Leptogenesis in the Early Universe*

Abstract

The presence of superheavy Majorana neutrinos is an important prediction of unified theories beyond the standard model. An integration of the heavy Majorana neutrinos induces very small masses for neutrinos via the seesaw mechanism. Thus, small neutrino masses is a reflection of a unification at very high energy scale.It is quite natural to consider that the heavy Majorana neutrinos are produced thermally in the early universe. Then, the heavy neutrinos begin to decay into Higgs and lepton, or Higgs and anti-lepton when the temperature of the universe cools down to the masses of heavy Majorana neutrinos. The decays of the heavy neutrinos may produce the lepton-number asymmetry if CP invariance is violated in the decay processes. This lepton-number asymmetry is converted into the baryon-number asymmetry in the present universe (leptogenesis) through nonperturbative effects of the electroweak gauge theory. We show, in this talk, that the neutrino masses suggested from atmospheric and solar neutrino-oscillation experiments are just in the range favorable for the thermal leptogenesis. With the aid of the observed neutrino masses we find that the thermal leptogenesis takes place at temperatures T ≳ 2 × 109 GeV.However, the above scenario suffers from the gravitino problem if one extends the standard model to the supergravity framework. This is because too many gravitinos are produced at the temperature required for the thermal leptogenesis and their decays destroy light elements created by the big-bang nucleosynthesis. We show that a leptogenesis via inflaton decay is an interesting alternative to the thermal leptogenesis. This scenario is free from the gravitino problem if the gravitino has a relatively large mass as m3/2 ≃ 3–6 TeV.The importance of neutrinoless double β decay experiments and measurement of CP violation in neutrino oscillation experiments is also emphasized to explain the present universe's baryon asymmetry.