210 research outputs found
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
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
Parton Distribution Functions with Twisted Mass Fermions
We present a first Wilson twisted mass fermion calculation of the matrix
element between pion states of the twist-2 operator, which is related to the
the lowest moment of the valence quark parton distribution function in a
pion. Using Wilson twisted mass fermions in the quenched approximation we
demonstrate that can be computed at small pseudoscalar meson masses down
to values of order 250 MeV. We investigate the scaling behaviour of this
physically important quantity by applying two definitions of the critical mass
and observe a scaling compatible with the expected O(a^2) behaviour in both
cases. A combined continuum extrapolation allows to obtain reliable results for
at very small pseudoscalar meson masses, which previously could not be
explored by lattice QCD simulations.Comment: 15 pages, 3 figure
Lattice QCD with a twisted mass term and a strange quark
There are three quarks with masses at or below the characteristic scale of QCD dynamics: up, down and strange. However, twisted mass lattice QCD relies on quark doublets. Various options for including three quark flavors within the twisted mass approach are explored by studying the kaon masses, both analytically (through chiral Lagrangians) and numerically (through lattice simulations). Advantages and disadvantages are revealed for each ``strange and twisted'' option
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
A first look at maximally twisted mass lattice QCD calculations at the physical point
In this contribution, a first look at simulations using maximally twisted
mass Wilson fermions at the physical point is presented. A lattice action
including clover and twisted mass terms is presented and the Monte Carlo
histories of one run with two mass-degenerate flavours at a single lattice
spacing are shown. Measurements from the light and heavy-light pseudoscalar
sectors are compared to previous results and their phenomenological
values. Finally, the strategy for extending simulations to is
outlined.Comment: presented at the 31st International Symposium on Lattice Field Theory
(Lattice 2013), 29 July - 3 August 2013, Mainz, German
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