inequality for matrices.
We demonstrate the accuracy and performance of the proposed method with
numerical examples, namely, the marginal maximum likelihood estimation for
linear Gaussian process regression and the estimation of the regularization
parameter of ridge regression with the generalized cross-validation method