4,050 research outputs found
Calculating the hadronic vacuum polarization and leading hadronic contribution to the muon anomalous magnetic moment with improved staggered quarks
We present a lattice calculation of the hadronic vacuum polarization and the
lowest-order hadronic contribution to the muon anomalous magnetic moment, a_\mu
= (g-2)/2, using 2+1 flavors of improved staggered fermions. A precise fit to
the low-q^2 region of the vacuum polarization is necessary to accurately
extract the muon g-2. To obtain this fit, we use staggered chiral perturbation
theory, including the vector particles as resonances, and compare these to
polynomial fits to the lattice data. We discuss the fit results and associated
systematic uncertainties, paying particular attention to the relative
contributions of the pions and vector mesons. Using a single lattice spacing
ensemble (a=0.086 fm), light quark masses as small as roughly one-tenth the
strange quark mass, and volumes as large as (3.4 fm)^3, we find a_\mu^{HLO} =
(713 \pm 15) \times 10^{-10} and (748 \pm 21) \times 10^{-10} where the error
is statistical only and the two values correspond to linear and quadratic
extrapolations in the light quark mass, respectively. Considering systematic
uncertainties not eliminated in this study, we view this as agreement with the
current best calculations using the experimental cross section for e^+e^-
annihilation to hadrons, 692.4 (5.9) (2.4)\times 10^{-10}, and including the
experimental decay rate of the tau lepton to hadrons, 711.0 (5.0)
(0.8)(2.8)\times 10^{-10}. We discuss several ways to improve the current
lattice calculation.Comment: 44 pages, 4 tables, 17 figures, more discussion on matching the chpt
calculation to lattice calculation, typos corrected, refs added, version to
appear in PR
Order of the Chiral and Continuum Limits in Staggered Chiral Perturbation Theory
Durr and Hoelbling recently observed that the continuum and chiral limits do
not commute in the two dimensional, one flavor, Schwinger model with staggered
fermions. I point out that such lack of commutativity can also be seen in
four-dimensional staggered chiral perturbation theory (SChPT) in quenched or
partially quenched quantities constructed to be particularly sensitive to the
chiral limit. Although the physics involved in the SChPT examples is quite
different from that in the Schwinger model, neither singularity seems to be
connected to the trick of taking the nth root of the fermion determinant to
remove unwanted degrees of freedom ("tastes"). Further, I argue that the
singularities in SChPT are absent in most commonly-computed quantities in the
unquenched (full) QCD case and do not imply any unexpected systematic errors in
recent MILC calculations with staggered fermions.Comment: 14 pages, 1 figure. v3: Spurious symbol, introduced by conflicting
tex macros, removed. Clarification of discussion in several place
K to pi and K to 0 in 2+1 Flavor Partially Quenched Chiral Perturbation Theory
We calculate results for K to pi and K to 0 matrix elements to
next-to-leading order in 2+1 flavor partially quenched chiral perturbation
theory. Results are presented for both the Delta I=1/2 and 3/2 channels, for
chiral operators corresponding to current-current, gluonic penguin, and
electroweak penguin 4-quark operators. These formulas are useful for studying
the chiral behavior of currently available 2+1 flavor lattice QCD results, from
which the low energy constants of the chiral effective theory can be
determined. The low energy constants of these matrix elements are necessary for
an understanding of the Delta I=1/2 rule, and for calculations of
epsilon'/epsilon using current lattice QCD simulations.Comment: 43 pages, 2 figures, uses RevTeX, added and updated reference
Heavy-Light Semileptonic Decays in Staggered Chiral Perturbation Theory
We calculate the form factors for the semileptonic decays of heavy-light
pseudoscalar mesons in partially quenched staggered chiral perturbation theory
(\schpt), working to leading order in , where is the heavy quark
mass. We take the light meson in the final state to be a pseudoscalar
corresponding to the exact chiral symmetry of staggered quarks. The treatment
assumes the validity of the standard prescription for representing the
staggered ``fourth root trick'' within \schpt by insertions of factors of 1/4
for each sea quark loop. Our calculation is based on an existing partially
quenched continuum chiral perturbation theory calculation with degenerate sea
quarks by Becirevic, Prelovsek and Zupan, which we generalize to the staggered
(and non-degenerate) case. As a by-product, we obtain the continuum partially
quenched results with non-degenerate sea quarks. We analyze the effects of
non-leading chiral terms, and find a relation among the coefficients governing
the analytic valence mass dependence at this order. Our results are useful in
analyzing lattice computations of form factors and when the
light quarks are simulated with the staggered action.Comment: 53 pages, 8 figures, v2: Minor correction to the section on finite
volume effects, and typos fixed. Version to be published in Phys. Rev.
Leptonic decay constants f_Ds and f_D in three flavor lattice QCD
We determine the leptonic decay constants in three flavor unquenched lattice
QCD. We use O(a^2)-improved staggered light quarks and O(a)-improved charm
quarks in the Fermilab heavy quark formalism. Our preliminary results, based
upon an analysis at a single lattice spacing, are f_Ds = 263(+5-9)(+/-24) MeV
and f_D = 225(+11-13)(+/-21) MeV. In each case, the first reported error is
statistical while the is the combined systematic uncertainty.Comment: Talk presented at Lattice2004(heavy), Fermilab, June 21-26, 2004. 3
pages, 2 figure
Light hadrons with improved staggered quarks: approaching the continuum limit
We have extended our program of QCD simulations with an improved
Kogut-Susskind quark action to a smaller lattice spacing, approximately 0.09
fm. Also, the simulations with a approximately 0.12 fm have been extended to
smaller quark masses. In this paper we describe the new simulations and
computations of the static quark potential and light hadron spectrum. These
results give information about the remaining dependences on the lattice
spacing. We examine the dependence of computed quantities on the spatial size
of the lattice, on the numerical precision in the computations, and on the step
size used in the numerical integrations. We examine the effects of
autocorrelations in "simulation time" on the potential and spectrum. We see
effects of decays, or coupling to two-meson states, in the 0++, 1+, and 0-
meson propagators, and we make a preliminary mass computation for a radially
excited 0- meson.Comment: 43 pages, 16 figure
Recommended from our members
Dissociating visuo-spatial and verbal working memory: It’s all in the features
Echoing many of the themes of the seminal work of Atkinson and Shiffrin (1968), this paper uses the Feature Model (Nairne, 1988, 1990; Neath & Nairne, 1995) to account for performance in working memory tasks. The Brooks verbal and visuo-spatial matrix tasks were performed alone, with articulatory suppression, or with a spatial suppression task; the results produced the expected dissociation. We used Approximate Bayesian Computation techniques to fit the Feature Model to the data and showed that the similarity-based interference process implemented in the model accounted for the data patterns well. We then fit the model to data from Guérard and Tremblay (2008); the latter study produced a double dissociation while calling upon more typical order reconstruction tasks. Again, the model performed well. The findings show that a double dissociation can be modelled without appealing to separate systems for verbal and visuo-spatial processing. The latter findings are significant as the Feature Model had not been used to model this type of dissociation before; importantly, this is also the first time the model is quantitatively fit to data. For the demonstration provided here, modularity was unnecessary if two assumptions were made: (1) the main difference between spatial and verbal working memory tasks is the features that are encoded; (2) secondary tasks selectively interfere with primary tasks to the extent that both tasks involve similar features. It is argued that a feature-based view is more parsimonious (see Morey, 2018) and offers flexibility in accounting for multiple benchmark effects in the field
Light extinction coefficients specific to the understory vegetation of the southern boreal forest, Quebec
This study was conducted in six different forest types in Abitibi, Que, (i) to identify the factors that most influence understory light transmission in the southern boreal forest and (ii) to develop light extinction coefficients (k), which could be used to simulate light transmission in the understory. Light availability and understory vegetation (cover, composition, vertical distribution, and leaf area index) were characterized within three strata (0.05-5 m) in a total of 180 quadrats. Calculated k values were based on measured light availability and leaf area index. These values varied among forest types, strata, understory vegetation types, and cover in the upper stratum The highest k values were generally associated with a dense stratum of Acer spicatum Lam. We developed five sets of k values based on the factors that most affected light transmission. Measured transmission (T(m)) was compared with transmission predicted (T(p)) from each set of k values Light transmission predicted using a single k value (mean k = 0 54) underestimated T(m). More accurate predictions were obtained when we used the other four sets of k values. Our results indicate that, in the southern boreal forest, the understory vegetation can be quite heterogeneous and patterns of light transmission cannot be accurately simulated using a unique k value. However, the various sets of k values developed in this study could be used in prediction models of forest dynamics to obtain relatively good predictions of understory light extinction in forest types similar to the ones studied here
Semileptonic D->pi/K and B->pi/D decays in 2+1 flavor lattice QCD
We present results for form factors of semileptonic decays of and
mesons in 2+1 flavor lattice QCD using the MILC gauge configurations. With an
improved staggered action for light quarks, we successfully reduce the
systematic error from the chiral extrapolation. The results for decays are
in agreement with experimental ones. The results for B decays are preliminary.
Combining our results with experimental branching ratios, we then obtain the
CKM matrix elements , , and . We also
check CKM unitarity, for the first time, using only lattice QCD as the
theoretical input.Comment: Talk presented at Lattice2004(heavy); 3 pages, 3 figure
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