The X⁻ Solution to the ⁶Li and ⁷Li Big Bang Nucleosynthesis Problems

The ⁶Li abundance observed in metal-poor halo stars exhibits a plateau as a function of metallicity similar to that for ⁷Li, suggesting a big bang origin. However, the inferred primordial abundance of ⁶Li is ~1000 times larger than that predicted by standard big bang nucleosynthesis for the WMAP baryon-to-photon ratio. In addition, the inferred ⁷Li primordial abundance is 3 times smaller than the big bang prediction. We describe a possible simultaneous solution to both of these lithium problems that is based on a hypothetical massive, negatively charged leptonic particle that binds to the light nuclei produced in big bang nucleosynthesis, but decays long before it can be detected. We consider only the X-nuclear reactions and assume that the effect of decay products is negligible, as would be the case if lifetime were large or the mass difference between the charged particle and its daughter were small. An interesting feature is that, because the particle gets bound to the existing nuclei after the cessation of the usual big bang nuclear reactions, a second longer epoch of nucleosynthesis can occur among X-nuclei with reduced Coulomb barriers. We confirm that reactions in which the hypothetical particle is transferred can greatly enhance the production of ⁶Li while depleting ⁷Li. Thus, big bang nucleosynthesis in the presence of these hypothetical particles, with or without an event of stellar processing, can simultaneously solve the two Li abundance problems.