Reannihilation of self-interacting dark matter

We explore the phenomenology of having a second epoch of dark matter annihilation into dark radiation long after the standard thermal freeze-out. Such a hidden reannihilation process could affect visible sectors only gravitationally. As a concrete realization we consider self-interacting dark matter (SIDM) with a light force mediator coupled to dark radiation. We demonstrate how resonantly Sommerfeld enhanced cross sections emerge to induce the reannihilation epoch. The effect is a temporally local modification of the Hubble expansion rate, and we show that the cosmic microwave background (CMB) measurements-as well as other observations-have a high sensitivity to observe this phenomenon. Special attention is given to the model region where late kinetic decoupling and strong self-interactions can alleviate several smallscale problems in the cold dark matter paradigm at the same time. Interestingly, we find that reannihilation might here also simultaneously lower the tension between CMB and low-redshift astronomical observations of H-0 and sigma(8). Moreover, we identify reannihilation as a clear signature to discriminate between the phenomenologically otherwise almost identical vector and scalar mediator realizations of SIDM.