A solution to lithium problem by long-lived stau

We review a nonstandard Big-Bang Nucleosynthesis (BBN) scenario within the minimal supersymmetric standard model, and propose an idea to solve both the Li-7 and Li-6 problems. Each problem is a discrepancy between the predicted abundance in the standard BBN and observed one. We focus on the stau, a supersymmetric partner of tau lepton, which is a long-lived charged particle when it is the next lightest supersymmetric particle and is degenerate in mass with the lightest supersymmetric particle. The long-lived stau forms a bound state with a nucleus, and provides nonstandard nuclear reactions. One of those, the internal conversion process, accelerates the destruction of Be-7 and Li-7, and leads to a solution to the Li-7 problem. On the other hand, the bound state of the stau and He-4 enhances productions of n, D, T and Li-6. The over-production of Li-6 could solve the Li-6 problem; while the over-productions of D and T could conflict with observations, and hence the relevant parameter space of the stau is strictly constrained. We therefore need to carefully investigate the stau-He-4 bound state to find a condition of solving the Li-6 problem. The scenario of the long-lived stau simultaneously and successfully fits the abundances of light elements (D, T, He-3, He-4, Li-6 and Li-7) and the neutralino dark matter to the observed ones. Consequently, the parameter space both of the stau and the neutralino is determined with excellent accuracy.