Speaker
Description
Dark matter remains one of the most compelling mysteries in modern physics. While WIMPs have long been leading candidates, the lack of experimental evidence has motivated alternatives such as SIMPs. We consider an MeV scale complex scalar SIMP dark matter candidate $\chi$, interacting via a massless dark photon $A^\mu$, which induces a Coulomb potential and forms bound states (SIMPonium).
We find that freeze out occurs at $x = m_\chi/T \approx 17$ ($T \sim \text{few MeV}$), coinciding with the epoch of Big Bang Nucleosynthesis (BBN). We analyze the impact of bound-state formation, ionization, and decay on light element abundances, including photodisintegration effects. The decay of light SIMPonium, as well as the annihilation of light $\chi$-$\chi^*$ pairs, results in reduction in abundance of $^{7}\mathrm{Li}$, offering a potential mechanism to alleviate the primordial lithium discrepancy. We further analyze the impact on the Cosmic Microwave Background (CMB) through exotic energy injection from SIMPonium formation and decay. The presence of dark radiation along with SIMPonium decay and annihilation of free dark matter into dark radiation, modifies the Hubble expansion rate, providing a solution to the Hubble tension.