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Project Summary

There are various components of matter in our universe: among them baryons, neutrinos, cold dark matter and dark energy. The nature of some of these components is well known (baryons, neutrinos), although the abundance might still require some explanation (e.g. through a mechanism like baryo- or leptogenesis). The nature of dark matter and dark energy is not understood. We thus have to answer two questions: What is the origin of dark matter and dark energy and what are their contributions to the current energy density? Attempts to clarify these questions typically concentrate on any one of these components, although one might think that they all should be related and that the relative values of their abundances should not be just an accident. The present project tries to explore possible relations between the various abundances of the components of matter. We would specifically like to investigate possible links between dark matter and dark energy. We shall consider several possible dark matter candidates, e.g. the axion-axino system, but also neutralinos. Axions are known to be a possible source of cold dark matter and/or quintessence; axinos could provide cold or warm dark matter and are in addition closely related to the properties of the neutrino sector. The existence of the axion-axino system is well motivated from particle physics considerations (the strong CP problem and supersymmetry) and they appear naturally in various unified models of fundamental interactions such as supersymmetric grand unified theories and superstring theories. Our investigations will be based on recent constructions of particle physics models in the framework of string theory that lead to an abundance of axion candidates. A thorough understanding of the (discrete) symmetries of string theory will be crucial for the analysis. We shall also try to analyze neutralinos which well motivated within the MSSM. They can be either cold or hot dark matter; only in the former case can they account for the structure formation of the universe. In the latter case they would be a sub-leading component of the dark matter. We consider what we can learn about such neutralinos from laboratory experiments and ways to distinguish them from axinos.

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