In a series of papers P. Kroll et al. investigated the effects of baryon wave functions with short-range correlations (diquarks). A systematic study of many exclusive photon-nucleon reactions has been carried out within that model: electromagnetic form factors in the space-like and time-like regions, real Compton scattering, two-photon annihilations into proton-antiproton, charmonium decays into proton-antiproton as well as photoproduction of mesons. A fair description of all the mentioned data has been achieved, utilizing in all cases the same nucleon wave function as well as the same values of the parameters specifying the diquarks. The pure quark Brodsky-Lepage model does not provide similarly good results for that class of reactions.
One can improve the standard Brodsky-Lepage model by taking into account the transverse degrees of freedom the constituents of a hadron possess as well as a Sudakov factor comprising gluonic radiative corrections. The Sudakov factor we use has been calculated by Sterman and collaborators to the next-to-leading log approximation using resummation techniques. It can be shown that this modification of the Brodsky-Lepage model allows to calculate the perturbative contribution to exclusive quantities in a theoretically self-consistent way. Previous objections (raised, for instance, by Isgur and Llewellyn-Smith) against the use of the standard Brodsky-Lepage model for momentum transfers of the order of a few GeV, do not apply to the modified model. To continue the research program we intend the following investigations in connection with "Hadronic Physics with High Energy Electromagnetic Probes": i) Extension of our analysis of the pion form factor to other mesons, for instance the kaon, and inclusion of non-zero orbital angular momentum components into the mesonic wave functions; ii) Construction of a realistic nucleon wave function; iii) Calculation of virtual Compton scattering off protons and of electroproduction of photons. The interference between the virtual Compton and the Bethe-Heitler amplitudes in the electroproduction case is particularly interesting since it is sensitive to the relative phases of the amplitudes. This research project will be carried out in collaboration with Dr. P. Guichon (Saclay).
Another possibility to investigate the onset of pQCD stems from the dispersion-theoretical analysis of the nucleon electromagnetic form factors. This allows for an essentially model-independent determination of these elastic form factors provided a sufficiently accurate data base is given. Based on the current world basis of data, it can be shown that the momentum transfers reached at CEBAF will not be sufficient to test the large-t behaviour suggested by pQCD. Data in the range of $20 - 30$ GeV$^2$ are absolutely necessary. These form factor studies can then further be extended to investigate the flavor structure of the nucleon.
A second intensive research activity includes the study of color transparency in various exclusive and semi inclusive processes. Since the analysis of the NE18 and E665 experiments many examples have been analyzed and special attention will be given to new experiments possible at CEBAF. Further study will also be devoted to the possibilities of a ELFE type facility to investigate such effects.
| Name | Occupation | Main activity |
|---|---|---|
| P. Kroll | Professor U. of Wuppertal | Exclusive reactions |
| U.Meißner | Professor U. of Bonn | Hadron structure, form factors |
| J.Speth | Professor U.Bonn and IKP KFA Jülich | Colour transparency |
| J.Wambach | Professor U.Bonn and IKP KFA Jülich | Hadron Structure |
| N.Nikolaev | Professor IKP Jülich | Colour Transparency |
| J. Bolz | Post-Doc U. of Wuppertal | |
| M. Raulfs | PhD Student U. of Wuppertal |
The team includes a non EU citizen: Prof. Nikolaev has a Russian passeport. Since two years, he has a staff position in Jülich, which will become permanent next year. He is recognized as one of the prominent theorists in the field.
The request is one post-doc during two years (24 man-months). 12 man-months will be allocated to the Wuppertal group and 12 man-months to the Bonn/Jülich group (which have a common visitor program). The travel fund will be shared approximately according to the commitment of each group (about 1/3 for Wuppertal and 2/3 for Bonn/Jülich).
Selected publications: