Effect of long-lived strongly interacting relic particles on big bang nucleosynthesis

Abstract

It has been suggested that relic long-lived strongly interacting massive particles (SIMPs, or $X$ particles) existed in the early universe. We study effects of such long-lived unstable SIMPs on big bang nucleosynthesis (BBN) assuming that such particles existed during the BBN epoch, but then decayed long before they could be detected. The interaction strength between an $X$ particle and a nucleon is assumed to be similar to that between nucleons. We then calculate BBN in the presence of the unstable neutral charged ${X}^{0}$ particles taking into account the capture of ${X}^{0}$ particles by nuclei to form $X$ nuclei. We also study the nuclear reactions and beta decays of $X$ nuclei. We find that SIMPs form bound states with normal nuclei during a relatively early epoch of BBN. This leads to the production of heavy elements which remain attached to them. Constraints on the abundance of ${X}^{0}$ particles during BBN are derived from observationally inferred limits on the primordial light element abundances. Particle models which predict long-lived colored particles with lifetimes longer than $\ensuremath{\sim}200\text{ }\text{ }\mathrm{s}$ are rejected based upon these constraints.