166 research outputs found
Finite-Size Scaling of Vector and Axial Current Correlators
Using quenched chiral perturbation theory, we compute the long-distance
behaviour of two-point functions of flavour non-singlet axial and vector
currents in a finite volume, for small quark masses, and at a fixed gauge-field
topology. We also present the corresponding predictions for the unquenched
theory at fixed topology. These results can in principle be used to measure the
low-energy constants of the chiral Lagrangian, from lattice simulations in
volumes much smaller than one pion Compton wavelength. We show that quenching
has a dramatic effect on the vector correlator, which is argued to vanish to
all orders, while the axial correlator appears to be a robust observable only
moderately sensitive to quenching.Comment: version to appear in NP
Determination of the weak Hamiltonian in the SU(4) chiral limit through topological zero-mode wave functions
A new method to determine the low-energy couplings of the weak
Hamiltonian is presented. It relies on a matching of the topological poles in
of three-point correlators of two pseudoscalar densities and a
four-fermion operator, measured in lattice QCD, to the same observables
computed in the -regime of chiral perturbation theory. We test this
method in a theory with a light charm quark, i.e. with an SU(4) flavour
symmetry. Quenched numerical measurements are performed in a 2 fm box, and
chiral perturbation theory predictions are worked out up to next-to-leading
order. The matching of the two sides allows to determine the weak low-energy
couplings in the SU(4) limit. We compare the results with a previous
determination, based on three-point correlators containing two left-handed
currents, and discuss the merits and drawbacks of the two procedures.Comment: 38 pages, 9 figure
Weak low-energy couplings from topological zero-mode wavefunctions
We discuss a new method to determine the low-energy couplings of the weak Hamiltonian in the -regime. It relies on a matching of the
topological poles in of three-point functions of two pseudoscalar
densities and a four-fermion operator computed in lattice QCD, to the same
observables in the Chiral Effective Theory. We present the results of a NLO
computation in chiral perturbation theory of these correlation functions
together with some preliminary numerical results.Comment: 7 pages. Contribution to Lattice 200
On the determination of low-energy constants for transitions
We present our preliminary results for three-point correlation functions
involving the operators entering the effective Hamiltonian with
an active charm quark, obtained using overlap fermions in the quenched
approximation. This is the first computation carried out for valence quark
masses small enough so as to permit a matching to Quenched Chiral Perturbation
Theory in the -regime. The commonly observed large statistical
fluctuations are tamed by means of low-mode averaging techniques, combined with
restrictions to individual topological sectors. We also discuss the matching of
the resulting hadronic matrix elements to the effective low-energy constants
for transitions. This involves (a) finite-volume corrections
which can be evaluated at NLO in Quenched Chiral Perturbation Theory, and (b)
the short-distance renormalization of the relevant four-quark operators in
discretizations based on the overlap operator. We discuss perturbative
estimates for the renormalization factors and possible strategies for their
non-perturbative evaluation. Our results can be used to isolate the
long-distance contributions to the rule, coming from physics
effects around the intrinsic QCD scale.Comment: 11 pages, 2 figures, talks presented at Lattice 2005 (Weak matrix
elements
Charm mass dependence of the weak Hamiltonian in chiral perturbation theory
Suppose that the weak interaction Hamiltonian of four-flavour SU(4) chiral
effective theory is known, for a small charm quark mass m_c. We study how the
weak Hamiltonian changes as the charm quark mass increases, by integrating it
out within chiral perturbation theory to obtain a three-flavour SU(3) chiral
theory. We find that the ratio of the SU(3) low-energy constants which mediate
Delta I=1/2 and Delta I=3/2 transitions, increases rather rapidly with m_c, as
\sim m_c ln (1/m_c). The logarithmic effect originates from "penguin-type"
charm loops, and could represent one of the reasons for the Delta I=1/2 rule.Comment: 20 pages. v2: references and clarifications added, published versio
Low-energy couplings of QCD from topological zero-mode wave functions
By matching 1/m^2 divergences in finite-volume two-point correlation
functions of the scalar or pseudoscalar densities with those obtained in chiral
perturbation theory, we derive a relation between the Dirac operator zero-mode
eigenfunctions at fixed non-trivial topology and the low-energy constants of
QCD. We investigate the feasibility of using this relation to extract the pion
decay constant, by computing the zero-mode correlation functions on the lattice
in the quenched approximation and comparing them with the corresponding
expressions in quenched chiral perturbation theory.Comment: 31 pages. v2: references and a small clarification added; published
versio
Low-energy couplings of QCD from current correlators near the chiral limit
We investigate a new numerical procedure to compute fermionic correlation
functions at very small quark masses. Large statistical fluctuations, due to
the presence of local ``bumps'' in the wave functions associated with the
low-lying eigenmodes of the Dirac operator, are reduced by an exact low-mode
averaging. To demonstrate the feasibility of the technique, we compute the
two-point correlator of the left-handed vector current with Neuberger fermions
in the quenched approximation, for lattices with a linear extent of L~1.5 fm, a
lattice spacing a~0.09 fm, and quark masses down to the epsilon-regime. By
matching the results with the corresponding (quenched) chiral perturbation
theory expressions, an estimate of (quenched) low-energy constants can be
obtained. We find agreement between the quenched values of F extrapolated from
the p-regime and extracted in the epsilon-regime.Comment: 20 pages, 5 figure
Observational evidence for AGN fueling. I. The merging of NGC6104 with a companion
We investigate in details the kinematics and morphology of the Seyfert galaxy
NGC6104 in order to identify the mechanism of gas transportation to the active
galactic nucleus (AGN). Our observational data were obtained at the 6-m Special
Astrophysical Observatory telescope with the MPFS integral-field spectrograph
and the SCORPIO universal device in three modes: direct imaging, a scanning
Fabry-Perot interferometer, and long-slit spectroscopy. Images from the HST
archive were invoked to study the structure of the circumnuclear region. An
analysis of deep images has revealed that NGC6104 is in the phase of active
merging with a companion galaxy. We have been able to study the detailed
picture of ionized gas motions up to galactocentric distances of 14 kpc and to
construct the stellar velocity field for the inner region. The radial gas
motions toward the AGN along the central bar play a significant role at
galactocentric distances of 1-5 kpc. In addition, we have detected an outflow
of ionized gas from the nucleus that presumably resulted from the intrusion of
a radio jet into the ambient interstellar medium. Using diagnostic diagrams, we
estimate the contributions from the AGN and star formation to the galactic gas
ionization. We estimate the bar pattern speed by the Tremaine-Weinberg method
and show that the inner ring observed in the galactic images has a resonant
nature. Two possible ring formation scenarios (before and during the
interaction with a companion) are discussed.Comment: 12 pages, 2 tables, 6 figures, accepted for publication in Astronomy
Letter
A new model for semantic photograph description combining basic levels and user-assigned descriptors
An economic model of long-term use of celecoxib in patients with osteoarthritis
<p>Abstract</p> <p>Background</p> <p>Previous evaluations of the cost-effectiveness of the cyclooxygenase-2 selective inhibitor celecoxib (Celebrex, Pfizer Inc, USA) have produced conflicting results. The recent controversy over the cardiovascular (CV) risks of rofecoxib and other coxibs has renewed interest in the economic profile of celecoxib, the only coxib now available in the United States. The objective of our study was to evaluate the long-term cost-effectiveness of celecoxib compared with nonselective nonsteroidal anti-inflammatory drugs (nsNSAIDs) in a population of 60-year-old osteoarthritis (OA) patients with average risks of upper gastrointestinal (UGI) complications who require chronic daily NSAID therapy.</p> <p>Methods</p> <p>We used decision analysis based on data from the literature to evaluate cost-effectiveness from a modified societal perspective over patients' lifetimes, with outcomes expressed as incremental costs per quality-adjusted life-year (QALY) gained. Sensitivity tests were performed to evaluate the impacts of advancing age, CV thromboembolic event risk, different analytic horizons and alternate treatment strategies after UGI adverse events.</p> <p>Results</p> <p>Our main findings were: 1) the base model incremental cost-effectiveness ratio (ICER) for celecoxib versus nsNSAIDs was 19,309 for a model in which UGI ulcer and ulcer complication event risks increased with advancing age; 3) the ICER per QALY was $17,120 in sensitivity analyses combining serious CV thromboembolic event (myocardial infarction, stroke, CV death) risks with base model assumptions.</p> <p>Conclusion</p> <p>Our model suggests that chronic celecoxib is cost-effective versus nsNSAIDs in a population of 60-year-old OA patients with average risks of UGI events.</p
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