Leptogenesis

A highly compelling and theoretically robust class of mechanisms for explaining the matter-antimatter asymmetry of the universe. It posits that the asymmetry originated first in the lepton sector (neutrinos and charged leptons) and was later converted into the observed baryon asymmetry (quarks) by sphaleron processes in the early universe. The most popular scenario involves the CP-violating decay of heavy, right-handed Majorana neutrinos in the early universe. These decays produce a net lepton number. The sphalerons, which violate both baryon and lepton number (but conserve B-L), then partially convert this lepton asymmetry into the final baryon asymmetry we observe today. Leptogenesis is deeply attractive because it seamlessly connects two major puzzles - the origin of matter and the tiny, non-zero masses of neutrinos - into a single, elegant framework.

Thus the popular hypothesis claims that heavier cousins of neutrinos filled the hot early universe. These would have decayed more often into antileptons than into leptons. (Leptons are the particle category that includes electrons.) Through high-temperature quantum tunneling events known as sphaleron processes, the surplus of antileptons would then have converted into the baryon surplus we see today. Experimental searches are being done for rare particle decays that would point to the existence of the heavy neutrinos.