Cold keV dark matter from decays and scatterings
  We explore ways of creating cold keV-scale dark matter by means of decays and
scatterings. The main observation is that certain thermal freeze-in processes
can lead to a cold dark matter distribution in regions with small available
phase space. In this way the free-streaming length of keV particles can be
suppressed without decoupling them too much from the Standard Model. In all
cases, dark matter needs to be produced together with a heavy particle that
carries away most of the initial momentum. For decays, this simply requires an
off-diagonal DM coupling to two heavy particles; for scatterings, the coupling
of soft DM to two heavy particles needs to be diagonal, in particular in spin
space. Decays can thus lead to cold light DM of any spin, while scatterings
only work for bosons with specific couplings. We explore a number of simple
models and also comment on the connection to the tentative 3.5 keV line.
