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M. Lüscher

Publications and source records attributed to M. Lüscher.

11 recordsLinked to original sources
arXiv2021

Master-field simulations of QCD

We report on the first master-field simulations of QCD with 2+1 dynamical quark flavours using non-perturbatively improved stabilised Wilson fermions. Our simulations are performed at a lattice spacing of 0.094 fm with 96 and 192 points in each direction. On both lattices, the pion and kaon masses are equal to 270 and 450 MeV, respectively, and $m_πL$ thus reaches an unprecedented value of 25 on the larger lattice. This setup matches a single point on a chiral trajectory with fixed trace of the quark mass matrix and allows for comparisons to standard large-scale simulations. We present our algorithmic setup and performance measures, and report about our experience in thermalising large master-field lattices with fermions.

hep-lat↗
arXiv2007

QCD with light Wilson quarks on fine lattices (II): DD-HMC simulations and data analysis

In this second report on our recent numerical simulations of two-flavour QCD, we provide further technical details on the simulations and describe the methods we used to extract the meson masses and decay constants from the generated ensembles of gauge fields. Among the topics covered are the choice of the DD-HMC parameters, the issue of stability, autocorrelations and the statistical error analysis. Extensive data tables are included as well as a short discussion of the quark-mass dependence in partially quenched QCD, supplementing the physics analysis that was presented in the first paper in this series.

hep-lat↗
arXiv2006

QCD with light Wilson quarks on fine lattices (I): first experiences and physics results

Recent conceptual, algorithmic and technical advances allow numerical simulations of lattice QCD with Wilson quarks to be performed at significantly smaller quark masses than was possible before. Here we report on simulations of two-flavour QCD at sea-quark masses from slightly above to approximately 1/4 of the strange-quark mass, on lattices with up to 64x32^3 points and spacings from 0.05 to 0.08 fm. Physical sea-quark effects are clearly seen on these lattices, while the lattice effects appear to be quite small, even without O(a) improvement. A striking result is that the dependence of the pion mass on the sea-quark mass is accurately described by leading-order chiral perturbation theory up to meson masses of about 500 MeV.

hep-lat↗
arXiv2005

Stability of lattice QCD simulations and the thermodynamic limit

We study the spectral gap of the Wilson--Dirac operator in two-flavour lattice QCD as a function of the lattice spacing $a$, the space-time volume $V$ and the current-quark mass $m$. It turns out that the median of the probability distribution of the gap scales proportionally to $m$ and that its width is practically equal to $a/\sqrt{V}$. In particular, numerical simulations are safe from accidental zero modes in the large-volume regime of QCD.

hep-lat↗
arXiv2002

Numerical techniques for lattice QCD in the $ε$--regime

In lattice QCD it is possible, in principle, to determine the parameters in the effective chiral lagrangian (including weak interaction couplings) by performing numerical simulations in the $ε$--regime, i.e. at quark masses where the physical extent of the lattice is much smaller than the Compton wave length of the pion. The use of a formulation of the lattice theory that preserves chiral symmetry is attractive in this context, but the numerical implementation of any such approach requires special care in this kinematical situation due to the presence of some very low eigenvalues of the Dirac operator. We discuss a set of techniques (low-mode preconditioning and adapted-precision algorithms in particular) that make such computations numerically safe and more efficient by a large factor.

hep-lat↗
arXiv2000

A note on the practical feasibility of domain-wall fermions

Domain-wall fermions preserve chiral symmetry up to terms that decrease exponentially when the lattice size in the fifth dimension is taken to infinity. The associated rates of convergence are given by the low-lying eigenvalues of a simple local operator in four dimensions. These can be computed using the Ritz functional technique and it turns out that the convergence tends to be extremely slow in the range of lattice spacings relevant to large-volume numerical simulations of lattice QCD. Two methods to improve on this situation are discussed.

hep-lat↗
arXiv1999

Locality properties of Neuberger's lattice Dirac operator

The gauge covariant lattice Dirac operator D which has recently been proposed by Neuberger satisfies the Ginsparg-Wilson relation and thus preserves chiral symmetry. The operator also avoids a doubling of fermion species, but its locality properties are not obvious. We now prove that D is local (with exponentially decaying tails) if the gauge field is sufficiently smooth at the scale of the cutoff. Further analytic and numerical studies moreover suggest that the locality of the operator is in fact guaranteed under far more general conditions.

hep-lat↗
arXiv1997

Theoretical advances in lattice QCD

The past few years have seen many interesting theoretical developments in lattice QCD. This talk (which is intended for non-experts) focuses on the problem of non-perturbative renormalization and the question of how precisely the continuum limit is reached. Progress in these areas is crucial in order to be able to compute quantities of phenomenological interest, such as the hadron spectrum, the running quark masses and weak transition matrix elements, with controlled systematic errors.

hep-ph↗
arXiv1996

Some new results in O(a) improved lattice QCD

It is shown how on-shell O(a) improvement can be implemented non-perturbatively in lattice QCD with Wilson quarks. Improvement conditions are obtained by requiring the PCAC relation to hold exactly in certain matrix elements. These are derived from the QCD Schrödinger functional which enables us to simulate directly at vanishing quark masses. In the quenched approximation and for bare couplings in the range $0\leq g_0\leq 1$, we determine the improved action, the improved axial current, the additive renormalization of the quark mass and the isospin current normalization constants Z_A and Z_V.

hep-lat↗
arXiv1996

O(a) improvement of the axial current in lattice QCD to one-loop order of perturbation theory

The conservation of the isovector axial current in lattice QCD with massless Wilson quarks is studied to one-loop order of perturbation theory. Following a strategy described in a previous publication, the O($a$) counterterm required for on-shell improvement of the current is computed. We also confirm an earlier result of Wohlert for the coefficient in front of the SW term in the improved action.

hep-lat↗