Demonstration of String Breaking in Quantum Chromodynamics by Large-Scale Eigenvalue Computations.

We present results of our ongoing determination of “string breaking” in quantum chromodynamics (QCD) including two dynamical light quarks. Our investigation of the fission of the string between a heavy (static) quark and a corresponding antiquark into a meson–antimeson system is based on dynamical configurations of size 243×40. The all-to-all light quark propagators occurring in the transition element are computed from a set of 200 low-lying eigenmodes of the Hermitian Wilson–Dirac matrix which encodes the effect of the dynamical quarks. These eigenmodes are calculated on the 1312-node IBM p690 system at the John von Neumann Institute in Jülich. Combining the eigenvalue computations with a variety of ground state enhancing optimization methods we determine the matrix elements of the two-by-two system with so far unprecedented accuracy. We observe—for the first time ever in a simulation of 4-dimensional lattice-QCD—level-splitting as the perfect signature for dynamical string breaking between ground state and excited potential.