Status and challenges of simulations with dynamical fermions

An overview over the current state of algorithms for dynamical fermion simulations is given. In particular some insight into the functioning of the determinant spitting techniques is discussed. The critical slowing down of the simulations towards the continuum limit and the role of the boundary conditions is also reviewed.Comment: 20 pages, 9 figures, plenary talk presented at the 30th International Symposium on Lattice Field Theory - Lattice 2012, June 24-29, 2012 Cairns, Australi

Algorithms for lattice QCD: progress and challenges

The development of improved algorithms for QCD on the lattice has enabled us to do calculations at small quark masses and get control over the chiral extrapolation. Also finer lattices have become possible, however, a severe slowing down associated with the topology of the gauge fields has been observed. This may prevent simulations of lattices fine enough for controlling the continuum extrapolation. This conference contribution introduces the basic concepts behind contemporary lattice algorithms, the current knowledge about their slowing down towards the continuum and its consequences for future lattice simulations.Comment: 6 pages, 3 figures, plenary talk given at the IX International Conference on Quark Confinement and Hadron Spectrum, Madrid, Spain, Aug 30th - Sept 3rd 201

Comparison of the mass preconditioned HMC and the DD-HMC algorithm for two-flavour QCD

Mass preconditioned HMC and DD-HMC are among the most popular algorithms to simulate Wilson fermions. We present a comparison of the performance of the two algorithms for realistic quark masses and lattice sizes. In particular, we use the locally deflated solver of the DD-HMC environment also for the mass preconditioned simulations.Comment: 7 pages, 2 figures. Presented at the XXVIII International Symposium on Lattice Field Theory (Lattice 2010), June 14-19 2010, Villasimius, Ital

A multilevel algorithm for flow observables in gauge theories

We study the possibility of using multilevel algorithms for the computation of correlation functions of gradient flow observables. For each point in the correlation function an approximate flow is defined which depends only on links in a subset of the lattice. Together with a local action this allows for independent updates and consequently a convergence of the Monte Carlo process faster than the inverse square root of the number of measurements. We demonstrate the feasibility of this idea in the correlation functions of the topological charge and the energy density.Comment: Minor modifications to the text. Version accepted to be published in PRD. 18 pages, 5 figure

Topological susceptibility and the sampling of field space in Nf=2N_f=2 lattice QCD simulations

We present a measurement of the topological susceptibility in two flavor QCD. In this observable, large autocorrelations are present and also sizable cutoff effects have to be faced in the continuum extrapolation. Within the statistical accuracy of the computation, the result agrees with the expectation from leading order chiral perturbation theory.Comment: 22 pages, 7 figures; References added, minor clarifications in the text, results unchange

CLS 2+1 flavor simulations at physical light- and strange-quark masses

We report recent efforts by CLS to generate an ensemble with physical light- and strange-quark masses in a lattice volume of 192x96^3 at $\beta=3.55$ corresponding to a lattice spacing of 0.064 fm. This ensemble is being generated as part of the CLS 2+1 flavor effort with improved Wilson fermions. Our simulations currently cover 5 lattice spacings ranging from 0.039 fm to 0.086 fm at various pion masses along chiral trajectories with either the sum of the quark masses kept fixed, or with the strange-quark mass at the physical value. The current status of simulations is briefly reviewed, including a short discussion of measured autocorrelation times and of the main features of the simulations. We then proceed to discuss the thermalization strategy employed for the generation of the physical quark-mass ensemble and present first results for some simple observables. Challenges encountered in the simulation are highlighted.Comment: 7 pages, 8 figures; Proceedings, 35th International Symposium on Lattice Field Theory (Lattice2017): Granada, Spai