Red-Giant Branch Stellar Cores as Macroscopic Dark Matter Detectors
  We show that macroscopic dark matter (DM) impacts on the degenerate helium
cores of red-giant branch (RGB) stars can ignite helium fusion via DM-baryon
elastic scattering. The onset of helium burning leads to a characteristic drop
in luminosity and rise in temperature that marks the transition to a horizontal
branch star. We show that such impacts can alter the RGB luminosity function of
globular clusters (GCs), focusing in particular on the GC M15. Using models of
M15 stars constructed with the stellar simulation code MESA, we compute the
expected DM-ignition event rates and the theoretical RGB luminosity functions
under the null and signal hypotheses. We constrain DM with masses $10^{17}\
{\rm g} \lesssim m_{\chi} \lesssim 10^{20}\ \rm{g}$ and geometric cross
sections $10^2\ {\rm cm}^2 \lesssim \sigma_{\chi n} \lesssim 10^{7}\ \rm{ cm}^2
$ assuming that the DM in M15 is sourced by the background Milky Way halo. We
also place more stringent constraints assuming that M15 formed in a DM subhalo
that survives today.
