6,267 research outputs found
Staggered fermion matrix elements using smeared operators
We investigate the use of two kinds of staggered fermion operators, smeared
and unsmeared. The smeared operators extend over a hypercube, and tend to
have smaller perturbative corrections than the corresponding unsmeared
operators. We use these operators to calculate kaon weak matrix elements on
quenched ensembles at , 6.2 and 6.4. Extrapolating to the continuum
limit, we find . The
systematic error is dominated by the uncertainty in the matching between
lattice and continuum operators due to the truncation of perturbation theory at
one-loop. We do not include any estimate of the errors due to quenching or to
the use of degenerate and quarks. For the
electromagnetic penguin operators we find
and . We also use the ratio of unsmeared to
smeared operators to make a partially non-perturbative estimate of the
renormalization of the quark mass for staggered fermions. We find that tadpole
improved perturbation theory works well if the coupling is chosen to be
\alpha_\MSbar(q^*=1/a).Comment: 22 pages, 1 figure, uses eps
Applications of Partially Quenched Chiral Perturbation Theory
Partially quenched theories are theories in which the valence- and sea-quark
masses are different. In this paper we calculate the nonanalytic one-loop
corrections of some physical quantities: the chiral condensate, weak decay
constants, Goldstone boson masses, B_K and the K+ to pi+ pi0 decay amplitude,
using partially quenched chiral perturbation theory. Our results for weak decay
constants and masses agree with, and generalize, results of previous work by
Sharpe. We compare B_K and the K+ decay amplitude with their real-world values
in some examples. For the latter quantity, two other systematic effects that
plague lattice computations, namely, finite-volume effects and unphysical
values of the quark masses and pion external momenta are also considered. We
find that typical one-loop corrections can be substantial.Comment: 22 pages, TeX, refs. added, minor other changes, version to appear in
Phys. Rev.
Effect of thermal expansion on the linear stability of planar premixed flames for a simple chain-branching model: The high activation energy asymptotic limit
The linear stability of freely propagating, adiabatic, planar premixed
ames is investigated in the context of a simple chain-branching
chemistry model consisting of a chain-branching reaction step and a completion reaction step. The role of chain-branching is governed
by a crossover temperature. Hydrodynamic effects, induced by thermal expansion, are taken into account and the results compared and
contrasted with those from a previous purely thermal-di�usive constant density linear stability study. It is shown that when thermal
expansion is properly accounted for, a region of stable
ames predicted by the constant density model disappears, and instead the
ame
is unstable to a long-wavelength cellular instability. For a pulsating mode, however, thermal expansion is shown to have only a weak
e�ect on the critical fuel Lewis number required for instability. These e�ects of thermal expansion on the two-step chain-branching
ame
are shown to be qualitatively similar to those on the standard one-step reaction model. Indeed, as found by constant density studies, in
the limit that the chain-branching crossover temperature tends to the adiabatic
ame temperature, the two-step model can be described
to leading order by the one-step model with a suitably de�ned e�ective activation energy
The Kaon -parameter with Wilson Fermions
We calculate the kaon -parameter in quenched lattice QCD at
using Wilson fermions at and . We use two kinds of
non-local (``smeared'') sources for quark propagators to calculate the matrix
elements between states of definite momentum. The use of smeared sources yields
results with much smaller errors than obtained in previous calculations with
Wilson fermions. By combining results for and , we show that one can carry out the non-perturbative subtraction
necessary to remove the dominant lattice artifacts induced by the chiral
symmetry breaking term in the Wilson action. Our final results are in good
agreement with those obtained using staggered fermions. We also present results
for -parameters of the part of the electromagnetic penguin
operators, and preliminary results for \bk\ in the presence of two flavors of
dynamical quarks.Comment: 39 pages, including 9 PS figures (LA UR-91-3522
Heavy-Meson Observables at One-Loop in Partially Quenched Chiral Perturbation Theory
I present one-loop level calculations of the Isgur-Wise functions for B ->
D^{(*)} + e + nu, of the matrix elements of isovector twist-2 operators in B
and D mesons, and the matrix elements for the radiative decays D^* -> D + gamma
in partially quenched heavy quark chiral perturbation theory. Such expressions
are required in order to extrapolate from the light quark masses used in
lattice simulations of the foreseeable future to those of nature.Comment: 13 pages, 3 fig
Weak matrix elements for CP violation
We present preliminary results of matrix elements of four-fermion operators
relevant to the determination of e and e'/e using staggered fermions.Comment: 3 pages, 4 figures, Lattice 2001 (Hadronic Matrix Elements
Linear stability of planar premixed flames: reactive Navier-Stokes equations with finite activation energy and arbitrary Lewis number
A numerical shooting method for performing linear stability analyses of travelling waves is described and applied to the problem of freely propagating planar premixed flames. Previous linear stability analyses of premixed flames either employ high activation temperature asymptotics or have been performed numerically with finite activation temperature, but either for unit Lewis numbers (which ignores thermal-diffusive effects) or in the limit of small heat release (which ignores hydrodynamic effects). In this paper the full reactive Navier-Stokes equations are used with arbitrary values of the parameters (activation temperature, Lewis number, heat of reaction, Prandtl number), for which both thermal-diffusive and hydrodynamic effects on the instability, and their interactions, are taken into account. Comparisons are made with previous asymptotic and numerical results. For Lewis numbers very close to or above unity, for which hydrodynamic effects caused by thermal expansion are the dominant destablizing mechanism, it is shown that slowly varying flame analyses give qualitatively good but quantitatively poor predictions, and also that the stability is insensitive to the activation temperature. However, for Lewis numbers sufficiently below unity for which thermal-diffusive effects play a major role, the stability of the flame becomes very sensitive to the activation temperature. Indeed, unphysically high activation temperatures are required for the high activation temperature analysis to give quantitatively good predictions at such low Lewis numbers. It is also shown that state-insensitive viscosity has a small destabilizing effect on the cellular instability at low Lewis numbers
Partially quenched chiral perturbation theory without
This paper completes the argument that lattice simulations of partially
quenched QCD can provide quantitative information about QCD itself, with the
aid of partially quenched chiral perturbation theory. A barrier to doing this
has been the inclusion of , the partially quenched generalization of
the , in previous calculations in the partially quenched effective
theory. This invalidates the low energy perturbative expansion, gives rise to
many new unknown parameters, and makes it impossible to reliably calculate the
relation between the partially quenched theory and low energy QCD. We show that
it is straightforward and natural to formulate partially quenched chiral
perturbation theory without , and that the resulting theory contains
the effective theory for QCD without the . We also show that previous
results, obtained including , can be reinterpreted as applying to the
theory without . We contrast the situation with that in the quenched
effective theory, where we explain why it is necessary to include .
We also compare the derivation of chiral perturbation theory in partially
quenched QCD with the standard derivation in unquenched QCD. We find that the
former cannot be justified as rigorously as the latter, because of the absence
of a physical Hilbert space. Finally, we present an encouraging result:
unphysical double poles in certain correlation functions in partially quenched
chiral perturbation theory can be shown to be a property of the underlying
theory, given only the symmetries and some plausible assumptions.Comment: 45 pages, no figure
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