The scalar pion form factor in two-flavor lattice QCD

We calculate the scalar form factor of the pion using two dynamical flavors of non-perturbatively $\mathcal{O}(a)$-improved Wilson fermions, including both the connected and the disconnected contribution to the relevant correlation functions. We employ the calculation of all-to-all propagators using stochastic sources and a generalized hopping parameter expansion. From the form factor data at vanishing momentum transfer, $Q^2=0$, and two non-vanishing $Q^2$ we obtain an estimate for the scalar radius \left^\pi_{_{\rm S}} of the pion at one value of the lattice spacing and for five different pion masses. Using Chiral Perturbation Theory at next-to-leading order, we find \left^\pi_{_{\rm S}}=0.635\pm0.016 fm$^2$ at the physical pion mass (statistical error only). This is in good agreement with the phenomenological estimate from $\pi\pi$-scattering. The inclusion of the disconnected contribution is essential for achieving this level of agreement.Comment: 15 pages, 10 pdf figures, uses revtex4-1; version to appear in PR

The Shape of Covariantly Smeared Sources in Lattice QCD

Covariantly smeared sources are commonly used in lattice QCD to enhance the projection onto the ground state. Here we investigate the dependence of their shape on the gauge field background and find that the presence of localized concentrations of magnetic field can lead to strong distortions which reduce the smearing radii achievable by iterative smearing prescriptions. In particular, as $a\to 0$, iterative procedures like Jacobi smearing require increasingly large iteration counts in order to reach physically-sized smearing radii $r_{sm}\sim$ 0.5 fm, and the resulting sources are strongly distorted. To bypass this issue, we propose a covariant smearing procedure (``free-form smearing'') that allows us to create arbitrarily shaped sources, including in particular Gaussians of arbitrary radius.Comment: 1+15 pages, 7 figures (24 pdf images

Wie vertragen sich Artenvielfalt und menschliche Besiedlung? : Städtische Biotope und gefährdete Arten im Rhein-Main-Gebiet

Ohne das Eingreifen des Menschen wäre Mitteleuropa fast ein reines Waldgebiet. Noch heute beheimaten die Wälder eine große Vielfalt an Pflanzen und Tieren, die für diese Region spezifisch sind. Regionale Besonderheiten gehen aber verloren, je mehr Menschen in die Ökosysteme eingreifen: So unterscheiden sich die Pflanzenarten auf der North Charles Street in Baltimore nur wenig von denjenigen der Mainzer Landstraße in Frankfurt. Gleichzeitig verdrängen zugewanderte und eingeschleppte Arten heimische Tiere und Pflanzen. Allerdings gibt es auch im Frankfurter Stadtgebiet echte Horte der Biodiversität

Scale setting via the \Omega\ baryon mass

We present the first results of an ongoing effort to determine the lattice scale on the N_f=2 CLS lattice ensembles via the mass of the \Omega\ baryon. Results from different methods are compared, and various sources of systematic uncertainty are discussed.Comment: 7 pages, 4 pdf figures; talk given at LATTICE 2011, XXIX International Symposium on Lattice Field Theory, July 11-16 2011, Squaw Valley, Lake Tahoe, Californi

Nucleon average quark momentum fraction with Nf=2+1N_\mathrm{f}=2+1 Wilson fermions

We report on an analysis of the average quark momentum fraction of the nucleon and related quantities using $N_\mathrm{f}=2+1$ Wilson fermions. Computations are performed on four CLS ensembles covering three values of the lattice spacing at pion masses down to $M_\pi \approx 200\,\mathrm{MeV}$. Several source-sink separations ($\sim 1.0\,\mathrm{fm}$ to $\sim 1.4\,\mathrm{fm}$) are used to assess the excited-state contamination. To gain further insight, the generalized pencil-of-functions approach has been implemented to reduce the excited-state contamination in the relevant two- and three-point functions. Preliminary results are shown for the isovector nucleon charges from vector, axial vector and tensor derivative (twist-2) operators.Comment: 8 pages, 3 figures, 2 tables. Talk presented at the 35th International Symposium on Lattice Field Theory, 18-24 June 2017, Granada, Spai