Manifest CP violation from Majorana phases -: art. no. 053004

We hunt for and discuss manifestly CP-violating effects which are mediated by Majorana phases. These phases are present if the standard model neutrinos are Majorana particles. We argue that while Majorana phases do affect the strength of neutrinoless double beta decay (a well known fact), they do so in a way that involves no manifest violation of CP. The conditions for manifestly CP-violating phenomena-differences between the rates for CP-mirror-image processes-are presented, and three examples are discussed: (i) neutrino <-> antineutrino oscillation; (ii) rare decays of K and B mesons and their antiparticles; and (iii) the lepton asymmetry generated by the decay of hypothetical very heavy right-handed "seesaw" neutrinos. We also find that, for the case of degenerate light neutrinos, manifestly CP-violating effects in neutrino <-> antineutrino oscillation vanish, although flavor-changing transitions do not. Finally, we comment on leptogenesis with degenerate right-handed neutrinos, and contrast it with the neutrino <-> antineutrino oscillation case.