941 research outputs found
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.
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
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
Light quarks with twisted mass fermions
We investigate Wilson twisted mass fermions in the quenched approximation
using different definitions of the critical bare quark mass m_c to realize
maximal twist and, correspondingly, automatic O(a) improvement for physical
observables. A particular definition of m_c is given by extrapolating the value
of m_c obtained from the PCAC relation at non-vanishing bare twisted quark mass
mu to mu=0. Employing this improved definition of the critical mass the Wilson
twisted mass formulation provides the possibility to perform reliable
simulations down to very small quark masses with correspondingly small pion
masses of m_pi \simeq 250 MeV, while keeping the cutoff effects of O(a^2) under
control.Comment: 10 pages, 8 figures, v2: version accepted for publication in PLB,
minor correction
Pion form factor with twisted mass QCD
The pion form factor is calculated using quenched twisted mass QCD with
beta=6.0 and maximal twisting angle omega=pi/2. Two pion masses and several
values of momentum transfer are considered. The momentum averaging procedure of
Frezzotti and Rossi is used to reduce lattice spacing errors, and numerical
results are consistent with the expected O(a) improvement.Comment: Talk presented at Lattice2004(spectrum), 3 pages, one reference added
and one typo fixe
Cutoff effects in twisted mass lattice QCD
We present a first numerical study of lattice QCD with O(a) improved Wilson
quarks and a chirally twisted mass term. Renormalized correlation functions are
derived from the Schroedinger functional and evaluated in an intermediate
space-time volume of size 0.75^3 x 1.5 fm^4. In the quenched approximation
precise results are then obtained with a moderate computational effort,
allowing for a detailed study of the continuum approach. The latter is
discussed in terms of observables which converge to meson masses and decay
constants in the limit of large space-time volume. In the O(a) improved theory
we find residual cutoff effects to be at the level of a few percent for lattice
spacings of about 0.1 fm.Comment: 20 pages, 5 figures. Comments about the uncertainties on improvement
coefficients adde
The phase diagram of twisted mass lattice QCD
We use the effective chiral Lagrangian to analyze the phase diagram of
two-flavor twisted mass lattice QCD as a function of the normal and twisted
masses, generalizing previous work for the untwisted theory. We first determine
the chiral Lagrangian including discretization effects up to next-to-leading
order (NLO) in a combined expansion in which m_\pi^2/(4\pi f_\pi)^2 ~ a \Lambda
(a being the lattice spacing, and \Lambda = \Lambda_{QCD}). We then focus on
the region where m_\pi^2/(4\pi f_\pi)^2 ~ (a \Lambda)^2, in which case
competition between leading and NLO terms can lead to phase transitions. As for
untwisted Wilson fermions, we find two possible phase diagrams, depending on
the sign of a coefficient in the chiral Lagrangian. For one sign, there is an
Aoki phase for pure Wilson fermions, with flavor and parity broken, but this is
washed out into a crossover if the twisted mass is non-vanishing. For the other
sign, there is a first order transition for pure Wilson fermions, and we find
that this transition extends into the twisted mass plane, ending with two
symmetrical second order points at which the mass of the neutral pion vanishes.
We provide graphs of the condensate and pion masses for both scenarios, and
note a simple mathematical relation between them. These results may be of
importance to numerical simulations.Comment: 13 pages, 5 figures, small clarifying comments added in introduction,
minor typos fixed. Version to be published in Phys. Rev.
Speeding up Lattice QCD simulations with clover-improved Wilson Fermions
We apply a recent proposal to speed up the Hybrid-Monte-Carlo simulation of
systems with dynamical fermions to two flavour QCD with clover-improvement. The
basic idea of our proposal is to split the fermion matrix into two factors with
a reduced condition number each. In the effective action, for both factors a
pseudo-fermion field is introduced. For our smallest quark masses we see a
speed-up of more than a factor of two compared with the standard algorithm.Comment: 26 pages, 2 figure
Non-perturbative scaling tests of twisted mass QCD
We present a scaling study of lattice QCD with O(a) improved Wilson fermions
and a chirally twisted mass term. In order to get precise results with a
moderate computational effort, we have considered a system of physical size of
0.75^3 * 1.5 fm^4 with Schroedinger functional boundary conditions in the
quenched approximation. Looking at meson observables in the pseudoscalar and
vector channels, we find that O(a) improvement is effective and residual cutoff
effects are fairly small.Comment: 5 pages, 4 figures, Lattice 2000 (Improvement and Renormalization),
two misprints correcte
Spectrum of quenched twisted mass lattice QCD at maximal twist
Hadron masses are computed from quenched twisted mass lattice QCD for a
degenerate doublet of up and down quarks with the twist angle set to pi/2,
since this maximally twisted theory is expected to be free of linear
discretization errors. Two separate definitions of the twist angle are used,
and the hadron masses for these two cases are compared. The flavor breaking,
that can arise due to twisting, is discussed in the context of mass splittings
within the Delta(1232) multiplet.Comment: 23 pages, 16 figures, added discussion of pion decay constan
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