Proton-Antiproton Annihilation into Meson Pairs

The subject of our proposed project is the study of the strong interaction in hard hadronic reactions. In particular we plan to describe the production of meson-pairs in proton-antiproton annihilations in the energy range of a few GeV. Our theoretical analyses are closely connected to experimental research conducted at particle accelerator facilities, especially to the planned experiments at the PANDA detector at FAIR-GSI, Darmstadt, Germany. In a broader context one aim of this project is to get a better insight how hadrons, such as protons and mesons, are made up of its constituents, quarks and gluons. The dynamics is supposed to be governed by Quantum Chromodynamics (QCD) as the underlying fundamental theory of the strong interaction. Depending on the energy scale at which one studies hadronic processes, it are two distinct features of QCD which are predominant. At very high energies (or equivalently at short distances) quarks and gluons interact only very weakly with each other. But at low energies (long distances) it is opposite, the coupling between them becomes large. Theoretical and experimental studies of proton-antiproton collisions in the energy range of a few GeV offer new possibilities to investigate strong-interaction dynamics in the kinematical regime where the transition from long-distance to short- distance dominated QCD phenomena sets in. We are interested in the perturbative treatment of meson-pair production in proton-antiproton collisions in the few-GeV region where, e.g., PANDA will operate. In this kinematic regime the energies are already high enough to justify a perturbative treatment of the interaction between (some) hadronic constituents. But nevertheless non-perturbative dynamics is still present such as, e.g., the long-distance transition from the initial- to the final-state hadrons. For our reactions under study three perturbative mechanisms can come into consideration. In each mechanism the perturbative short- distance dynamics has to be separated from the long-distance effects containing the non-perturbative formation of the hadrons. But how this splitting has to be performed is different within each method. Up to now a detailed investigation, which of those perturbative approaches is dominating our hadronic reactions under consideration in the few-GeV region, is missing. Thus, we propose to investigate this kind of reactions within those competing mechanisms. Our project does not only aim to clarify which mechanism dominates meson-pair production in proton-antiproton collisions. From our theoretical investigations, together with corresponding measurements, we also expect considerable progress in the theoretical understanding of proton-to-meson transitions and thus of the proton structure in terms of quarks and gluons. Furthermore, this project would help to develop a main pillar of Austrian research in the environment of the physics program of PANDA.

In the coming years the construction of the Facility for Antiproton and lone Research (FAIR) In Darmstadt Germany will be finished. Its particle accelerator will start operations and let antiprotons and ions collide with protons or atomic nuclei. These unique experiments will deliver new findings about the evolution of the early universe or the different states of hadronic matter (matter whose fundamental building blocks are quarks and gluons). Our project Proton-Antiproton Annihilation into Meson Pairs was intended to do theoretical groundwork in view of the upcoming proton-antiproton scattering experiments performed at FAIR. In particular through our studies of meson-pair production in proton-antiproton collisions we could gain deeper Insight in the characteristics of quark-gluon interactions and how the hadrons - in our case the (anti)proton and the produced mesons - are formed by their fundamental constituents.In principle the underlying theory of the so-called strong force, which governs the interaction between quarks, gluons and their compounds, has been formulated a long time ago. A general solution over the whole energy range or all space-time distances is non the less still missing. It are two distinct features of the strong force which are competing with each other and making a full solution of the theory intractable: asymptotic freedom and confinement. At very large energies or, equivalently, small distances It Is the former one which dominates. Quarks and gluons behave as 1f they were quasi-free. The low-energy regime where the distances between the hadronic constituents become large, is dominated by the latter feature. There the quarks and gluons are strongly bound together and confined to hadrons which become the relevant degrees of freedom. At interaction energies in the region of a few GeV, where collider experiments of nowadays are performed, it is not clear from the beginning which of the two characteristics of the strong force is dominating.Therefore one has to examine each class of particle reaction in which one is interested Individually In order to find the appropriate mechanism for its description.For the proton-antiproton annihilations into meson pairs which we want to describe we assume a mechanism based on the existence of a well-defined separation between the short- and long- distance effects during the reaction. Furthermore, in our case t is the high-energy domain which is supposed to dominate the overall process. This means the relevant scattering takes place on the level of the hadronic constituents. Our approach is in competition with approaches where already the hadrons are the relevant degrees of freedom and as such, they perform the elementary scattering. Such concepts are for example applied by colleagues at the institute for Nuclear Physics of the Forschungszentrum Jülich in Germany. When calculating cross sections for meson-pair production within our mechanism and comparing them with the results of the Jülich group one sees that we get much higher even of the order of two in magnitude cross-section predictions.The experimental findings, which will be provided by FAIR in the near future, will offer valuable clues to find out which mechanisms effectively are at work at proton-antiproton reactions when comparing their data with the theoretical predictions. This will also shed new light on the specific structure of hadrons and on the characteristics of the strong force itself.Last but not least, the chance given to realize our project Proton-Antiproton Annihilation IntoMeson Pairs also marked one of the starting points of the Austrian endeavors with reference to FAIR physics.