273 research outputs found
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
Strange quarks in quenched twisted mass lattice QCD
Two twisted doublets, one containing the up and down quarks and the other
containing the strange quark with an SU(2)-flavor partner, are used for studies
in the meson sector. The relevant chiral perturbation theory is presented, and
quenched QCD simulations (where the partner of the strange quark is not active)
are performed. Pseudoscalar meson masses and decay constants are computed; the
vector and scalar mesons are also discussed. A comparison is made to the case
of an untwisted strange quark, and some effects due to quenching,
discretization, and the definition of maximal twist are explored.Comment: 37 pages, 12 figures, accepted for publicatio
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
Deflation for inversion with multiple right-hand sides in QCD
Most calculations in lattice Quantum Chromodynamics (QCD) involve the solution of a series of linear systems of equations with exceedingly large matrices and a large number of right hand sides. Iterative methods for these problems can be sped up significantly if we deflate approximations of appropriate invariant spaces from the initial guesses. Recently we have developed eigCG, a modification of the Conjugate Gradient (CG) method, which while solving a linear system can reuse a window of the CG vectors to compute eigenvectors almost as accurately as the Lanczos method. The number of approximate eigenvectors can increase as more systems are solved. In this paper we review some of the characteristics of eigCG and show how it helps remove the critical slowdown in QCD calculations. Moreover, we study scaling with lattice volume and an extension of the technique to nonsymmetric problems
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