Corrections to Bino Annihilation II: One-Loop Contribution to   $\widetilde B \widetilde B \to Z^*$
  We calculate the one-loop contribution to the bino annihilation rate due to the process $\widetilde B \widetilde B \to Z^*$, which vanishes at tree level. 
Toby Falk, Richard Madden, Keith A. Olive, and Mark Srednicki