Solving the Crisis in Big-Bang Nucleosynthesis by the Radiative Decay of an Exotic Particle

A new mechanism which can solve the crisis in the standard big-bang nucleosynthesis scenario is discussed. If we assume an unstable particle $X$ with a long lifetime ($10^4{\rm sec}\lesssim\tau_X\lesssim 10^6{\rm sec}$) which decays into photon(s), then cascade photons induced by the radiative decay of $X$ can destroy significant amounts of D and $^3$He without affecting $^4$He, resulting in a better agreement between the theoretical and observational values of the primordial abundances of D, $^3$He, and $^4$He. We numerically investigate this process and derive a constraint on the properties of $X$ in order to make the big-bang nucleosynthesis scenario viable. We also present some candidates for the unstable particle $X$.