B−Lcosmic strings and baryogenesis

Abstract

Cosmic strings arising from breaking of the $\mathrm{U}(1{)}_{B\ensuremath{-}L}$ 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 ``$B\ensuremath{-}L$'' cosmic strings can be a nonthermal 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 $B\ensuremath{-}L$ cosmic strings are expected in grand unified theory (GUT) models such as SO(10), where they can be formed at an intermediate stage of symmetry breaking well below the GUT scale $\ensuremath{\sim}{10}^{16}\text{ }\text{ }\mathrm{G}\mathrm{e}\mathrm{V}$; such light strings are not excluded by the cosmic microwave background (CMB) anisotropy data and may well exist. We estimate the contribution of $B\ensuremath{-}L$ 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 $B\ensuremath{-}L$ cosmic string loops can contribute significantly to the BAU for $\mathrm{U}(1{)}_{B\ensuremath{-}L}$ symmetry-breaking scale ${\ensuremath{\eta}}_{B\ensuremath{-}L}\ensuremath{\gtrsim}1.7\ifmmode\times\else\texttimes\fi{}{10}^{11}\text{ }\text{ }\mathrm{G}\mathrm{e}\mathrm{V}$. At the same time, in order for the contribution of decaying $B\ensuremath{-}L$ cosmic string loops not to exceed the observed baryon-to-photon ratio inferred from the recent Wilkinson microwave anisotropy probe (WMAP) results, the lightest heavy right-handed Majorana neutrino mass ${M}_{1}$ must satisfy the constraint ${M}_{1}\ensuremath{\le}2.4\ifmmode\times\else\texttimes\fi{}{10}^{12}({\ensuremath{\eta}}_{B\ensuremath{-}L}/{10}^{13}\text{ }\text{ }\mathrm{G}\mathrm{e}\mathrm{V}{)}^{1/2}\text{ }\text{ }\mathrm{G}\mathrm{e}\mathrm{V}$. This may have interesting implications for the associated Yukawa couplings in the heavy neutrino sector and consequently for the light neutrino masses generated through seesaw mechanism.