Massive Gravitons as Feebly Interacting Dark Matter Candidates

We detail our discovery of a chiral enhancement in the production cross sections of massive spin-2 gravitons, below the electroweak symmetry breaking scale, that makes them ideal dark matter candidates for the freeze-in mechanism. The result is independent of the physics at high scales and points toward masses in the keV–MeV range. The graviton is, therefore, a sub-MeV dark matter particle, as favored by the small scale galaxy structures. We apply the novel calculation to a Randall-Sundrum model with multiple branes, showing a significant parameter space where the first two massive gravitons saturate the dark matter relic density.

Figure 1

The lifetime for the lightest KK graviton (red lines) and the IR brane radion (blue line) as a function of $k{r}_2$, with $k{r}_1=11.0$. The solid line corresponds to $m_r>2m_G^{(1)}$, whereas the dashed line is for $m_r=3.45{\mathrm{\Lambda }}_{\mathrm{IR}}$. The region with lifetime below the age of the Universe ($\sim H_0^{-1}$) is shaded in green.

Figure 2

The contour of relic density in the plane of $(k{r}_1,k{r}_2)$, where the blank region is viable for the graviton as long-lived DM. The boundary of the pink region satisfies $\mathrm{\Omega }{h}^2=0.12$ by summing the direct and subsequent freeze-in. The light blue region is excluded by the indirect direction.