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Starting with high scale mixing unification hypothesis, we investigate the renormalization group evolution of mixing parameters and masses for Dirac type neutrinos. Following this hypothesis, the PMNS mixing angles and phase are taken to be identical to the CKM ones at a unifying high scale. Then, they are evolved to a low scale using renormalization-group equations. The notable feature of this hypothesis is that renormalization group evolution with quasi-degenerate mass pattern can explain largeness of leptonic mixing angles even for Dirac neutrinos. The renormalization group evolution naturally results in a non-zero and small value of leptonic mixing angle $theta_{13}$. One of the important predictions of this work is that the mixing angle $theta_{23}$ is non-maximal and lies only in the second octant. We also derive constraints on the allowed parameter range for the SUSY breaking and unification scales for which this hypothesis works. The results are novel and can be tested by present and future experiments.
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We investigate the renormalization group evolution of masses and mixing angles of Majorana neutrinos under the `High Scale Mixing Unification hypothesis. Assuming the unification of quark-lepton mixing angles at a high scale, we show that all the experimentally observed neutrino oscillation parameters can be obtained, within 3-$sigma$ range, through the running of corresponding renormalization group equations provided neutrinos have same CP parity and are quasi-degenerate. One of the novel results of our analysis is that $theta_{23}$ turns out to be non-maximal and lies in the second octant. Furthermore, we derive new constraints on the allowed parameter space for the unification scale, SUSY breaking scale and $tan beta$, for which the `High Scale Mixing Unification hypothesis works.
If neutrinos are Dirac, the conditions for cobimaximal mixing, i.e. $theta_{23}=pi/4$ and $delta_{CP}=pm pi/2$ in the $3 times 3$ neutrino mixing matrix, are derived. One example with $A_4$ symmetry and radiative Dirac neutrino masses is presented.
The origin of small mixing among the quarks and a large mixing among the neutrinos has been an open question in particle physics. In order to answer this question, we postulate general relations among the quarks and the leptonic mixing angles at a high scale, which could be the scale of Grand Unified Theories. The central idea of these relations is that the quark and the leptonic mixing angles can be unified at some high scale either due to some quark-lepton symmetry or some other underlying mechanism and as a consequence, the mixing angles of the leptonic sector are proportional to that of the quark sector. We investigate the phenomenology of the possible relations where the leptonic mixing angles are proportional to the quark mixing angles at the unification scale by taking into account the latest experimental constraints from the neutrino sector. These relations are able to explain the pattern of leptonic mixing at the low scale and thereby hint that these relations could be possible signatures of a quark-lepton symmetry or some other underlying quark-lepton mixing unification mechanism at some high scale linked to Grand Unified Theories.
If the sterile neutrino mass matrix in an otherwise conventional seesaw model has a rank less than the number of flavors, it is possible to produce pseudo-Dirac neutrinos. In a two-flavor, sterile rank 1 case, we demonstrate analytic conditions for large active mixing induced by the existence of (and coupling to) the sterile neutrino components. For the three-flavor, rank 1 case, ``3+2 scenarios with large mixing also devolve naturally as we show by numerical examples. We observe that, in this approach, small mass differences can develop naturally without any requirement that masses themselves are small. Additionally, we show that significant three channel mixing and limited experimental resolution can combine to produce extracted two channel mixing parameters at variance with the actual values.
We propose a simple renormalizable grand unified theory based on the $SU(5)$ gauge symmetry where the neutrino masses are generated at the quantum level through the Zee mechanism. In this model the same Higgs needed to correct the mass relation between charged leptons and down-type quarks plays a crucial role to generate neutrino masses. We show that in this model one can satisfy the constrains coming from the unification of gauge couplings and the mechanism for neutrino masses is discussed in detail. The predictions for proton decay are discussed in order to understand the testability at current and future experiments such as Hyper-Kamiokande. This simple theory predicts a light colored octet which could give rise to exotic signatures at the LHC.
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