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.