  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.
