16,274 research outputs found
Chiral Corrections to the Hyperon Vector Form Factors
We present the complete calculation of the SU(3)-breaking corrections to the
hyperon vector form factors up to O(p^4) in the Heavy Baryon Chiral
Perturbation Theory. Because of the Ademollo-Gatto theorem, at this order the
results do not depend on unknown low energy constants and allow to test the
convergence of the chiral expansion. We complete and correct previous
calculations and find that O(p^3) and O(1/M_0) corrections are important. We
also study the inclusion of the decuplet degrees of freedom, showing that in
this case the perturbative expansion is jeopardized. These results raise doubts
on the reliability of the chiral expansion for hyperons.Comment: 20 pages, 4 figures, v2: published versio
Vector form factor in K_l3 semileptonic decay with two flavors of dynamical domain-wall quarks
We calculate the vector form factor in K \to \pi l \nu semileptonic decays at
zero momentum transfer f_+(0) from numerical simulations of two-flavor QCD on
the lattice. Our simulations are carried out on 16^3 \times 32 at a lattice
spacing of a \simeq 0.12 fm using a combination of the DBW2 gauge and the
domain-wall quark actions, which possesses excellent chiral symmetry even at
finite lattice spacings. The size of fifth dimension is set to L_s=12, which
leads to a residual quark mass of a few MeV. Through a set of double ratios of
correlation functions, the form factor calculated on the lattice is accurately
interpolated to zero momentum transfer, and then is extrapolated to the
physical quark mass. We obtain f_+(0)=0.968(9)(6), where the first error is
statistical and the second is the systematic error due to the chiral
extrapolation. Previous estimates based on a phenomenological model and chiral
perturbation theory are consistent with our result. Combining with an average
of the decay rate from recent experiments, our estimate of f_+(0) leads to the
Cabibbo-Kobayashi-Maskawa (CKM) matrix element |V_{us}|=0.2245(27), which is
consistent with CKM unitarity. These estimates of f_+(0) and |V_{us}| are
subject to systematic uncertainties due to the finite lattice spacing and
quenching of strange quarks, though nice consistency in f_+(0) with previous
lattice calculations suggests that these errors are not large.Comment: 23 pages, 11 figures, 7 tables, RevTeX4; v3: one table added, results
and conclusions unchanged, final version to appear in Phys.Rev.
Lattice study of semileptonic form factors with twisted boundary conditions
We apply twisted boundary conditions to lattice QCD simulations of
three-point correlation functions in order to access spatial components of
hadronic momenta different from the integer multiples of 2 pi / L. We calculate
the vector and scalar form factors relevant to the K -> pi semileptonic decay
and consider all the possible ways of twisting one of the quark lines in the
three-point functions. We show that the momentum shift produced by the twisted
boundary conditions does not introduce any additional noise and easily allows
to determine within a few percent statistical accuracy the form factors at
quite small values of the four-momentum transfer, which are not accessible when
periodic boundary conditions are considered. The use of twisted boundary
conditions turns out to be crucial for a precise determination of the form
factor at zero-momentum transfer, when a precise lattice point sufficiently
close to zero-momentum transfer is not accessible with periodic boundary
conditions.Comment: latex 15 pages, 4 figures and 3 tables; modified intro and
discussions of the results; version to appear in PR
A Complete Atlas of HI Absorption toward HII Regions in the Southern Galactic Plane Survey (SGPS1)
We present a complete catalog of H I emission and absorption spectrum pairs,
toward H II regions, detectable within the boundaries of the Southern Galactic
Plane Survey (SGPS I), a total of 252 regions. The catalog is presented in
graphical, numerical and summary formats. We demonstrate an application of this
new dataset through an investigation of the locus of the Near 3kpc Arm.Comment: Accepted for publication by ApJS Feb 6, 2014. Data files and Figure
Set (252 images) to appear in the on-line version of the journa
Proton decay matrix elements with domain-wall fermions
Hadronic matrix elements of operators relevant to nucleon decay in grand
unified theories are calculated numerically using lattice QCD. In this context,
the domain-wall fermion formulation, combined with non-perturbative
renormalization, is used for the first time. These techniques bring reduction
of a large fraction of the systematic error from the finite lattice spacing.
Our main effort is devoted to a calculation performed in the quenched
approximation, where the direct calculation of the nucleon to pseudoscalar
matrix elements, as well as the indirect estimate of them from the nucleon to
vacuum matrix elements, are performed. First results, using two flavors of
dynamical domain-wall quarks for the nucleon to vacuum matrix elements are also
presented to address the systematic error of quenching, which appears to be
small compared to the other errors. Our results suggest that the representative
value for the low energy constants from the nucleon to vacuum matrix elements
are given as |alpha| simeq |beta| simeq 0.01 GeV^3. For a more reliable
estimate of the physical low energy matrix elements, it is better to use the
relevant form factors calculated in the direct method. The direct method tends
to give smaller value of the form factors, compared to the indirect one, thus
enhancing the proton life-time; indeed for the pi^0 final state the difference
between the two methods is quite appreciable.Comment: 56 pages, 17 figures, a comment and two references added in the
introduction, typo corrected in Eq.1
Allopurinol use yields potentially beneficial effects on inflammatory indices in those with recent ischemic stroke: a randomized, double-blind, placebo-controlled trial
<p><b>Background and Purpose</b>: Elevated serum uric acid level is associated with poor outcome and increased risk of recurrent events after stroke. The xanthine oxidase inhibitor allopurinol lowers uric acid but also attenuates expression of inflammatory adhesion molecules in murine models, reduces oxidative stress in the vasculature, and improves endothelial function. We sought to investigate whether allopurinol alters expression of inflammatory markers after acute ischemic stroke.</p>
<p><b>Methods</b>: We performed a randomized, double-blind, placebo-controlled trial to investigate the safety, tolerability, and effect of 6 weeks’ treatment with high- (300 mg once a day) or low- (100 mg once a day) dose allopurinol on levels of uric acid and circulating inflammatory markers after ischemic stroke.</p>
<p><b>Results</b>: We enrolled 50 patients with acute ischemic stroke (17, 17, and 16 in the high, low, and placebo groups, respectively). Mean (±SD) age was 70 (±13) years. Groups had similar characteristics at baseline. There were no serious adverse events. Uric acid levels were significantly reduced at both 7 days and 6 weeks in the high-dose group (by 0.14 mmol/L at 6 weeks, P=0.002). Intercellular adhesion molecule-1 concentration (ng/mL) rose by 51.2 in the placebo group, rose slightly (by 10.6) in the low-dose allopurinol group, but fell in the high-dose group (by 2.6; difference between groups P=0.012, Kruskal-Wallis test).</p>
<p><b>Conclusion</b>: Allopurinol treatment is well tolerated and attenuates the rise in intercellular adhesion molecule-1 levels seen after stroke. Uric acid levels were lowered with high doses. These findings support further evaluation of allopurinol as a preventive measure after stroke.</p>
The Kaon B-parameter from Quenched Domain-Wall QCD
We present numerical results for the kaon B-parameter, B_K, determined in the
quenched approximation of lattice QCD. Our simulations are performed using
domain-wall fermions and the renormalization group improved, DBW2 gauge action
which combine to give quarks with good chiral symmetry at finite lattice
spacing. Operators are renormalized non-perturbatively using the RI/MOM scheme.
We study scaling by performing the simulation on two different lattices with
a^{-1} = 1.982(30) and 2.914(54) GeV. We combine this quenched scaling study
with an earlier calculation of B_K using two flavors of dynamical, domain-wall
quarks at a single lattice spacing to obtain
B_K(MS,NDR,mu=2GeV)=0.563(21)(39)(30), were the first error is statistical, the
second systematic (without quenching errors) and the third estimates the error
due to quenching.Comment: 77 pages, 44 figures, to be published in Phys. Rev.
Immediate effects of diamond burr debridement in patients with spontaneous chronic corneal epithelial defects, light and electron microscopic evaluation
The history of the CATH structural classification of protein domains
This article presents a historical review of the protein structure classification database CATH. Together with the SCOP database, CATH remains comprehensive and reasonably up-to-date with the now more than 100,000 protein structures in the PDB. We review the expansion of the CATH and SCOP resources to capture predicted domain structures in the genome sequence data and to provide information on the likely functions of proteins mediated by their constituent domains. The establishment of comprehensive function annotation resources has also meant that domain families can be functionally annotated allowing insights into functional divergence and evolution within protein families
Scalar K pi form factor and light quark masses
Recent experimental improvements on K-decay data allow for a precise
extraction of the strangeness-changing scalar K pi form factor and the related
strange scalar spectral function. On the basis of this scalar as well as the
corresponding pseudoscalar spectral function, the strange quark mass is
determined to be m_s(2 GeV) = 92 +- 9 MeV. Further taking into account chiral
perturbation theory mass ratios, the light up and down quark masses turn out to
be m_u(2 GeV) = 2.7 +- 0.4 MeV as well as m_d(2 GeV) = 4.8 +- 0.5 MeV. As a
by-product, we also find a value for the Cabibbo angle |V_{us}| = 0.2236(29)
and the ratio of meson decay constants F_K/F_\pi = 1.203(16). Performing a
global average of the strange mass by including extractions from other channels
as well as lattice QCD results yields m_s(2 GeV) = 94 +- 6 MeV.Comment: 5 pages, 2 figures; comparison with lattice and global average added;
version to appear in Phys. Rev.
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