500 research outputs found
Non-perturbative renormalization constants on the lattice from flavour non-singlet Ward identities
By imposing axial and vector Ward identities for flavour-non-singlet
currents, we estimate in the quenched approximation the non-perturbative values
of combinations of improvement coefficients, which appear in the expansion
around the massless case of the renormalization constants of axial,
pseudoscalar, vector, scalar non-singlet currents and of the renormalized mass.
These coefficients are relevant for the completion of the improvement
programme to O(a) of such operators.
The simulations are performed with a clover Wilson action non-perturbatively
improved.Comment: 9 pages, latex file + 4 eps files of figure
On the extraction of zero momentum form factors on the lattice
We propose a method to expand correlation functions with respect to the
spatial components of external momenta. From the coefficients of the expansion
it is possible to extract Lorentz-invariant form factors at zero spatial
momentum transfer avoiding model dependent extrapolations. These objects can be
profitably calculated on the lattice. We have explicitly checked the validity
of the proposed procedure by considering two-point correlators with insertions
of the axial current, the form factors of the semileptonic decay of
pseudoscalar mesons, and the hadronic vacuum polarization tensor entering, for
example, the lattice calculation of the anomalous magnetic moment of the muon.Comment: 10 pages, 5 figures, published versio
Quenched lattice calculation of the B --> D l nu decay rate
We calculate, in the continuum limit of quenched lattice QCD, the form factor
that enters in the decay rate of the semileptonic decay B --> D l nu. Making
use of the step scaling method (SSM), previously introduced to handle two scale
problems in lattice QCD, and of flavour twisted boundary conditions we extract
G(w) at finite momentum transfer and at the physical values of the heavy quark
masses. Our results can be used in order to extract the CKM matrix element Vcb
by the experimental decay rate without model dependent extrapolations.Comment: 5 pages, 4 figures, accepted for publication on Phys. Lett. B,
corrected one typ
Determination of the strong coupling from semi-leptonic decay
According to heavy-meson chiral perturbation theory, the vector form factor
of exclusive semi-leptonic decay is closely
related, at least in the soft-pion region (i.e., ),
to the strong coupling or the normalized coupling .
Combining the precisely measured spectrum of decay by
the BaBar and Belle collaborations with several parametrizations of the form
factor , we can extract these couplings from the residue of the form
factor at the pole, which relies on an extrapolation of the form factor
from the semi-leptonic region to the unphysical point .
Comparing the extracted values with the other experimental and theoretical
estimates, we can test these various form-factor parametrizations, which differ
from each other by the amount of physical information embedded in. It is found
that the extracted values based on the BK, BZ and BCL parametrizations are
consistent with each other and roughly in agreement with the other theoretical
and lattice estimates, while the BGL ansatz, featured by a spurious, unwanted
pole at the threshold of the cut, gives a neatly larger value.Comment: 19 pages, no figure. Revise
Lattice Determination of the Coupling
The coupling is related to the form factor at zero
momentum of the axial current between and states. Moreover it is
related to the effective coupling between heavy mesons and pions that appear
the heavy meson chiral Lagrangian. This coupling has been evaluated on the
lattice using static heavy quarks and light quark propagators determined by a
stochastic inversion of the fermionic bilinear. We found the value
. Beside its theoretical interest, this quantity has
phenomenological implications in decays.Comment: Lattice 99, 3 page
BB Potentials in Quenched Lattice QCD
The potentials between two B-mesons are computed in the heavy-quark limit
using quenched lattice QCD at . Non-zero central
potentials are clearly evident in all four spin-isospin channels, (I,s_l) =
(0,0) , (0,1) , (1,0) , (1,1), where s_l is the total spin of the light degrees
of freedom. At short distance, we find repulsion in the channels and
attraction in the I=s_l channels. Linear combinations of these potentials that
have well-defined spin and isospin in the t-channel are found, in three of the
four cases, to have substantially smaller uncertainties than the potentials
defined with the s-channel (I,s_l), and allow quenching artifacts from single
hairpin exchange to be isolated. The BB*\pi coupling extracted from the
long-distance behavior of the finite-volume t-channel potential is found to be
consistent with quenched calculations of the matrix element of the isovector
axial-current. The tensor potentials in both of the s_l = 1 channels are found
to be consistent with zero within calculational uncertainties.Comment: 30 page
Determination of B*B pi coupling in unquenched QCD
The coupling is a fundamental parameter of chiral effective
Lagrangian with heavy-light mesons and can constrain the chiral behavior of
, and the form factor in the soft pion limit. We
compute the coupling with the static heavy quark and the
-improved Wilson light quark. Simulations are carried out with
unquenched lattices at and
lattices at generated by CP-PACS collaboration. To improve the
statistical accuracy, we employ the all-to-all propagator technique and the
static quark action with smeared temporal link variables following the quenched
study by Negishi {\it et al.}. These methods successfully work also on
unquenched lattices, and determine the coupling with 1--2%
statistical accuracy on each lattice spacing.Comment: 19pages,26figure
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