2,272 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($a$) improved pseudoscalar and vector meson masses and the
pseudoscalar decay constant down to $M_{\rm PS}/M_{\rm V} = 0.467(13)$ at
$\beta=6$ in large volume. We check the absence of exceptional configurations
and -- by further data at $\beta=6.2$ -- the size of scaling violations. The
CPU time cost for reaching a given accuracy is close to that with ordinary
Wilson quarks at $M_{\rm PS}/M_{\rm V} \simeq 0.6$ and grows smoothly as
$M_{\rm PS}/M_{\rm V}$ decreases.Comment: 4 pages, 3 figures, to appear in Nucl. Phys. B (Proc. Suppl.

### Twisted-mass lattice QCD with mass non-degenerate quarks

The maximally twisted lattice QCD action of an $SU_f(2)$ doublet of mass
degenerate Wilson quarks gives rise to a real positive fermion determinant and
it is invariant under the product of standard parity times the change of sign
of the coefficient of the Wilson term. The existence of this spurionic symmetry
implies that O($a$) improvement is either automatic or achieved through simple
linear combinations of quantities taken with opposite external three-momenta.
We show that in the case of maximal twist all these nice results can be
extended to the more interesting case of a mass non-degenerate quark pair.Comment: 10 pages (due to different LateX style), Latex file, based on a talk
presented by G.C. Rossi at LHP2003 - Cairns. Reasons for replacement:
Correction of the transformation properties of energies under r --> -r. Minor
changes in Appendix

### 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

### How the PHMC algorithm samples configuration space

We show that in practical simulations of lattice QCD with two dynamical light
fermion species the PHMC algorithm samples configuration space differently from
the commonly used HMC algorithm.Comment: 3 pages, 2 figures, LATTICE98 (Algorithms

### Twisted mass Lattice QCD

I review the main theoretical properties and some recent analytical and
numerical investigations of the formulations of lattice QCD with chirally
twisted Wilson quarks (also known as twisted mass lattice QCD).Comment: Lattice 04 (plenary), 7 pages. Corrected two typos (one immediately
before eq. 18); improved wording of a sentence in last paragraph of sect. 1.

### 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 $a$.Comment: 7 pages, LaTeX2e, final published versio

### 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

### Observations on discretization errors in twisted-mass lattice QCD

I make a number of observations concerning discretization errors in
twisted-mass lattice QCD that can be deduced by applying chiral perturbation
theory including lattice artifacts. (1) The line along which the PCAC quark
mass vanishes in the twisted mass-twisted mass plane makes an angle to the
untwisted mass axis which is a direct measure of O(a) terms in the chiral
Lagrangian, and is found numerically to be large; (2) Numerical results for
pionic quantities in the mass plane show the qualitative properties predicted
by chiral perturbation theory, in particular an asymmetry in slopes between
positive and negative untwisted quark masses; (3) By extending the description
of the ``Aoki regime'' (where m_q is of size a^2 Lambda_QCD^3) to
next-to-leading order in chiral perturbation theory I show how the phase
transition lines and lines of maximal twist (using different definitions)
extend into this region, and give predictions for the functional form of pionic
quantities; (4) I argue that the recent claim that lattice artifacts at maximal
twist have apparent infrared singularities in the chiral limit results from
expanding about the incorrect vacuum state. Shifting to the correct vacuum (as
can be done using chiral perturbation theory) the apparent singularities are
summed into non-singular, and furthermore predicted, forms. I further argue
that there is no breakdown in the Symanzik expansion in powers of lattice
spacing, and no barrier to simulating at maximal twist in the Aoki regime.Comment: 20 pages, 6 figures. Published version. More typos corrected, and
summary paragraph added to sections II and I

### Solving the Boltzmann Equation on GPU

We show how to accelerate the direct solution of the Boltzmann equation using
Graphics Processing Units (GPUs). In order to fully exploit the computational
power of the GPU, we choose a method of solution which combines a finite
difference discretization of the free-streaming term with a Monte Carlo
evaluation of the collision integral. The efficiency of the code is
demonstrated by solving the two-dimensional driven cavity flow. Computational
results show that it is possible to cut down the computing time of the
sequential code of two order of magnitudes. This makes the proposed method of
solution a viable alternative to particle simulations for studying unsteady low
Mach number flows.Comment: 18 pages, 3 pseudo-codes, 6 figures, 1 tabl

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