B-L Cosmic strings and Baryogenesis
Abstract
Cosmic strings arising from breaking of the
U(1
)
B−L
gauge symmetry that occurs in a wide variety of unified models can carry zero modes of heavy Majorana neutrinos. Decaying and/or repeatedly self-interacting closed loops of these ``
B−L
'' cosmic strings can be a non-thermal source of heavy right-handed Majorana neutrinos whose decay can contribute to the observed baryon asymmetry of the Universe (BAU) via the leptogenesis route. The
B−L
cosmic strings are expected in GUT models such as SO(10), where they can be formed at an intermediate stage of symmetry breaking well below the GUT scale
∼
10
16
GeV; such light strings are not excluded by the CMB anisotropy data and may well exist. We estimate the contribution of
B−L
cosmic string loops to the baryon-to-photon ratio of the Universe in the light of current knowledge on neutrino masses and mixings implied by atmospheric and solar neutrino measurements. We find that
B−L
cosmic string loops can contribute significantly to the BAU for
U(1
)
B−L
symmetry breaking scale $\eta_{B-L}\gsim 1.7\times 10^{11}\gev$. At the same time, in order for the contribution of decaying
B−L
cosmic string loops not to exceed the observed baryon-to-photon ratio inferred from the recent WMAP results, the lightest heavy right-handed Majorana neutrino mass
M
1
must satisfy the constraint $M_1 \leq 2.4 \times 10^{12}(\eta_{B-L}/10^{13}\gev)^{1/2}\gev$. This may have interesting implications for the associated Yukawa couplings in the heavy neutrino sector and consequently for the light neutrino masses generated through see-saw mechanism.