Constraint on B-L cosmic string from leptogenesis with degenerate neutrinos

In the early universe, as a consequence of U(1)B-L gauge symmetry-breaking, the so-called B-L cosmic strings are expected to be produced at the breaking scale ηB-L according to the Kibble mechanism. The decaying, collapsing closed loops of these strings can release the right-handed neutrinos, whose subsequent decay can contribute to the baryon asymmetry of the universe (BAU), through the “slow death” (SD) process and/or the “quick death” (QD) process. In this Letter, we assume that the decay of the lightest heavy Majorana neutrinos released from the B-L cosmic string loops can produce a baryon asymmetry consistent with the cosmic microwave background (CMB) observations. Considering the fact that both the neutrinoless double beta decay experiment and the cosmological data show a preference for degenerate neutrinos, we give the lower limits for the breaking scale ηB-L with the neutrino masses 0.06eV⩽m¯=(m12+m22+m32)1/2⩽1.0eV, where the full possible cases of degenerate neutrinos are included. We obtain ηB-L≳3.3×1015GeV, 5.3×1015GeV and 9.5×1015GeV for m¯=0.2eV, 0.4eV and 1.0eV respectively in the SD process, and find the B-L cosmic string has a very small contribution to the BAU in the QD process.