Leptogenesis, Z' bosons, and the reheating temperature of the Universe
  We study the impact for leptogenesis of new U(1) gauge bosons coupled to the heavy Majorana neutrinos. They can significantly enhance the efficiency of thermal scenarios in the weak washout regime as long as the Z' masses are not much larger than the reheating temperature ($M_{Z'}<10 T_{rh}$), with the highest efficiencies obtained for Z' bosons considerably heavier than the heavy neutrinos ($M_{Z'} > 100 M_1$). We show how the allowed region of the parameter space is modified in the presence of a Z' and we also obtain the minimum reheating temperature that is required for these models to be successful. 
J. Racker and E. Roulet