ar X iv : h ep - p h / 04 06 05 4 v 3 2 N ov 2 00 4 B − L Cosmic strings and Baryogenesis

Pijushpani Bhattacharjee, Narendra Sahu, and Urjit A. Yajnik Department of Physics, Indian Institute of Technology, Bombay, Mumbai 400076, India and Indian Institute of Astrophysics, Bangalore-560 034, India 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 ∼ 1016 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 baryonto-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 ηB−L > ∼ 1.7× 1011 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 M1 must satisfy the constraint M1 ≤ 2.4 × 1012 (