Big-Bang Nucleosynthesis with Negatively-Charged Massive Particles as a Cosmological Solution to the 6 Li and 7 Li Problems

Observations of metal poor halo stars exhibit a possible plateau of 6Li abundance 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 (BBN) for the baryon-to-photon ratio inferred from the WMAP data. On the other hand, the inferred 7Li primordial abundance is about 3 times smaller than the prediction. We study a possible simultaneous solution to both the problems of underproduction of 6Li and overproduction of 7Li in BBN. This solution involves a hypothetical massive, negatively-charged leptonic particle that would bind to the light nuclei produced in BBN, but would decay long before it could be detected. 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 which have reduced Coulomb barriers. We numerically carry out a fully dynamical BBN calculation, simultaneously solving the recombination and ionization processes of negatively-charged particles by normal and X-nuclei as well as many possible nuclear reactions among them. We confirm that a reaction in which the hypothetical particle is transferred can occur that greatly enhance the production of 6Li while a reaction through an atomic excited state of X-nucleus depletes 7Li. It is confirmed that BBN in the presence of these hypothetical particles, together with or without an event of stellar burning process, can simultaneously solve the two Li abundance problems.