637 research outputs found
Twisted-mass QCD, O(a) improvement and Wilson chiral perturbation theory
We point out a caveat in the proof for automatic O(a) improvement in twisted
mass lattice QCD at maximal twist angle. With the definition for the twist
angle previously given by Frezzotti and Rossi, automatic O(a) improvement can
fail unless the quark mass satisfies m_q >> a^2 Lambda_QCD^3. We propose a
different definition for the twist angle which does not require a restriction
on the quark mass for automatic O(a) improvement. In order to illustrate
explicitly automatic O(a) improvement we compute the pion mass in the
corresponding chiral effective theory. We consider different definitions for
maximal twist and show explicitly the absence or presence of the leading O(a)
effect, depending on the size of the quark mass.Comment: 27 pages, no figure
Twisted mass QCD for the pion electromagnetic form factor
The pion form factor is computed using quenched twisted mass QCD and the
GMRES-DR matrix inverter. The momentum averaging procedure of Frezzotti and
Rossi is used to remove leading lattice spacing artifacts, and numerical
results for the form factor show the expected improvement with respect to the
standard Wilson action. Although some matrix inverters are known to fail when
applied to twisted mass QCD, GMRES-DR is found to be a viable and powerful
option. Results obtained for the pion form factor are consistent with the
published results from other O(a) improved actions and are also consistent with
the available experimental data.Comment: 19 pages, 12 figure
Quenched twisted mass QCD at small quark masses and in large volume
As a test of quenched lattice twisted mass QCD, we compute the
non-perturbatively O() improved pseudoscalar and vector meson masses and the
pseudoscalar decay constant down to at
in large volume. We check the absence of exceptional configurations
and -- by further data at -- the size of scaling violations. The
CPU time cost for reaching a given accuracy is close to that with ordinary
Wilson quarks at and grows smoothly as
decreases.Comment: 4 pages, 3 figures, to appear in Nucl. Phys. B (Proc. Suppl.
Chiral perturbation theory for partially quenched twisted mass lattice QCD
Partially quenched Quantum Chromodynamics with Wilson fermions on a lattice
is considered in the framework of chiral perturbation theory. Two degenerate
quark flavours are associated with a chirally twisted mass term. The pion
masses and decay constants are calculated in next-to-leading order including
terms linear in the lattice spacing .Comment: 7 pages, LaTeX2e, final published versio
Nucleon and Delta masses in twisted mass chiral perturbation theory
We calculate the masses of the nucleons and deltas in twisted mass heavy
baryon chiral perturbation theory. We work to quadratic order in a power
counting scheme in which we treat the lattice spacing and the quark masses to
be of the same order. We give expressions for the mass and the mass splitting
of the nucleons and deltas both in and away from the isospin limit. We give an
argument using the chiral Lagrangian treatment that, in the strong isospin
limit, the nucleons remain degenerate and the delta multiplet breaks into two
degenerate pairs to all orders in chiral perturbation theory. We show that the
mass splitting between the degenerate pairs of the deltas first appears at
quadratic order in in the lattice spacing. We discuss the subtleties in the
effective chiral theory that arise from the inclusion of isospin breaking.Comment: 21 pages, 4 figures, version published in PR
The PHMC algorithm for simulations of dynamical fermions: II - Performance analysis
We compare the performance of the PHMC algorithm with the one of the HMC
algorithm in practical simulations of lattice QCD. We show that the PHMC
algorithm can lead to an acceleration of numerical simulations. It is
demonstrated that the PHMC algorithm generates configurations carrying small
isolated eigenvalues of the lattice Dirac operator and hence leads to a
sampling of configuration space that is different from that of the HMC
algorithm.Comment: Latex2e file, 6 figures, 31 page
Chirally improving Wilson fermions II. Four-quark operators
In this paper we discuss how the peculiar properties of twisted lattice QCD
at maximal twist can be employed to set up a consistent computational scheme in
which, despite the explicit breaking of chiral symmetry induced by the presence
of the Wilson and mass terms in the action, it is possible to completely bypass
the problem of wrong chirality and parity mixings in the computation of the
CP-conserving matrix elements of the effective weak Hamiltonian
and at the same time have a positive determinant for non-degenerate quarks as
well as full O() improvement in on-shell quantities with no need of
improving the lattice action and the operators.Comment: Replaced with published version in JHEP style: 43 pages, no figures.
Added few references and discussion on "critical mass and O(a) improvement"
as well as on "tests and numerical issues" in the Conclusions (sect. 6
Twisted mass lattice QCD with non-degenerate quark masses
Quantum Chromodynamics on a lattice with Wilson fermions and a chirally
twisted mass term is considered in the framework of chiral perturbation theory.
For two and three numbers of quark flavours, respectively, with non-degenerate
quark masses the pseudoscalar meson masses and decay constants are calculated
in next-to-leading order including lattice effects quadratic in the lattice
spacing a.Comment: 9 pages, LaTeX2e, reference adde
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