Mapping the allowed parameter space for decaying dark matter models
  I consider constraints on a phenomenological decaying-dark-matter model, in
which two weakly-interacting massive particle (WIMP) species have a small mass
splitting, and in which the heavier particle decays to the lighter particle and
a massless particle on cosmological timescales. The decay parameter space is
parameterized by $v_k$, the speed of the lighter particle in the center-of-mass
frame of the heavier particle prior to decay, and the decay time $\tau$. Since
I consider the case in which dark-matter halos have formed before there has
been significant decay, I focus on the effects of decay in already-formed
halos. I show that the $v_k-\tau$ parameter space may be constrained by
observed properties of dark-matter halos. I highlight which set of observations
is likely to yield the cleanest constraints on $v_k-\tau$ parameter space, and
calculate the constraints in those cases in which the effect of decay on the
observables can be calculated without N-body simulations of decaying dark
matter. I show that for $v_k \gtrsim 5\times 10^3$ km s$^{-1}$, the z=0 galaxy
cluster mass function and halo mass-concentration relation constrain $\tau
\gtrsim$ 40 Gyr, and that precise constraints on $\tau$ for smaller $v_k$ will
require N-body simulations.
