Probing Gravitational Dark Matter

So far all evidences of dark matter (DM) come from astrophysical and cosmological observations, due to gravitational interactions of the DM. It is possible that the true DM particle in the universe joins gravitational interactions only, but nothing else. Such a Gravitational DM (GDM) acts as a weakly interacting massive particle (WIMP), which is conceptually simple and attractive. In this work, we explore this direction by constructing the simplest scalar GDM particle $\chi_s$. It is a $Z_2$ odd singlet under the standard model (SM) gauge group, and naturally joins the unique dimension-4 interaction with Ricci curvature, $\xi_s \chi_s^2 R$, where $\xi_s$ is the dimensionless nonminimal coupling. We demonstrate that this gravitational interaction $\xi_s \chi_s^2 R$, together with Higgs-curvature nonminimal coupling term $\xi_h H^\dag H R$, induces effective couplings between $\chi_s^2$ and SM fields which can account for the observed DM thermal relic abundance. We analyze the annihilation cross sections of GDM particles and derive the viable parameter space for realizing the DM thermal relic density. We further study the direct/indirect detections and the collider signatures of such a scalar GDM. These turn out to be highly predictive and testable.