Common Origin of Soft mu-tau and CP Breaking in Neutrino Seesaw and the Origin of Matter
Shao-Feng Ge, Hong-Jian He, Fu-Rong Yin
备注：
60pp, JCAPin Press, v2: only minor stylistic refinements (added Daya Bay's future sensitivity in Figs.2+8, shortened some eqs, added new Appendix-A and some references), comments are welcome.
Neutrino oscillation data strongly support mu-tau symmetry as a good approximate flavor symmetry of the neutrino sector, which has to appear in any viable theory for neutrino mass-generation. The mu-tau breaking is not only small, but also the source of Dirac CP-violation. We conjecture that both discrete mu-tau and CP symmetries are fundamental symmetries of the seesaw Lagrangian (respected by interaction terms), and they are only softly broken, arising from a common origin via a unique dimension-3 Majorana mass-term of the heavy right-handed neutrinos. From this conceptually attractive and simple construction, we can predict the soft mu-tau breaking at low energies, leading to quantitative correlations between the apparently two small deviations θ_{23} - 45^o and θ_{13} - 0^o. This nontrivially connects the on-going measurements of mixing angle θ_{23} with the upcoming experimental probes of θ_{13}. We find that any deviation of θ_{23} - 45^o must put a lower limit on θ_{13}. Furthermore, we deduce the low energy Dirac and Majorana CP violations from a common soft-breaking phase associated with mu-tau breaking in the neutrino seesaw. Finally, from the soft CP breaking in neutrino seesaw we derive the cosmological CP violation for the baryon asymmetry via leptogenesis. We fully reconstruct the leptogenesis CP-asymmetry from the low energy Dirac CP phase and establish a direct link between the cosmological CP-violation and the low energy Jarlskog invariant. We predict new lower and upper bounds on the θ_{13} mixing angle, 1^o < θ_{13} < 6^o. In addition, we reveal a new hidden symmetry that dictates the solar mixing angle θ_12 by its group-parameter, and includes the conventional tri-bimaximal mixing as a special case, allowing deviations from it.
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The most general supersymmetric seesaw mechanism has too many parameters to be predictive and thus can not be excluded by any measurements of lepton flavour violating (LFV) processes. We focus on the simplest version of the type-I seesaw mechanism assuming minimal supergravity boundary conditions. We compute branching ratios for the LFV scalar tau decays, ${\tilde τ}_2 \to (e,μ) + χ^0_1$, as well as loop-induced LFV decays at low energy, such as $l_i \to l_j + γ$ and $l_i \to 3 l_j$, exploring their sensitivity to the unknown seesaw parameters. We find some simple, extreme scenarios for the un…
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