Abstract
In the type-I seesaw mechanism the Casas-Ibarra (CI) parametrization provides a convenient description of the Dirac neutrino mass matrix in terms of the light and heavy Majorana neutrino masses, the lepton flavor mixing matrix U and an unknown complex orthogonal matrix O. If O is assumed to be real, it will be impossible to generate unflavored thermal leptogenesis via the lepton-number-violating and CP-violating decays of the lightest heavy Majorana neutrino. We find that this observation can be invalidated after small but unavoidable quantum corrections to the CI parametrization are taken into account with the help of the one-loop renormalization-group equations (RGEs) between the seesaw and electroweak scales. We illustrate a novel and viable unflavored leptogenesis scenario of this kind based on the RGEs in the seesaw-extended standard model, and show its direct link to the CP-violating phases of U at low energies.
Highlights
JHEP04(2020)179 with MD = Yνv being the Dirac neutrino mass matrix
If O is assumed to be real, it will be impossible to generate unflavored thermal leptogenesis via the lepton-number-violating and CP-violating decays of the lightest heavy Majorana neutrino. We find that this observation can be invalidated after small but unavoidable quantum corrections to the CI parametrization are taken into account with the help of the one-loop renormalization-group equations (RGEs) between the seesaw and electroweak scales
Adopting the parametrization of the PMNS matrix U given in eq (2.8), we find that there are totally twelve parameters involved in our unflavored leptogenesis scenario: the heavy neutrino mass M1 which determines the values of I0 and ∆τ ; three light neutrino masses mi; three lepton flavor mixing angles θ12, θ13 and θ23; three CPviolating phases δ, ρ and σ; and two free parameters θ and φ used to parametrize Oi1
Summary
The product MD† MD becomes U -dependent This new observation motivates us to reexamine whether unflavored leptogenesis has something to do with leptonic CP violation at low energies when O is taken to be real. Each vertex involving the τ -flavored lepton doublet in the Feynman diagrams of N1 → α+H and N1 → α+H decays is slightly modified, making it possible to trigger the interference between their tree and one-loop amplitudes at the leading order of ∆τ and result in the unflavored CP-violating asymmetry ε1 as shown in eq (2.7). Eq (2.7) provides us with a direct link between unflavored leptogenesis at the seesaw scale and the CP-violating phases of U at low energies, since O has been assumed to be real. To make the unflavored leptogenesis scenario under consideration viable in interpreting the observed baryon-antibaryon asymmetry of the Universe, the value of M1 must be big enough such that both the magnitudes of ∆τ and ε1 can be properly enhanced
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