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Register a new userCP Violations in Lepton Number Violation Processes and Neutrino Oscillations
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We examine the constraints on the MNS lepton mixing matrix from the present and future experimental data of the neutrino oscillation and lepton number violation processes. We introduce a graphical representation of the CP violation phases which appear in the lepton number violation processes such as neutrinoless double beta decay, the $mu^--e^+$ conversion, and the K decay, $K^-topi^+mu^-mu^-.$ Using this graphical representation, we derive the constraints on the CP violation phases in the lepton sector.
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Measurements of CP--violating observables in neutrino oscillation experiments have been studied in the literature as a way to determine the CP--violating phase in the mixing matrix for leptons. Here we show that such observables also probe new neutrino interactions in the production or detection processes. Genuine CP violation and fake CP violation due to matter effects are sensitive to the imaginary and real parts of new couplings. The dependence of the CP asymmetry on source--detector distance is different from the standard one and, in particular, enhanced at short distances. We estimate that future neutrino factories will be able to probe in this way new interactions that are up to four orders of magnitude weaker than the weak interactions. We discuss the possible implications for models of new physics.
If neutrinos get mass via the seesaw mechanism the mixing matrix describing neutrino oscillations can be effectively non-unitary. We show that in this case the neutrino appearance probabilities involve a new CP phase, phi, associated to non-unitarity. This leads to an ambiguity in extracting the standard three--neutrino phase delta_CP, which can survive even after neutrino and antineutrino channels are combined. Its existence should be taken into account in the planning of any oscillation experiment aiming at a robust measurement of delta_CP.
We consider a version of the low-scale type I seesaw mechanism for generating small neutrino masses, as an alternative to the standard seesaw scenario. It involves two right-handed (RH) neutrinos $ u_{1R}$ and $ u_{2R}$ having a Majorana mass term with mass $M$, which conserves the lepton charge $L$. The RH neutrino $ u_{2R}$ has lepton-charge conserving Yukawa couplings $g_{ell 2}$ to the lepton and Higgs doublet fields, while small lepton-charge breaking effects are assumed to induce tiny lepton-charge violating Yukawa couplings $g_{ell 1}$ for $ u_{1R}$, $l=e,mu,tau$. In this approach the smallness of neutrino masses is related to the smallness of the Yukawa coupling of $ u_{1R}$ and not to the large value of $M$: the RH neutrinos can have masses in the few GeV to a few TeV range. The Yukawa couplings $|g_{ell 2}|$ can be much larger than $|g_{ell 1}|$, of the order $|g_{ell 2}| sim 10^{-4} - 10^{-2}$, leading to interesting low-energy phenomenology. We consider a specific realisation of this scenario within the Froggatt-Nielsen approach to fermion masses. In this model the Dirac CP violation phase $delta$ is predicted to have approximately one of the values $delta simeq pi/4,, 3pi/4$, or $5pi/4,, 7pi/4$, or to lie in a narrow interval around one of these values. The low-energy phenomenology of the considered low-scale seesaw scenario of neutrino mass generation is also briefly discussed.
We explore the generation of the baryon asymmetry in an extension of the Standard Model where the lepton number is promoted to a $U(1)_ell$ gauge symmetry with an associated $Z^prime$ gauge boson. This is based on a novel electroweak baryogenesis mechanism first proposed by us in Ref. cite{Carena:2018cjh}. Extra fermionic degrees of freedom - including a fermionic dark matter $chi$ - are introduced in the dark sector for anomaly cancellation. Lepton number is spontaneously broken at high scale and the effective theory, containing the Standard Model, the $Z^prime$, the fermionic dark matter, and an additional complex scalar field $S$, violates CP in the dark sector. The complex scalar field couples to the Higgs portal and is essential in enabling a strong first order phase transition. Dark CP violation is diffused in front of the bubble walls and creates a chiral asymmetry for $chi$, which in turn creates a chemical potential for the Standard Model leptons. Weak sphalerons are then in charge of transforming the net lepton charge asymmetry into net baryon number. We explore the model phenomenology related to the leptophilic $Z^prime$, the dark matter candidate, the Higgs boson and the additional scalar, as well as implications for electric dipole moments. We also discuss the case when baryon number $U(1)_B$ is promoted to a gauge symmetry, and discuss electroweak baryogenesis and its corresponding phenomenology.
We study lepton number violation in Little Higgs model and find that the choice of putting triplet Higgs vev equal to zero so as not to have any tree level neutrino Majorana mass is not natural in the sense that such a term is generated at the one loop level. We investigate the contribution of exotic lepton number violating terms on neutrinoless double beta decay, K meson decay and on trilepton production in $ u$-N scattering.
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