Revisiting the pion's scalar form factor in chiral perturbation theory

The quark-connected and the quark-disconnected Wick contractions contributing to the pion's scalar form factor are computed in the two and in the three flavour chiral effective theory at next-to-leading order. While the quark-disconnected contribution to the form factor itself turns out to be power-counting suppressed its contribution to the scalar radius is of the same order of magnitude as the one of the quark-connected contribution. This result underlines that neglecting quark-disconnected contributions in simulations of lattice QCD can cause significant systematic effects. The technique used to derive these predictions can be applied to a large class of observables relevant for QCD-phenomenology.Comment: 16 pages, 5 figure

Quark disconnected diagrams in chiral perturbation theory

We show how quark-disconnected and quark-connected contributions to hadronic n-point functions can be written as independent correlators for which one can derive expressions in partially quenched chiral effective theory. As an example we apply the idea to the case of the hadronic vacuum polarisation. In particular, we consider the cases of the Nf = 2 theory without and with a partially quenched strange quark and also the Nf = 2 + 1 theory. In the latter two cases a parameter-free prediction for the disconnected contribution at NLO in the effective theory is given. Finally we show how twisted boundary conditions can then be used in lattice QCD to improve the q^2 resolution in the connected contributions even when flavour singlet operators are considered.Comment: 19 pages, 4 figures; results unchanged; typos corrected, references added and discussion extended; accepted for publication in JHE

Precision Lattice Computations in the Heavy Quark Sector

The phenomenology of the pseudo scalar mesons D_s and B_s and of the vector mesons D_s* and B_s* was investigated in simulations of quenched lattice QCD. The work is particularly focused on the continuum limit and the minimisation of all systematic errors within the quenched approximation. The decay constants F_Ds and F_Ds* and the difference in the masses between the D_s and the D_s* were determined from the direct computer simulation of lattice QCD in large physical volume (L ~ 1.5 fm). As an aside, the renormalisation group invariant charm quark mass M_c could be obtained from the simulation results. For the B_s- and the B_s*-meson, an interpolation in the meson mass to its physical point was carried out for the decay constant and the mass splitting in the continuum limit. The functional form of the interpolation to the static limit was guided by the prediction of the Heavy Quark Effective Theory (HQET). A set of conversion functions between HQET and QCD were derived and evaluated numerically with input from perturbation theory.Comment: Ph.D. Thesi

Calculation of the pion electromagnetic form factor from lattice QCD

We present a lattice calculation of the vector form factor of the pion for two flavours of non-perturbatively O(a) improved Wilson fermions. For the measurements we utilise the CLS ensembles which include various lattice spacings and pion masses down to about 250 MeV. To obtain a fine momentum resolution near zero momentum transfer (q^2) partially twisted boundary conditions are employed using several twist angles. Due to the fine resolution around q^2=0 we are able to determine the slope of the form factor and, in turn, extract the charge radius of the pion without any model dependence. The results for the form factor and the charge radius are then compared to chiral perturbation theory and phenomenological models which are used to extrapolate the results to the physical point.Comment: 4 pages, 4 figures, talk presented at Hadron 2011: 14th International Conference on Hadron Spectroscopy, Munich, German