Parametrizations of Quark Confinement in Production Processes

Baryons are described as bound states of quarks and scalar as well as axialvector diquarks. In order to effectively parametrize confinement several {\it ansaetze} for the dressing functions of the constituent propagators are used. The corresponding results for electromagnetic and strong form factors differ only slightly. On the other hand observables from production processes show the permissibility of different dressing functions.Comment: 2 pages, 2 figures, talk given at the workshop {\it The Spin Structure of the Proton}, Trento, July 23rd to 28th, 200

Mode expansion for the density profile of crystal-fluid interfaces: Hard spheres as a test case

We present a technique for analyzing the full three-dimensional density profiles of a planar crystal-fluid interface in terms of density modes. These density modes can also be related to crystallinity order parameter profiles which are used in coarse-grained, phase field type models of the statics and dynamics of crystal-fluid interfaces and are an alternative to crystallinity order parameters extracted from simulations using local crystallinity criteria. We illustrate our results for the hard sphere system using finely-resolved, three-dimensional density profiles from density functional theory of fundamental measure type.Comment: submitted for the special issue of the CODEF III conferenc

Effective forces between colloids at interfaces induced by capillary wave-like fluctuations

We calculate the effective force mediated by thermally excited capillary waves between spherical or disklike colloids trapped at a fluid interface. This Casimir type interaction is shown to depend sensitively on the boundary conditions imposed at the three-phase contact line. For large distances between the colloids an unexpected cancellation of attractive and repulsive contributions is observed leading to a fluctuation force which decays algebraically very rapidly. For small separations the resulting force is rather strong and it may play an important role in two-dimensional colloid aggregation if direct van der Waals forces are weak.Comment: 7 pages, 3 figures, minor revisions, one additional figur

Nucleon form factors from a covariant quark core: limits in their description

In treating the relativistic three-quark problem, a dressed-quark propagator parameterization is used which is compatible with recent lattice data and pion observables. Furthermore two-quark correlations are modeled as a series of quark loops in the scalar and axialvector channel. The resulting reduced Faddeev equations are solved for nucleon and delta. Nucleon electromagnetic form factors are calculated in a fully covariant and gauge--invariant scheme. Whereas the proton electric form factor $G_E$ and the nucleon magnetic moments are described correctly, the neutron electric form factor and the ratio $G_E/G_M$ for the proton appear to be quenched. The influence of vector mesons on the form factors is investigated which amounts to a 25 percent modification of the electromagnetic proton radii within this framework.Comment: 28 pages, 10 figures, 4 table

Nucleon Form Factors in the Covariant Diquark-Quark Model

The nucleon is described as a bound state of a quark and an extended diquark. Hereby the notion ``diquark'' refers to the modelling of separable correlations in the two-quark Green's functions. Binding of quarks and diquarks takes place via an exchange interaction and is therefore related to the Pauli principle for three-quark states. Fully Poincare covariant nucleon amplitudes are calculated for free constituent propagators as well as for dressed propagators which parameterise confinement. The corresponding results for space-like form factors differ quantitatively but not qualitatively for various ans\"atze for the propagators. These results do not allow to draw definite conclusions on the permissibility of different dressing functions. Results for kaon photoproduction, on the other hand, exclude a whole class of constituent propagators.Comment: 8 pages, 4 figures; to appear in the proceedings of the workshop on Lepton scattering, Hadrons and QCD, Adelaide, March 26 to April 6, 200

The Importance of Boundary Conditions for Fluctuation Induced Forces between Colloids at Interfaces

We calculate the effective fluctuation induced force between spherical or disk-like colloids trapped at a flat, fluid interface mediated by thermally excited capillary waves. This Casimir type force is determined by the partition function of the system which in turn is calculated in a functional integral approach, where the restrictions on the capillary waves imposed by the colloids are incorporated by auxiliary fields. In the long-range regime the fluctuation induced force is shown to depend sensitively on the boundary conditions imposed at the three-phase contact line between the colloids and the two fluid phases. The splitting of the fluctuating capillary wave field into a mean-field and a fluctuation part leads to competing repulsive and attractive contributions, respectively, which give rise to cancellations of the leading terms. In a second approach based on multipole expansion of the Casimir interaction, these cancellations can be understood from the vanishing of certain multipole moments enforced by the boundary conditions. We also discuss the connection of the different types of boundary conditions to certain external fields acting on the colloids which appear to be realizable by experimental techniques such as the laser tweezer method.Comment: 24 pages, 4 figure