| dc.description.abstract | In quantum chromodynamics (QCD), the static potential V (r) is defined as the energy of the ground state of the system containing a static quark and a static antiquark, separated by a distance r. As a consequence of confinement, the energy between the quark-antiquark pair is contained inside a color flux tube, the string. As soon as the energy is high enough, the gluonic string connecting the quarks breaks due to creation of a pair of light quarks, which recombine with the static quarks and form two static-light mesons. This so called string breaking provides an intuitive example of a strong decay and is one of the defining characteristics of a confining gauge theory with dynamical matter fields. Since it is a low energy phenomenon not accessible by perturbative QCD, it can only be examined by nonperturbative methods. We investigate string breaking using Lattice QCD, a well-established non-perturbative approach to solving QCD.
In the theory with dynamical quarks, string breaking is manifested as a quantum-mechanical mixing phenomenon. This means that the two states, the string state and the two meson state, are both needed to describe the potential. After the string is broken, the meson state dominates the new ground state of this system. In the neighborhood of the critical separation, the two states mix. If there is mixing, the ground state and first excited state are superpositions of the string state and the two meson state. The system undergoes an avoided level crossing, giving rise to an energy gap between the states.
So far, string breaking on the lattice has been observed in the N f =2 theory, but not for the N f =2+1 theory. In the latter case, when the strange quark is included in the sea, two separate thresholds are expected, one for the decay into two static-light mesons and one for the decay into two static-strange mesons.
In this work, the phenomenon is investigated with N f =2+1 flavors of non-perturbatively O(a)-improved dynamical Wilson fermions using an ensemble of gauge configurations generated through the Coordinated Lattice Simulations (CLS) effort. The ensemble has an estimated isotropic lattice spacing of a ≈ 0.064fm, pion mass m π = 280MeV and kaon mass m K = 460MeV. We employ the stochastic LapH method in order to calculate correlation functions required for string breaking efficiently and perform a variational analysis to extract the ground state as well as the first and second excited state of the system containing two static quarks. A large set of off-axis distances is used in order to achieve the resolution needed to observe both mixing phenomena.
We see the effect of the strange quark, which results in a second mixing-phenomenon due to the formation of a strange-antistrange pair. Two avoided level crossings can be resolved clearly. We employ a simple model to define two distinct string breaking distances for the light and the strange mixing phenomenon. | en |
