Updated nucleosynthesis constraints on unstable relic particles

We reexamine the upper limits on the abundance of unstable massive relic particles provided by the success of big-bang nucleosynthesis calculations. We use the cosmic microwave background data to constrain the baryon-to-photon ratio, and incorporate an extensively updated compilation of cross sections into a new calculation of the network of reactions induced by electromagnetic showers that create and destroy the light elements deuterium, ³He, ⁴He, ⁶Li and ⁷Li. We derive analytic approximations that complement and check the full numerical calculations. Considerations of the abundances of ⁴He and ⁶Li exclude exceptional regions of parameter space that would otherwise have been permitted by deuterium alone. We illustrate our results by applying them to massive gravitinos. If they weigh ∼100GeV, their primordial abundance should have been below about 10^-13 of the total entropy. This would imply an upper limit on the reheating temperature of a few times 10⁷GeV, which could be a potential difficulty for some models of inflation. We discuss possible ways of evading this problem.