14 research outputs found
Disconnected hadronic vacuum polarization contribution to the muon g-2 with HISQ
7 pages, 4 figures, The 36th Annual International Symposium on Lattice Field Theory - LATTICE2018We describe a computation of the contribution to the anomalous magnetic moment of the muon from the disconnected part of the hadronic vacuum polarization. We use the highly-improved staggered quark (HISQ) formulation for the current density with gauge configurations generated with four flavors of HISQ sea quarks. The computation is performed by stochastic estimation of the current density using the truncated solver method combined with deflation of low-modes. The parameters are tuned to minimize the computational cost for a given target uncertainty in the current-current correlation function. The calculation presented here is carried out on a single gauge-field ensemble of size with an approximate lattice spacing of fm and with physical sea-quark masses. We describe the methodology and the analysis procedur
Strange nucleon electromagnetic form factors from lattice QCD
We evaluate the strange nucleon electromagnetic form factors using an
ensemble of gauge configurations generated with two degenerate maximally
twisted mass clover-improved fermions with mass tuned to approximately
reproduce the physical pion mass. In addition, we present results for the
disconnected light quark contributions to the nucleon electromagnetic form
factors. Improved stochastic methods are employed leading to high-precision
results. The momentum dependence of the disconnected contributions is fitted
using the model-independent z-expansion. We extract the magnetic moment and the
electric and magnetic radii of the proton and neutron by including both
connected and disconnected contributions. We find that the disconnected light
quark contributions to both electric and magnetic form factors are non-zero and
at the few percent level as compared to the connected. The strange form factors
are also at the percent level but more noisy yielding statistical errors that
are typically within one standard deviation from a zero value.Comment: 10 pages, 11 figure
A Mixed Action Analysis of and Mesons
We study and mesons and their mixing angle in a mixed action
approach with so-called Osterwalder-Seiler valence quarks on a Wilson twisted
mass sea. The gauge configurations have been generated by ETMC for
dynamical quark flavours and for three values of the lattice spacing. The main
results are that differences in between the mixed action and the unitary
approach vanish towards the continuum limit with the expected rate of
. The individual size of the lattice artifacts depends,
however, strongly on the observable used to match unitary and valence actions.
Moreover, we show that for the mass valence and valence plus sea quark
mass dependence differ significantly. Hence, in this case a re-tuning of the
simulation parameters in the valence sector only is not sufficient to
compensate for mismatches in the original quark masses.Comment: 30 pages, 8 figures, version accepted for publicatio
First moment of the flavour octet nucleon parton distribution function using lattice QCD
18 pages, 7 figuresWe perform a lattice computation of the flavour octet contribution to the average quark momentum in a nucleon, \la x\ra^{(8)}_{\mu^2 = 4 \gev^2}. In particular, we fully take the disconnected contributions into account in our analysis for which we use a generalization of the technique developed in \cite{Dinter:2012tt}. We investigate systematic effects with a particular emphasis on the excited states contamination. We find that in the renormalization free ratio \frac{\la x \ra^{(3)}}{\la x \ra^{(8)}} (with \la x \ra^{(3)} the non-singlet moment) the excited state contributions cancel to a large extend making this ratio a promising candidate for a comparison to phenomenological analyses. Our final result for this ratio is in agreement with the phenomenological value and we find, including systematic errors, \frac{\la x \ra^{(3)}}{\la x \ra^{(8)}} = 0.39(1)(4)
Nucleon scalar and tensor charges using lattice QCD simulations at the physical value of the pion mass
We present results on the light, strange and charm nucleon scalar and tensor
charges from lattice QCD, using simulations with flavors of twisted
mass Clover-improved fermions with a physical value of the pion mass. Both
connected and disconnected contributions are included, enabling us to extract
the isoscalar, strange and charm charges for the first time directly at the
physical point. Furthermore, the renormalization is computed non-perturbatively
for both isovector and isoscalar quantities. We investigate excited state
effects by analyzing several sink-source time separations and by employing a
set of methods to probe ground state dominance. Our final results for the
scalar charges are , ,
, and for the tensor charges
, ,
, in the scheme at 2~GeV. The first error is statistical, the second is the
systematic error due to the renormalization and the third the systematic
arising from possible contamination due to the excited states.Comment: 20 pages and 13 figure
Lattice calculation of parton distributions
We report on our exploratory study for the direct evaluation of the parton
distribution functions from lattice QCD, based on a recently proposed new
approach. We present encouraging results using Nf = 2 + 1 + 1 twisted mass
fermions with a pion mass of about 370 MeV. The focus of this work is a
detailed description of the computation, including the lattice calculation, the
matching to an infinite momentum and the nucleon mass correction. In addition,
we test the effect of gauge link smearing in the operator to estimate the
influence of the Wilson line renormalization, which is yet to be done.Comment: Minor changes in the text. Version published in PRD. 19 pages, 6
figure