### Refine

- Scalar mesons and tetraquarks by means of lattice QCD (2012)
- We study the light scalar mesons a_0(980) and kappa using N_f = 2+1+1 flavor lattice QCD. In order to probe the internal structure of these scalar mesons, and in particular to identify, whether a sizeable tetraquark component is present, we use a large set of operators, including diquark-antidiquark, mesonic molecule and two-meson operators. The inclusion of disconnected diagrams, which are technically rather challenging, but which would allow us to extend our work to e.g. the f_0(980) meson, is introduced and discussed.

- Static-light meson masses from twisted mass lattice QCD (2008)
- We compute the static-light meson spectrum using two-flavor Wilson twisted mass lattice QCD. We have considered five different values for the light quark mass corresponding to 300MeV<~ mPS<~ 600MeV. We have extrapolated our results, to make predictions regarding the spectrum of B and Bs mesons.

- Forces between static-light mesons (2010)
- The isospin, spin and parity dependent potential of a pair of static-light mesons is computed using Wilson twisted mass lattice QCD with two flavors of degenerate dynamical quarks. From the results a simple rule can be deduced stating, which isospin, spin and parity combinations correspond to attractive and which to repulsive forces.

- The adjoint potential in the pseudoparticle approach: string breaking and Casimir scaling (2008)
- We perform a detailed study of the adjoint static potential in the pseudoparticle approach, which is a model for SU(2) Yang-Mills theory. We find agreement with the Casimir scaling hypothesis and there is clear evidence for string breaking. At the same time the potential in the fundamental representation is linear for large separations. Our results are in qualitative agreement with results from lattice computations.

- Light hadrons from Nf = 2+1+1 dynamical twisted mass fermions (2010)
- We present results of lattice QCD simulations with mass-degenerate up and down and mass-split strange and charm (Nf = 2+1+1) dynamical quarks using Wilson twisted mass fermions at maximal twist. The tuning of the strange and charm quark masses is performed at three values of the lattice spacing a ~ 0:06 fm, a ~ 0:08 fm and a ~ 0:09 fm with lattice sizes ranging from L ~ 1:9 fm to L ~ 3:9 fm. We perform a preliminary study of SU(2) chiral perturbation theory by combining our lattice data from these three values of the lattice spacing.

- Strange and charm meson masses from twisted mass lattice QCD (2012)
- We present first results of a 2+1+1 flavor twisted mass lattice QCD computation of strange and charm meson masses. We focus on D and D_s mesons with spin J = 0,1 and parity P = -,+.

- Dynamical lattice computation of the Isgur-Wise functions τ1/2 and τ3/2 (2009)
- We perform a two-flavor dynamical lattice computation of the Isgur-Wise functions t1/2 and t3/2 at zero recoil in the static limit. We find t1/2(1) = 0.297(26) and t3/2(1) = 0.528(23) fulfilling Uraltsev’s sum rule by around 80%. We also comment on a persistent conflict between theory and experiment regarding semileptonic decays of B mesons into orbitally excited P wave D mesons, the so-called “1/2 versus 3/2 puzzle”, and we discuss the relevance of lattice results in this context.

- Definitions of a static SU(2) color triplet potential (2012)
- We discuss possibilities and problems to non-perturbatively define and compute a static color triplet potential in SU(2) gauge theory. Numerical lattice results are presented and compared to analytical perturbative results.

- Kaon and D meson masses with Nf = 2+1+1 twisted mass lattice QCD (2010)
- We discuss the computation of the kaon and D meson masses in the Nf = 2+1+1 twisted mass lattice QCD setup, where explicit heavy flavor and parity breaking occurs at finite lattice spacing. We present three methods suitable in this context and verify their consistency.

- The pseudoparticle approach for solving path integrals in gauge theories (2005)
- We present a numerical technique for calculating path integrals in non-compact U(1) and SU(2) gauge theories. The gauge fields are represented by a superposition of pseudoparticles of various types with their amplitudes and color orientations as degrees of freedom. Applied to Maxwell theory this technique results in a potential which is in excellent agreement with the Coulomb potential. For SU(2) Yang-Mills theory the same technique yields clear evidence of confinement. Varying the coupling constant exhibits the same scaling behavior for the string tension, the topological susceptibility and the critical temperature while their dimensionless ratios are similar to those obtained in lattice calculations.