We study the big-bang nucleosynthesis (BBN) with the long-lived exotic particle, called X. If the lifetime of X is longer than ∼ 0.1 sec, its decay may cause nonthermal nuclear reactions during or after the BBN, altering the predictions of the standard BBN scenario. We pay particular attention to its hadronic decay modes and calculate the primordial abundances of the light elements. Using the result, we derive constraints on the primordial abundance of X.Compared to the previous studies, we have improved the following points in our analysis: The JETSET 7.4 Monte Carlo event generator is used to calculate the spectrum of hadrons produced by the decay of X; The evolution of the hadronic shower is studied taking account of the details of the energy-loss processes of the nuclei in the thermal bath; We have used the most recent observational constraints on the primordial abundances of the light elements; In order to estimate the uncertainties, we have performed the Monte Carlo simulation which includes the experimental errors of the cross sections and transfered energies.We will see that the non-thermal productions of D, 3 He, 4 He and 6 Li provide stringent upper bounds on the primordial abundance of late-decaying particle, in particular when the hadronic branching ratio of X is sizable. We apply our results to the gravitino problem, and obtain upper bound on the reheating temperature after inflation.

Various lepton-flavor violating (LFV) processes in the supersymmetric standard model with right-handed neutrino supermultiplets are investigated in detail. It is shown that large LFV rates are obtained when tan β is large. In the case where the mixing matrix in the lepton sector has a similar structure as the KobayashiMaskawa matrix and the third-generation Yukawa coupling is as large as that of the top quark, the branching ratios can be as large as Br(µ → eγ) ≃ 10 −11 and Br(τ → µγ) ≃ 10 −7 , which are within the reach of future experiments. If we assume a large mixing angle solution to the atmospheric neutrino problem, rate for the process τ → µγ becomes larger. We also discuss the difference between our case and the case of the minimal SU (5) grand unified theory.

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