In-medium Spectral functions
Vector Mesons
Once hadrons are put in a nuclear environment their properties like masses and life times change due to the reactions with the medium. To be detected, however, the hadron has to leave the medium and reach the detector. On this journey the mass, e.g., of the hadron changes from its in-medium to its vacuum value. Thus at least a part of the in-medium information is lost. A promising alternative is to observe not the hadron directly but rather a decay product which does not interact any more. Such a scenario is realized by vector mesons decaying into dileptons which carry information about the in-medium vector meson mass to the detector. This has triggered a lot of theoretical interest in the in-medium properties of vector mesons. In our approach to this problem we study the properties of rho and omega vector mesons put (for simplicity) in infinite nuclear matter. We are aiming at the in-medium mass distributions (spectral function) of the vector mesons. The most important reaction mechanism for nucleon-rho meson collisions is the formation of baryonic resonances in the 1.5 to 2 GeV region. These lead to a significant broadening of the rho spectral function and to interesting new structures. Recently we have extended this approach to a relativistic treatment of the interactions between nucleons, rhos and resonances. We also study the consequences of the fact that in-medium modified vector meson properties in turn change the properties of the baryonic resonances.
Nucleons
Nuclear spectral functions, i.e. the energy-momentum distributions of nucleons in nuclear matter and finite nuclei, have long been studied experimentally in (e, e'p) reactions. Theoretical studies of these functions have been performed in the framework of state-of-the-art, complex and time-consuming many-body calculations. We have now shown that these spectral functions in the hole sector can be surprisingly well described by just one parameter, the average strength of the short-range correlations. This strength determines the interaction rate in a selfconsistent treatment of the scattering rates between collision-broadened particles. Thus, hole spectral functions and the connected nucleon momentum distributions in nuclei do not depend on any details of the interaction and the correlation, but offer instead a direct access to the average short-range correlations.
Quarks
We have studied in-medium strong interactions on a more elementary level: While free quarks, if they existed as stable states, would by construction have a delta type spectral function, quarks in a medium experience collisional broadening. The scattering rates which determine the spectral function depend recursively on the spectral functions of the interacting particles. This recursion problem is solved in a self-consistent way for a simple four-point type quark interaction. This enables us to study the properties of quarks in hadronic matter, i.e. in-medium effects on the elementary degrees of freedom.
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