New mechanism for leptogenesis

Unified theories containing a $U(1)_{B-L}$ gauge symmetry predict heavy Majorana right-handed neutrinos. In such theories, cosmic strings may form at the $B-L$ breaking scale. If the Higgs field forming the strings is also the Higgs field which gives mass to the right-handed neutrinos, there are right-handed neutrinos trapped as transverse zero modes in the core of the strings. When cosmic string loops decay, they release these neutrinos. This is an out-of-equilibrium process. The released neutrinos acquire heavy Majorana mass and decay into massless leptons and electroweak Higgs particles to produce a lepton asymmetry, which is converted into a baryon asymmetry via sphaleron transitions.