797 research outputs found
The role of the double pole in lattice QCD with mixed actions
We investigate effects resulting from the use of different discretizations
for the valence and the sea quarks in lattice QCD, considering Wilson and/or
Ginsparg-Wilson fermions. We assume that such effects appear through scaling
violations that can be studied using effective lagrangian techniques. We show
that a double pole is present in flavor-neutral Goldstone meson
propagators,even if the charged Goldstone mesons made out of valence quarks and
those made out of sea quarks have equal masses. We then consider some
observables known to be anomalously sensitive to the presence of a double pole.
For these observables, we find that the double-pole enhanced scaling violations
may turn out to be rather small in practice.Comment: 13 page
On the effects of (partial) quenching on penguin contributions to K-> pi pi
Recently, we pointed out that chiral transformation properties of strong
penguin operators change in the transition from unquenched to (partially)
quenched QCD. As a consequence, new penguin-like operators appear in the
(partially) quenched theory, along with new low-energy constants, which should
be interpreted as a quenching artifact. Here, we extend the analysis to the
contribution of the new low-energy constants to the K^0 -> pi^+ pi^- amplitude,
at leading order in chiral perturbation theory, and for arbitrary (momentum
non-conserving) kinematics. Using these results, we provide a detailed
discussion of the intrinsic systematic error due to this (partial) quenching
artifact. We also give a simple recipe for the determination of the
leading-order low-energy constant parameterizing the new operators in the case
of strong penguins.Comment: 17 pages, 1 figure, minor correction
Lattice extraction of amplitudes to NLO in partially quenched and in full chiral perturbation theory
We show that it is possible to construct to NLO
using partially quenched chiral perturbation theory (PQChPT) from amplitudes
that are computable on the lattice. We demonstrate that none of the needed
amplitudes require three-momentum on the lattice for either the full theory or
the partially quenched theory; non-degenerate quark masses suffice.
Furthermore, we find that the electro-weak penguin ( and 1/2)
contributions to in PQChPT can be determined to NLO
using only degenerate () computations without momentum
insertion. Issues pertaining to power divergent contributions, originating from
mixing with lower dimensional operators, are addressed. Direct calculations of
at unphysical kinematics are plagued with enhanced finite volume
effects in the (partially) quenched theory, but in simulations when the sea
quark mass is equal to the up and down quark mass the enhanced finite volume
effects vanish to NLO in PQChPT. In embedding the QCD penguin left-right
operator onto PQChPT an ambiguity arises, as first emphasized by Golterman and
Pallante. With one version (the "PQS") of the QCD penguin, the inputs needed
from the lattice for constructing at NLO in PQChPT coincide with
those needed for the full theory. Explicit expressions for the finite
logarithms emerging from our NLO analysis to the above amplitudes are also
given.Comment: 54 pages, 3 figures; Important revisions: Corrections to formulas for
K->pi pi with degenerate quark masses have been mad
Before sailing on a domain-wall sea
We discuss the very different roles of the valence-quark and the sea-quark
residual masses ( and ) in dynamical domain-wall fermions
simulations. Focusing on matrix elements of the effective weak hamiltonian
containing a power divergence, we find that can be a source of a
much bigger systematic error. To keep all systematic errors due to residual
masses at the 1% level, we estimate that one needs
and , at a lattice spacing fm. The
practical implications are that (1) optimal use of computer resources calls for
a mixed scheme with different domain-wall fermion actions for the valence and
sea quarks; (2) better domain-wall fermion actions are needed for both the sea
and the valence sectors.Comment: latex, 25 pages. Improved discussion in appendix, including
correction of some technical mistakes; ref. adde
On Lattice Computations of K+ --> pi+ pi0 Decay at m_K =2m_pi
We use one-loop chiral perturbation theory to compare potential lattice
computations of the K+ --> pi+ pi0 decay amplitude at m_K=2m_pi with the
experimental value. We find that the combined one-loop effect due to this
unphysical pion to kaon mass ratio and typical finite volume effects is still
of order minus 20-30%, and appears to dominate the effects from quenching.Comment: 4 pages, revte
Finite-volume two-pion energies and scattering in the quenched approximation
We investigate how L\"uscher's relation between the finite-volume energy of two pions at rest and pion scattering lengths has to be modified in quenched QCD. We find that this relation changes drastically, and in particular, that ``enhanced finite-volume corrections" of order L^0=1 and L^{-2} occur at one loop (L is the linear size of the box), due to the special properties of the \eta' in the quenched approximation. We define quenched pion scattering lengths, and show that they are linearly divergent in the chiral limit. We estimate the size of these various effects in some numerical examples, and find that they can be substantial
Possible duality violations in tau decay and their impact on the determination of alpha_s
We discuss the issue of duality violations in hadronic tau decay. After
introducing a physically motivated ansatz for duality violations, we estimate
their possible size by fitting this ansatz to the tau experimental data
provided by the ALEPH collaboration. Our conclusion is that these data do not
exclude significant duality violations in tau decay. This may imply an
additional systematic error in the value of alpha_s(m_tau), extracted from tau
decay, as large as \delta alpha_s(m_tau) \sim 0.003-0.010 .Comment: 20 pages, 4 figures. Minor fixes in the Appendi
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