Bounds on charged lepton mixing with exotic charged leptons

We examine the effects of mixing induced light heavy charged lepton neutral currents on the partial wave amplitude for the process l⁺l⁻ →ZZ (withl = e,μ or τ). By imposing the constraints that the amplitude should not exceed the perturbative unitarity limit at high energy (√s = Λ), we obtain bounds on light heavy charged lepton mixing parameter sin²(2θ _L ^a ) where θ _L ^a is the mixing angle of the ordinary charged lepton with its exotic partner. For Λ = 1 TeV, no bound is obtained on sin² (2θ _L ^a ) form _E < 0.69 TeV. However, sin² (2θ _L ^a ) ≤ 1.52×10⁻⁵ form _E = 5 TeV, sin² (2θ _L ^a ) ≤ 2.41 ×10⁻⁷ form _E = 10 TeV. Similarity for Λ = ∞ no bound is obtained on sin² (2θ _L ^a ) for m_E < 1.97 TeV and sin² (2θ _L ^a ) ≤ 0.15 form _E = 5 TeV and sin² (2θ _L ^a ) ≤ 3.88×10^-2 form _E = 10 TeV.     

Standard leptons and the next lepton in the byuon theory

A physical picture of the origin of lepton mass is presented on the basis of the byuon theory (BT). The BT is the theory of the life of special unobservable discrete objects: byuons, from which the world surrounding us are formed (physical space; fundamental constants h, e, c; the mass of elementary particles; the four fundamental interactions; the predicted new force in nature, and so on). A key difference between this theory and current models of the classical and quantum field theories is that the potentials of physical fields acquire precise fixable, measurable values. The definition of byuon contains a new fundamental vector constant: the cosmological vector potential (A_g ≈ 1.95 × 10¹¹ G cm). The BT contains only three constants: \(\tilde x_0 \) ≈ 2.78 × 10^–33 cm, τ₀ ≈ 0.927 × 10^–43 s, and a modulus of A_g that allows us to obtain the masses of all known leptons and predict the mass of the next lepton (80.4 GeV).     

Finite modular groups and lepton mixing

We study lepton mixing patterns which are derived from finite modular groups ΓN, requiring subgroups Gν and Ge to be preserved in the neutrino and charged lepton sectors, respectively. We show that only six groups ΓN with N=3, 4, 5, 7, 8, 16 are relevant. A comprehensive analysis is presented for Ge arbitrary and Gν=Z₂×Z₂, as demanded if neutrinos are Majorana particles. We discuss interesting patterns arising from both groups Ge and Gν being arbitrary. Several of the most promising patterns are specific deviations from tri-bimaximal mixing, all predicting θ₁₃ non-zero as favoured by the latest experimental data. We also comment on prospects to extend this idea to the quark sector.     

Baryogenesis and lepton number violation

The cosmological baryon asymmetry can be explained by the nonperturbative electroweak reprocessing of a lepton asymmetry generated in the out-of-equilibrium decay of heavy right-handed Majorana neutrinos. We analyze this mechanism in detail in the framework of a SO(10)-subgroup. We take three right-handed neutrinos into account and discuss physical neutrino mass matrices.     

Sterile neutrinos in lepton number and lepton flavor violating decays

We study the contribution of massive dominantly sterile neutrinos, N, to the lepton number and lepton flavor violating semileptonic decays of τ and B, D, K-mesons. We focus on special domains of sterile neutrino masses mN where it is close to its mass-shell. This leads to an enormous resonant enhancement of the decay rates of these processes. This allows us to derive stringent limits on the sterile neutrino mass mN and its mixing UαN with active flavors. We apply a joint analysis of the existing experimental bounds on the decay rates of the studied processes. In contrast to other approaches in the literature our limits are free from ad hoc assumptions on the relative size of the sterile neutrino mixing parameters. We analyze the impact of this sort of assumptions on the extraction of the limits on mN and UαN, and discuss the effect of finite detector size. Special attention was paid to the limits on meson decays with e^±e^± in final state, derived from non-observation of 0νββ-decay. We point out that observation of these decays may, in particular, shed light on the Majorana phases of the neutrino mixing matrix.     

Baryogenesis from mixing of lepton doublets

It is shown that the mixing of lepton doublets of the Standard Model can yield sizable contributions to the lepton asymmetry, that is generated through the decays of right-handed neutrinos at finite temperature in the early Universe. When calculating the flavour-mixing correlations, we account for the effects of Yukawa as well as of gauge interactions. We compare the freeze-out asymmetry from lepton-doublet mixing to the standard contributions from the mixing and direct decays of right-handed neutrinos. The asymmetry from lepton mixing is considerably large when the mass ratio between the right-handed neutrinos is of order of a few, while it becomes Maxwell-suppressed for larger hierarchies. For an intermediate range between the case of degenerate right-handed neutrinos (resonant leptogenesis) and the hierarchical case, lepton mixing can yield the main contribution to the lepton asymmetry.     

Parametrizing the lepton mixing matrix in terms of charged lepton corrections

We consider a parametrization of the lepton mixing matrix in which the deviations from maximal atmospheric mixing and vanishing reactor mixing are obtained in terms of small corrections from the charged lepton sector. Relatively large deviations for the reactor mixing angle from zero as indicated by T2K experiment can be obtained in this parametrization. We are able to further reduce the number of complex phases, thus, simplifying the analysis. In addition, we have obtained the sides of unitarity triangles and the vacuum oscillation probabilities in this parametrization. The Jarlskog rephasing invariant measure of CP violation at the leading order has a single phase difference which can be identified as Dirac-type CP violating phase in this parametrization.     

Unitarity boomerangs of quark and lepton mixing matrices

The most popular way to present mixing matrices of quarks (CKM) and leptons (PMNS) is the parametrization with three mixing angles and one CP-violating phase. There are two major options in this kind of parametrizations, one is the original Kobayashi–Maskawa (KM) matrix, and the other is the Chau–Keung (CK) matrix. In a new proposal by Frampton and He, a unitarity boomerang is introduced to combine two unitarity triangles, and this new presentation displays all four independent parameters of the KM parametrization in the quark sector simultaneously. In this Letter, we study the relations between KM and CK parametrizations, and also consider the quark–lepton complementarity (QLC) in the KM parametrization. The unitarity boomerang is discussed in the situation of the CK parametrization for comparison with that in the KM parametrization in the quark sector. Then we extend the idea of unitarity boomerang to the lepton sector, and check the corresponding unitarity boomerangs in the two cases of parametrizations.     

On non-unitary lepton mixing and neutrino mass observables

There are three observables related to neutrino mass, namely the kinematic mass in direct searches, the effective mass in neutrino-less double beta decay, and the sum of neutrino masses in cosmology. In the limit of exactly degenerate neutrinos there are very simple relations between those observables, and we calculate corrections due to non-zero mass splitting. We discuss how the possible non-unitarity of the lepton mixing matrix may modify these relations and find in particular that corrections due to non-unitarity can exceed the corrections due to mass splitting. We furthermore investigate constraints from neutrino-less double beta decay on mass and mixing parameters of heavy neutrinos in the type I see-saw mechanism. There are constraints from assuming that heavy neutrinos are exchanged, and constraints from assuming light neutrino exchange, which arise from an exact see-saw relation. The latter has its origin in the unitarity violation arising in see-saw scenarios. We illustrate that the limits from the latter approach are much stronger. The drastic impact of the new limit on inverse neutrino-less double beta decay (e⁻e⁻→W⁻W⁻)$(e^{-}{e}^{-}\to W^{-}{W}^{-})$ is studied. We furthermore discuss neutrino mixing in case there is one or more light sterile neutrinos. Neutrino oscillation probabilities for long baseline neutrino oscillation experiments are considered, and the analogy to general non-unitarity phenomenology, such as zero-distance effects, is pointed out.     

Charged lepton contributions to bipair neutrino mixing

The bipair neutrino mixing describes the observed solar and atmospheric mixings; however, it predicts vanishing reactor mixing angle, which is inconsistent with the observed data. We explore the ways of minimally modifying the bipair neutrino mixing by including charged lepton contributions. There are two categories of the bipair neutrino mixing which are referred to as case 1 and case 2. It turns out that, without arbitrary phases, a minimal modification is realized by just considering one e–τ$e\text{–}\tau$ contribution from the charged lepton sector in the case 1. On the other hand, not only e–τ$e\text{–}\tau$ contribution but also μ–τ$\mu \text{–}\tau$ contribution is required to realize a minimal modification in the case 2.     

Relations between quark and lepton mixing angles and matrices

We discuss the relations between the mixing angles and the mixing matrices of quarks and leptons. With Raidal’s numerical relations, we parameterize the lepton mixing (PMNS) matrix with the parameters of the quark mixing (CKM) matrix, and calculate the products of V_CKMU_PMNS and U_PMNSV_CKM. Also, under the conjectures V_CKMU_PMNS = U_bimax or U_PMNSV_CKM = U_bimax, we get the PMNS matrix naturally, and test Raidal’s relations in these two different versions. The similarities and the differences between the different versions are discussed in detail.Received: 6 April 2005, Published online: 18 May 2005PACS: 14.60.Pq, 12.15.Ff     

Mixing of neutral atoms and lepton number oscillations

We discuss oscillations of two neutral atoms which proceed with the violation of lepton number. One of the neutral atoms is stable, the other one represents a quasistationary state subjected to electromagnetic deexcitation. The system of neutral atoms exhibits oscillations similar to those of the system of neutral kaons and neutron-antineutron oscillations in the nuclear medium. The underlying mechanism is a transition of two protons and two bound electrons to two neutrons p + p + e _b ⁻ + e _b ⁻ ↔ n + n. A signature of the oscillations might be an electromagnetic deexcitation of the involved unstable nucleus and atomic shell with the electron holes. A resonant enhancement of the neutrinoless double electron capture takes place when the atomic masses tend to be degenerate. Qualitative estimates show that in searches for lepton number violation oscillations of atoms might be a possible alternative to the conventional mechanism of the neutrinoless double β decay process with emission of two electrons. The article is published in the original.     

Neutrino-pair bremsstrahlung and the number of lepton generations

Neutrino pair creation in bremsstrahlung processes of the type \(l \to l{\text{ }}v{\text{ }}\bar v\) contains vital information on the number of lepton generations, and is catalyzed by the coherent nuclear Coulomb effect or other forms of intense fields. Of particular interest is the ratio \(R_{v\bar v} = \sigma [1\mathop \to \limits_A l(v\bar v)]/\sigma [1\mathop \to \limits_A l'(v\bar v)]\) (wherel, l′ are distinct charged leptons). It is sensitive to the number of neurino types and their couplings in the same way that the ratio \(R_{q\bar q} = \sigma [e^ + e^ - \to {\text{hadrons}}]/\sigma [e^ + e^ - \to \mu ^ + \mu ^ - ]\) is to those of quarks. In the Weinberg-Salam model withN lepton generations, the ratio \(R_{v\bar v}\) is approximately given by \([(N + 4) + 4(1 - 4\sin ^2 \theta _W )]/8\) .