Finite-Size Dark Matter and its Effect on Small-Scale Structure
Xiaoyong Chu (Institute of High Energy Physics, Austrian Academy of Sciences, Nikolsdorfer Gasse 18, 1050 Vienna, Austria); Camilo Garcia-Cely (Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany); Hitoshi Murayama (Kavli Institute for the Physics and Mathematics of the Universe (WPI), University of Tokyo, Kashiwa 277-8583, Japan, Department of Physics, University of California, Berkeley, California 94720, USA, Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA, Deutsches Elektronen-Synchrotron DESY, Notkestrasse 85, 22607 Hamburg, Germany)
If dark matter has a finite size that is larger than its Compton wavelength, the corresponding self-interaction cross section decreases with the velocity. We investigate the implications of this puffy dark matter for addressing the small-scale problems of the cold dark matter model and show that the way the nonrelativistic cross section varies with the velocity is largely independent of the dark matter internal structure. Even in the presence of a light particle mediating self-interactions, we find that the finite-size effect may dominate the velocity dependence. We present an explicit example in the context of a QCD-like theory and discuss possible ways to differentiate puffy dark matter from the usual light-mediator scenarios. Particularly relevant for this are low-threshold direct-detection experiments and indirect signatures associated with the internal structure of dark matter.
