Justus H. Koch

Pion structure from improved lattice QCD: form factor and charge radius at low masses

The charge form factor of the pion is calculated in lattice QCD. The nonperturbatively improved Sheikholeslami-Wohlert action is used together with the O(a) improved vector current. Other choices for the current are examined. The form factor is extracted for pion masses from 970 down to 360 MeV and for momentum transfers Q(2)less than or equal to2 GeV2. The mean square charge radius is extracted and compared to previous determinations, and its extrapolation to lower masses discussed.

The Pion form-factor in improved lattice QCD

Abstract We calculate the electromagnetic form factor of the pion in lattice gauge theory. The non-perturbatively improved Sheikoleslami–Wohlert lattice action is used together with the O (a) improved current. The form factor is compared to results for other choices for the current and features of the structure of the pion deduced from the ‘Bethe–Salpeter wave function’ are discussed.

Production of vector mesons by real and virtual photons at high energies

Abstract A model based on Reggeon field theory is used for the description of the photo- and electroproduction of vector mesons (ϱ0, ω, φ, J ψ ) on the proton. Its main feature is the dependence of the Pomeron intercept on the virtuality of the photon. The few parameters of the model were determined by fitting the total cross sections for real and virtual photons on the proton. This allows a parameter-free description of the energy dependence of vector meson production. Very good agreement with the existing high energy data is obtained.

THE BFKL POMERON WITH RUNNING COUPLING CONSTANT : HOW MUCH OF ITS HARD NATURE SURVIVES ?

Abstract We discuss the BFKL equation with a running gauge coupling and identify in its solutions the contributions originating from different transverse momentum scales. We show that for a running coupling constant the distribution of the gluons making up the BFKL Pomeron shifts to smaller transverse momenta so that the dominant part of Pomeron can have a non-perturbative origin. It is demonstrated how this soft physics enters into the BFKL solution through the boundary condition. We consider two kinematical regimes leading to different behaviour of the rapidity and transverse momentum dependence of the gluon distribution. In the diffusion approximation to the BFKL kernel with running α s , we find a sequence of poles which replaces the cut for fixed α s . The second regime corresponds to the singular part of the kernel, which gives the dominant contribution in the limit of very large transverse momenta. Finally, a simple more general picture is obtained for the QCD Pomeron in hard processes: it is of soft, non-perturbative nature, but has hard ends of DGLAP type.

Evidence for meson-exchange currents in the radiative thermal neutron capture by deuterium nuclei

Abstract The circular polarization of photons from the radiative capture of polarized neutrons in deuterium is measured. Comparison to existing model calculations shows the importance of meson-exchange currents in this reaction.

A measurement of the photopion reaction 13C(γ, π−)13Ng.s. at a pion kinetic energy of 48 MeV

Abstract The Amsterdam electron accelerator facility was used to measure the differential cross section for pions of 48 MeV kinetic energy produced in the reaction between mirror states 13 C (γ, π − ) 13 N g.s. , at pion angles 65°, 100° and 125°. The measured cross sections are larger than previously reported measurements at θ π = 900 deg ; and T π = 42 MeV, but smaller than theoretical calculations. Excited state transitions were also measured.

A study of the role of the Δ-resonance in the reaction 13C(γ,π−)13Ngs

Abstract Cross sections for the exclusive photoproduction reaction 13 C(γ,π − ) 13 N gs were measured at energies across the resonance region. To focus on the resonant production mechanism the measurements were carried out at a fixed nuclear momentum transfer of Q = 1 fm −1 . For a test of the non-resonant production and of the nuclear transition matrix element, data at Q = 1.25 fm −1 and a differential cross section at 223 MeV photon energy were taken. There are sizeable discrepancies between our measurements and the theoretical predictions.

Charm production in deep inelastic and diffractive scattering

Abstract. We consider the production of charm by real and virtual photons. Special attention is paid to diffractive charm production, which provides information on the gluonic content of the Pomeron. Our calculations are based on the gluon distributions of the CKMT-model, which is shown to lead to agreement with the data on open charm production in deep inelastic scattering. We compare predictions for diffractive charm production of different models for the distribution of gluons in the Pomeron. Experiments at HERA should be able to discriminate between them. Predictions for beauty production in diffractive and non-diffractive interactions of photons are also given.

Stochastic methods for zero energy quantum scattering

We investigate the use of stochastic methods for zero energy quantum scattering based on a path integral approach. With the application to the scattering of a projectile from a nuclear many-body target in mind, we use the potential scattering of a particle as a test for the accuracy and efficiency of several methods. To be able to deal with complex potentials, we introduce a path sampling action and a modified scattering observable. The approaches considered are the random walk, where the points of a path are sequentially generated, and the Langevin algorithm, which updates an entire path. Several improvements are investigated. A cluster algorithm for dealing with scattering problems is finally proposed, which shows the best accuracy and stability.

Monte Carlo calculations for the hard Pomeron

Abstract Starting from the same input as the standard BFKL Pomeron, we directly calculate the “hard” Pomeron as a gluonic ladder by using Monte Carlo methods. We reproduce the characteristic features of the the BFKL Pomeron and are now also able to evaluate new observables. The applicability of the BFKL approach under realistic kinematical conditions can be tested and the influence of the running coupling constant examined.