Self-interacting dark matter

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Dark matter (DM) is usually assumed to be collisionless, however this is necessary only for its interactions with baryonic matter. In principle dark matter may have strong interactions with itself. This has in fact been suggested to alleviate some problems of the collisionless cold dark matter hypothesis when confronted with observational data on small scales. Moreover there is observational indication for self-interactions in the cluster A3827 where a galaxy falling into the cluster is observed to be displaced from its (gravitationally lensed) dark matter halo. This would require a large self-interaction cross-section of order 1 cm^2/gm.We will study angular distributions of DM-DM scattering for a number of different models (e.g. for Dirac DM, how would the angular distribution for a vector exchange differ from the angular distribution from an axial-vector exchange? Would there be differences between particle-particle scattering and particle-antiparticle scattering?) Forthcoming observational data will be analysed in the light of these findings. The implications of such large cross-sections for collider signatures will also be studied in the framework of 'simplified models' being considered by the ATLAS/CMS working group.