2,272 research outputs found

    Quenched twisted mass QCD at small quark masses and in large volume

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    As a test of quenched lattice twisted mass QCD, we compute the non-perturbatively O(aa) improved pseudoscalar and vector meson masses and the pseudoscalar decay constant down to MPS/MV=0.467(13)M_{\rm PS}/M_{\rm V} = 0.467(13) at ő≤=6\beta=6 in large volume. We check the absence of exceptional configurations and -- by further data at ő≤=6.2\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 MPS/MV‚ČÉ0.6M_{\rm PS}/M_{\rm V} \simeq 0.6 and grows smoothly as MPS/MVM_{\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

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    The maximally twisted lattice QCD action of an SUf(2)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(aa) 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

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    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

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    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

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    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

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    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

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    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 aa.Comment: 7 pages, LaTeX2e, final published versio

    Light quarks with twisted mass fermions

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    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

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    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

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    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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