Freeze-in production of scalaron dark matter in f ( R ) gravity

We demonstrate that the scalaron, a scalar degree of freedom, emerging from the f ( R ) theory of gravity, can account for the observed dark matter (DM) abundance if its mass is around the MeV scale, to ensure its cosmological stability. Focusing on two well-known f ( R ) gravity models, we systematically show that if scalaron production proceeds via the freeze-in mechanism, the right relic abundance is satisfied over a very narrow window of reheating temperature 10 14 ≲ T rh ≲ 10 16 GeV . We delineate the viable parameter space of the f ( R ) models consistent with the observed DM abundance, and highlight relevant experimental constraints from searches targeting DM decay signatures.