Constantia Alexandrou

B-meson properties from lattice QCD☆

Abstract A high statistics computation of the decay constant ƒ B is presented to study scaling and finite size effects. Smooth wave functions for the B-meson allow to suppress excited state contributions effectively. We observe the scaling violation in the static approximation to be 15% between β = 5.7 and 6.0. The conventional light quark methods appear to remain applicable, at β = 6.0 even beyond the D-meson mass. The value obtained in the static approximation, ƒ B = 366 ± 22( stat. ) ± 55 ( syst. ) MeV , cannot be reached by smooth extrapolation from the D-meson range, where we find ƒ D = 198 ± 17 MeV .

Lattice calculation of parton distributions

We report on our exploratory study for the direct evaluation of the parton distribution functions from lattice QCD, based on a recently proposed new approach. We present encouraging results using Nf = 2 + 1 + 1 twisted mass fermions with a pion mass of about 370 MeV. The focus of this work is a detailed description of the computation, including the lattice calculation, the matching to an infinite momentum and the nucleon mass correction. In addition, we test the effect of gauge link smearing in the operator to estimate the influence of the Wilson line renormalization, which is yet to be done.

The ground state of three quarks

We measure the static three-quark potential in SU(3) lattice gauge theory with improved accuracy, by using all available technical refinements, including Luscher-Weisz exponential variance reduction. Together with insight gained from 3-state Potts model simulations, our results allow us to sort out the merits of the Delta- and Y-ansaetze.

Low-lying baryon spectrum with two dynamical twisted mass fermions

The masses of the low-lying baryons are evaluated using two degenerate flavors of twisted mass sea quarks corresponding to pseudoscalar masses in the range of about 270\char21{}500 MeV. The strange valence quark mass is tuned to reproduce the mass of the kaon in the physical limit. The tree-level Symanzik improved gauge action is employed. We use lattices of spatial size 2.1 and 2.7 fm at two values of the lattice spacing with

N-to-delta electromagnetic-transition form factors from lattice QCD.

{r}_{0}/a=5.22(2)

Axial nucleon form factors from lattice QCD

and

Disconnected quark loop contributions to nucleon observables in lattice QCD

{r}_{0}/a=6.61(3)

Baryon spectrum with

. We check for both finite volume and cutoff effects on the baryon masses. We performed a detailed study of the chiral extrapolation of the octet and decuplet masses using SU(2)

N_f = 2 + 1 + 1

\ensuremath{\chi}\mathrm{PT}

twisted mass fermions

. The lattice spacings determined using the nucleon mass at the physical point are consistent with the values extracted using the pion decay constant. We examine the issue of isospin symmetry breaking for the octet and decuplet baryons and its dependence on the lattice spacing. We show that in the continuum limit isospin breaking is consistent with zero, as expected. The baryon masses that we find after taking the continuum limit and extrapolating to the physical limit are in good agreement with experiment.