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

We discuss a new mechanism which can solve the crisis in standard big-bang nucleosynthesis. A long-lived particle $X$ $({10}^4\sec \lesssim {\tau }_X\lesssim {10}^6\sec )$ which decays into photon(s) will induce cascade photons, and destroy significant amounts of D and ${}^3$He without destroying ${}^4$He or too much ${}^7$Li. We numerically investigate this process and derive a constraint on the properties of $X$ such that the theoretical values of the primordial light-element abundances agree with observation. We also present some candidates for the unstable particle $X$.