Gu, PH (reprint author), Chinese Acad Sci, Inst High Energy Phys, POB 918-4, Beijing 100049, Peoples R China.
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 eta(B-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 eta(B-L) with the neutrino masses 0.06 eV <= (m) over bar = (m(1)(2) + m(2)(2) + m(3)(2))(1/2) <= 1.0 eV, where the full possible cases of degenerate neutrinos are included. We obtain eta(B-L) greater than or similar to 3.3 x 10(15) GeV, 5.3 x 10(15) GeV and 9.5 x 10(15) GeV for (m) over bar = 0.2 eV, 0.4 eV and 1.0 eV respectively in the SD process, and find the B-L cosmic string has a very small contribution to the BAU in the QD process. (c) 2005 Elsevier B.V. All rights reserved.