213 research outputs found
Lattice QCD with open boundary conditions and twisted-mass reweighting
Lattice QCD simulations at small lattice spacings and quark masses close to
their physical values are technically challenging. In particular, the
simulations can get trapped in the topological charge sectors of field space or
may run into instabilities triggered by accidental near-zero modes of the
lattice Dirac operator. As already noted in ref. [1], the first problem is
bypassed if open boundary conditions are imposed in the time direction, while
the second can potentially be overcome through twisted-mass determinant
reweighting [2]. In this paper, we show that twisted-mass reweighting works out
as expected in QCD with open boundary conditions and 2+1 flavours of O(a)
improved Wilson quarks. Further algorithmic improvements are tested as well and
a few physical quantities are computed for illustration.Comment: Plain TeX source, 27 pages, 7 figure
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
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
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
On the extraction of spectral quantities with open boundary conditions
We discuss methods to extract decay constants, meson masses and gluonic
observables in the presence of open boundary conditions. The ensembles have
been generated by the CLS effort and have 2+1 flavors of O(a)-improved Wilson
fermions with a small twisted-mass term as proposed by L\"uscher and Palombi.
We analyse the effect of the associated reweighting factors on the computation
of different observables.Comment: 7 pages, talk presented at the 32nd International Symposium on
Lattice Field Theory - Lattice 2014, Columbia University New Yor
Topological susceptibility and the sampling of field space in 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
The CLS 2+1 flavor simulations
We report on the status of large volume simulations with 2+1 dynamical
fermions which are being performed by the CLS initiative. The algorithmic
details include: open boundary conditions, twisted mass reweighting and RHMC,
whereas the main feature of the simulation strategy is the approach to the
physical point along a trajectory of constant trace of the mass matrix. We
comment on the practical side of the above issues using as examples some of the
newly generated ensembles, which presently cover lattice spacings between 0.05
fm and 0.11 fm and pion masses between 150 MeV and 415 MeV.Comment: 8 pages, 6 figures, talk presented at the 32nd International
Symposium on Lattice Field Theory, 23-28 June, 2014, Columbia University New
York, NY. appears in PoS(LATTICE2014)02
New linearization and reweighting for simulations of string sigma-model on the lattice
We study the discretized worldsheet of Type IIB strings in the Gubser-Klebanov-Polyakov background in a new setup, which eliminates a complex phase previously detected in the fermionic determinant. A sign ambiguity remains, which a study of the fermionic spectrum shows to be related to Yukawa-like terms, including those present in the original Lagrangian before the linearization standard in a lattice QFT approach. Monte Carlo simulations are performed in a large region of the parameter space, where the sign problem starts becoming severe and instabilities appear due to the zero eigenvalues of the fermionic operator. To face these problems, simulations are conducted using the absolute value of a fermionic Pfaffian obtained introducing a small twisted-mass term, acting as an infrared regulator, into the action. The sign of the Pfaffian and the low modes of the quadratic fermionic operator are then taken into account by a reweighting procedure of which we discuss the impact on the measurement of the observables. In this setup we study bosonic and fermionic correlators and observe a divergence in the latter, which we argue - also via a one-loop analysis in lattice perturbation theory - to originate from the U(1)-breaking of our Wilson-like discretization for the fermionic sector
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