A Solution to Lithium Problem by Long-Lived Stau
  We review a non-standard Big-Bang nucleosynthesis (BBN) scenario within the
minimal supersymmetric standard model, and propose an idea to solve both
${}^{7}$Li and ${}^{6}$Li 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 provide non-standard nuclear reactions. One of those,
the internal conversion process, accelerates the destruction of ${}^{7}$Be and
${}^{7}$Li, and leads to a solution to the ${}^{7}$Li problem. On the other
hand, the bound state of the stau and ${}^{4}$He enhances productions of n, d,
t, and ${}^{6}$Li. The over-production of ${}^{6}$Li could solve the ${}^{6}$Li
problem. While, the over-productions of d and t could conflict with
observations, and the relevant parameter space of the stau is strictly
constrained. We therefore need to carefully investigate the stau-${}^{4}$He
bound state to find a condition of solving the ${}^{6}$Li problem. The scenario
of the long-lived stau simultaneously and successfully fit the abundances of
light elements (d, t, ${}^{3}$He, ${}^{4}$He, ${}^{6}$Li, and ${}^{7}$Li) and
the neutralino dark matter to the observed ones. Consequently parameter space
both of the stau and the neutralino is determined with excellent accuracy.
