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M. V. Medvedev (CITA)
Dark matter constitutes most of the mass in the Universe, but its nature and properties remain largely unknown. A model of structure formation in a universe with CDM is in excellent agreement with observations on large scales but fails on small (galactic) scales. This suggests that the CDM model is a good first approximation, but it has to be corrected on a small scale. It seems that the resolution of the problem lies in specific physical properties of dark matter particles. The presently most attractive models, namely self-interacting (SIDM) and annihilating (ACDM), seem to capture important physics but still have some problems. In particular, SIDM requires fine tuning of the collision time in both dwarf galaxies and in clusters to be comparable to the Hubble time, which is unlikely to fit simultaneously, whereas ACDM suffers from the annihilation catastrophe in the early universe.
We argue that the difficulties of SIDM and ACDM may be resolved by postulating that dark matter particles are mixed states of several quantum flavors (similar to neutrinos or quarks). Elastic scattering of the particles results in a conversion of eigenstates, akin to annihilation. We demonstrate that the proposed model accumulates most of the attractive features of the SIDM and ACDM models, but is unlikely to suffer from their problems. In addition, the model is robust and does not require fine tuning. Depending on whether a significant fraction of dark matter has converted since the beginning of structure formation, our model may have some features of the decaying dark matter as well. We additionally considered the physics of the flavor-mixed dark matter in the early universe, which places strong constraints on the particle masses to be in or below the GeV/sub-GeV range.
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