Simulation of QCD with N_f=2+1 flavors of non-perturbatively improved Wilson fermions (1411.3982v2)

Abstract: We describe a new set of gauge configurations generated within the CLS effort. These ensembles have N_f=2+1 flavors of non-perturbatively improved Wilson fermions in the sea with the Luescher-Weisz action used for the gluons. Open boundary conditions in time are used to address the problem of topological freezing at small lattice spacings and twisted-mass reweighting for improved stability of the simulations. We give the bare parameters at which the ensembles have been generated and how these parameters have been chosen. Details of the algorithmic setup and its performance are presented as well as measurements of the pion and kaon masses alongside the scale parameter t_0.

• The paper introduces a robust simulation framework that leverages non-perturbatively improved Wilson fermions, open boundary conditions, and twisted-mass reweighting to mitigate topological freezing.
• It employs an advanced algorithmic setup with detailed determinant decompositions and molecular dynamics integration via openQCD to achieve precise measurements of pion and kaon masses.
• The study establishes a new precision benchmark in lattice QCD by incorporating 2+1 flavors, setting the stage for improved continuum extrapolations and future work on electromagnetic effects.

Simulation of QCD with $N_f=2+1$ Flavors of Non-Perturbatively Improved Wilson Fermions

The paper presents a comprehensive paper aimed at creating and analyzing gauge configurations for Quantum Chromodynamics (QCD) using $N_f = 2+1$ flavors of non-perturbatively improved Wilson fermions. These configurations were generated within the Coordinated Lattice Simulations (CLS) effort, a collaborative initiative addressing some of the key challenges in lattice QCD simulations, such as topological freezing and stability issues arising from small lattice spacings.

Key Methodological Details

1. Fermion and Gauge Action: The gauge fields are simulated using the Lüscher-Weisz action with tree-level coefficients, while the fermions are implemented using the improved Wilson Dirac operator, essential for $\mathcal{O}(a)$ improvement of the action. This paper employs open boundary conditions to mitigate issues associated with topological freezing.
2. Twisted-Mass Reweighting: Twisted-mass reweighting is utilized to maintain stability during simulations by circumventing the challenges posed by near-zero eigenvalues of the Wilson Dirac operator. This process requires careful compensation through reweighting during measurements.
3. Open Boundary Conditions: Open boundary conditions facilitate decorrelation of global topological charge, which is significant at small lattice spacings where periodic boundary conditions typically induce topological freezing.
4. Algorithmic Setup: The simulations were executed using the openQCD code version 1.2, with detailed determinant decompositions and molecular dynamics integration, ensuring precise and stable computation of observables such as the pion and kaon masses.

Numerical Results

The paper reports measurements of key physical observables such as pion and kaon masses and the scale parameter $t_0$, obtained through Wilson flow techniques. Careful tuning of bare parameters was performed to ensure accurate simulations pertinent for QCD observables.

Implications and Future Directions

This work positions itself as foundational in the evolution of lattice QCD simulations, particularly at finer lattice spacings. By incorporating an additional flavor in the fermion sea, the paper reached a higher precision benchmark than earlier two-flavor simulations. The avoidance of catastrophic scaling without including a dynamical charm quark sets a new precedent for upcoming studies aiming for more precise calculations while managing computational demands.

The implications of this paper are both theoretical and practical. Theoretically, it provides a robust framework enabling more accurate continuum extrapolations in QCD. Practically, the methodologies applied here can be leveraged in future computational projects, potentially extending this work to address electromagnetic and isospin breaking effects not covered in the current setup.

Speculations on Future Developments

Given the success of these simulations in addressing major issues in lattice QCD, future work may expand this framework to include domain wall fermions or incorporate more flavors to further validate and refine QCD theories. Enhanced computational resources and algorithmic innovations will likely be driven by this groundwork, paving the path for even more precise simulations that could eventually include all standard model parameters in lattice computations.

In conclusion, this paper contributes significantly to lattice QCD methodologies, providing a new dataset and benchmarks for precise simulations and setting the stage for future advancements in the field.