Constraint on slepton intergenerational mixing from big-bang nucleosynthesis

We find constraint on intergenerational mixing of slepton from big-bang nucleosynthesis (BBN). Today, we know that there exist lepton flavor violation (LFV) from the observation of neutrino oscillation, though there do not exist LFV in the standard model of particle physics (SM). LFV in charged lepton sector (cLFV) have also been expected to exist. From theoretical point of view, the effects of long-lived stau on BBN have been investigated and it is known that the stau can solve the consmological Li-7 problem. However, in the study so far, tau flavor is exactly conserved and it contradict with the existence of cLFV. In this study, we generalize the flavor to be violated and call the stau as slepton. Even if the violation is tiny, it drastically changes the lifetime and the evolution of relic density of the slepton. Thus we analyze the effects of the long-lived slepton on BBN, and constrain the magnitude of the cLFV.