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Register a new userPartial mass-degeneracy and spontaneous CP violation in the lepton sector
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Inspired by the small mass-squared difference measured in the solar neutrino oscillation experiments and by the testability, we suggest that a limit of the partial mass degeneracy, in which masses of the first two generation fermions are degenerate, may be a good starting point for understanding the observed fermion mass spectra and mixing patterns. The limit indicates the existence of a two-dimensional rotation symmetry, such as $O(2)$, $D_N$ and so on, in flavor space of the first two generations. We propose simple models for the lepton sector based on $D_N$ and show that the models can successfully reproduce the experimental data without imposing unnatural hierarchies among dimensionless couplings, although at least $10%$ tuning is necessary in order to explain a large atmospheric mixing. It is especially found that the $Z_2$ subgroup of the $D_N$ symmetry plays an important role in understanding the smallness of the electron mass and $theta_{13}^{rm PMNS}$. We also discuss testability of the models by the future neutrinoless-double-beta-decay experiments and cosmological observations.
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We study the contributions of supersymmetric models with a $U(1)$ horizontal symmetry and only spontaneous CP breaking to various lepton flavor observables, such as $mu to egamma$ and the electron electric dipole moment. We show that both a horizontal symmetry and a lack of explicit CP violation can alleviate the existing bounds from such observables. The undetermined $mathcal{O}(1)$ coefficients in such mass matrix models muddle the interpretation of the bounds from various flavor observables. To overcome this, we define a new fine-tuning measure for different observables in such setups. This allows us to study how naturally the observed IR flavor observables can emerge from a given mass matrix model. We use our flavor-naturalness measure in study of our supersymmetric models and quantify the degree of fine tuning required by the bounds from various lepton flavor observables at each mass scale of sleptons, neutralinos, and charginos.
We discuss the prospects - within several models - for the observation of CP-violation (CPV) in top decays and production. The outlook looks best for t -> bW at the LHC (MSSM CPV), t -> b tau u_tau at TeV3, LHC and NLC (H^+ CPV), p p-bar -> t b-bar + X at TeV3 (MSSM CPV), p p -> t t-bar + X at the LHC (MSSM CPV and neutral Higgs CPV) and for e^+ e^- -> t t-bar h, t t-bar Z, where h is the lowest mass neutral Higgs boson, at an NLC with energy geq 1 TeV (neutral Higgs CPV).
We derive sufficient conditions that guarantee a robust solution of the strong CP problem in theories with spontaneous CP violation, and introduce a class of models satisfying these requirements. In the simplest scenarios the dominant contribution to the topological angle arises at 3-loop order in the Yukawa couplings. A variety of realizations are possible on a warped extra dimension, which can simultaneously address the Planck-TeV hierarchy. Experimental signatures of this approach to the strong CP problem include flavor violation and vector-like partners of the top or bottom quarks.
We present the latest Hyper-Kamiokande sensitivity study showing that, with a total exposure of 13 MW $times 10^{7}$ seconds integrated beam power, the CP phase - $delta_{CP}$ - can be determined better than 21 degrees for all possible values of $delta_{CP}$ and CP violation can be established with a significance of more than 3$sigma$ (5$sigma$) for 78% (62%) of the $delta_{CP}$ parameter space.
The recent results from T2K and MINOS experiments point towards a relatively large value of the reactor angle theta_13 in the lepton sector. In this paper we show how a large theta_13 can arise from the charged lepton sector alone in the context of an SU(5) GUT. In such a scenario (tri-)bimaximal mixing in the neutrino sector is still a viable possibility. We also analyse the general implications of the considered scenario for the searches of CP violation in neutrino oscillations.
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