Heavy Particles from Inflation
  We describe a simple and efficient mechanism by which very heavy particles are copiously created from a primordial inflationary epoch. It works for scalar fields which are massless or very light during inflation and acquire a large mass right after the end of inflation. Such particles can exist in realistic scenarios, as we illustrate with several examples in the context of both field and string theory. Long-wavelength fluctuations of these fields are generated during inflation with an almost scale-invariant spectrum and may give the dominant contribution to the energy density of the heavy fields at late times. Applications of our results to superheavy dark matter and leptogenesis are discussed. 
G.F. Giudice, A. Riotto, A. Zaffaroni