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We present results of our ongoing determination of string breaking in full QCD with N_f=2 Wilson fermions. Our investigation of the fission of the static quark-antiquark string into a static-light meson-antimeson system is based on dynamical configurations of size 24^3 x 40 produced by the TxL collaboration. Combining various optimization methods we determine the matrix elements of the two-by-two system with so far unprecedented accuracy. The all-to-all light quark propagators occurring in the transition element are computed from eigenmodes of the Hermitian Wilson-Dirac matrix complemented by stochastic estimates in the orthogonal subspace. We observe a clear signature for level-splitting between ground state and excited potential. Thus, for the first time, string breaking induced by sea quarks is observed in a simulation of 4-dimensional lattice-QCD.
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An uncontroversial observation of adjoint string breaking is proposed, while measuring the static potential from Wilson loops only. The overlap of the Wilson loop with the broken-string state is small, but non-vanishing, so that the broken-string groundstate can be seen if the Wilson loop is long enough. We demonstrate this in the context of the (2+1)d SU(2) adjoint static potential, using an improved version of the Luscher-Weisz exponential variance reduction. To complete the picture we perform the more usual multichannel analysis with two basis states, the unbroken-string state and the broken-string state (two so-called gluelumps). As by-products, we obtain the temperature-dependent static potential measured from Polyakov loop correlations, and the fundamental SU(2) static potential with improved accuracy. Comparing the latter with the adjoint potential, we see clear deviations from Casimir scaling.
The first observation is made of hadronic string breaking due to dynamical fermions in zero temperature lattice QCD. The simulations are done for SU(2) color in three dimensions, with two flavors of staggered fermions. The results have clear implications for the large scale simulations that are being done to search (so far, without success) for string breaking in four-dimensional QCD. In particular, string breaking is readily observed using only Wilson loops to excite a static quark-antiquark pair. Improved actions on coarse lattices are used, providing an extremely efficient means to access the quark separations and propagation times at which string breaking occurs.
As computing resources are limited, choosing the parameters for a full Lattice QCD simulation always amounts to a compromise between the competing objectives of a lattice spacing as small, quarks as light, and a volume as large as possible. Aiming to push unquenched simulations with the Wilson action towards the computationally expensive regime of small quark masses we address the question whether one can possibly save computing time by extrapolating results from small lattices to the infinite volume, prior to the usual chiral and continuum extrapolations. In the present work the systematic volume dependence of simulated pion and nucleon masses is investigated and compared with a long-standing analytic formula by Luescher and with results from Chiral Perturbation Theory. We analyze data from Hybrid Monte Carlo simulations with the standard (unimproved) two-flavor Wilson action at two different lattice spacings of a=0.08fm and 0.13fm. The quark masses considered correspond to approximately 85 and 50% (at the smaller a) and 36% (at the larger a) of the strange quark mass. At each quark mass we study at least three different lattices with L/a=10 to 24 sites in the spatial directions (L=0.85-2.08fm).
QCD is investigated at finite temperature using Wilson fermions in the fixed scale approach. A 2+1 flavor stout and clover improved action is used at four lattice spacings allowing for control over discretization errors. The light quark masses in this first study are fixed to heavier than physical values. The renormalized chiral condensate, quark number susceptibility and the Polyakov loop is measured and the results are compared with the staggered formulation in the fixed N_t approach. The Wilson results at the finest lattice spacing agree with the staggered results at the highest N_t.
We investigate the Kondo effect with Wilson fermions. This is based on a mean-field approach for the chiral Gross-Neveu model including four-point interactions between a light Wilson fermion and a heavy fermion. For massless Wilson fermions, we demonstrate the appearance of the Kondo effect. We point out that there is a coexistence phase with both the light-fermion scalar condensate and Kondo condensate, and the critical chemical potentials of the scalar condensate are shifted by the Kondo effect. For negative-mass Wilson fermions, we find that the Kondo effect is favored near the parameter region realizing the Aoki phase. Our findings will be useful for understanding the roles of heavy impurities in Dirac semimetals, topological insulators, and lattice QCD simulations.
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