Non-decoupling of heavy neutrinos and lepton flavour violation

We consider a class of models predicting new heavy neutral fermionic states, whose mixing with the light neutrinos can be naturally significant and produce observable effects below the threshold for their production. We update the indirect limits on the flavour non-diagonal mixing parameters that can be derived from unitarity, and show that significant rates are in general expected for one-loop-induced rare processes due to the exchange of virtual heavy neutrinos, involving the violation of the muon and electron lepton numbers. In particular, the amplitudes for μ-e conversion in nuclei and for μ → ee⁺e⁻ show a non-decoupling quadratic dependence on the heavy neutrino mass M, while μ → eγ is almost independent of the heavy scale above the electroweak scale. These three processes are then used to set stringent constraints on the flavour-violating mixing angles. In all the cases considered, we point out explicitly that the non-decoupling behaviour is strictly related to the spontaneous breaking of the SU(2) symmetry.     

Lepton flavour violation in the constrained MSSM with constrained sequential dominance

We consider charged lepton flavour violation (LFV) in the constrained minimal supersymmetric Standard Model, extended to include the see-saw mechanism with constrained sequential dominance (CSD), where CSD provides a natural see-saw explanation of tri-bimaximal neutrino mixing. When charged lepton corrections to tri-bimaximal neutrino mixing are included, we discover characteristic correlations among the LFV branching ratios, depending on the mass ordering of the right-handed neutrinos, with a pronounced dependence on the leptonic mixing angle θ₁₃${\theta }_{13}$ (and in some cases also on the Dirac CP phase δ).     

Theoretical aspects of neutrino mass and lepton flavour violation

We consider lepton flavour violation (LFV) in the charged lepton sector both from the bottom-up effective Lagrangian approach and from the top-down approach via various case studies that have been analysed. The implications for LFV studies at the LHC is briefly discussed. Finally the nature of LFV in the neutrino sector is considered, paying particular regard to the implications of the recent measurements of ?? ₁₃.     

Probing the violation of equivalence principle at a muon storage ring via neutrino oscillation

We examine the possible tests of violation of the gravitational equivalence principle (VEP) at a muon storage ring via neutrino oscillation experiments. If the gravitational interactions of the neutrinos are not diagonal in the flavour basis and the gravitational interaction eigenstates have different couplings to the gravitational field, this leads to the neutrino oscillation. If one starts with μ⁺ beam then appearance of τ^±, e⁺ and μ⁻ in the final state are the signals for neutrino oscillation. We have estimated the number of μ⁻ events in this scenario in ν_μ–N deep inelastic scattering. Final state lepton energy distribution can be used to distinguish the VEP scenario from the others. A large area of VEP parameter space can be explored at a future muon storage ring facility with moderate beam energy.     

Search for lepton flavour violation in ep collisions at HERA

A search for the lepton flavour violating processes ep→μX and ep→τX is performed with the H1 experiment at HERA. Final states with a muon or tau and a hadronic jet are searched for in a data sample corresponding to an integrated luminosity of 66.5 pb^-1 for e⁺p collisions and 13.7 pb^-1 for e^-p collisions at a centre-of-mass energy of 319 GeV. No evidence for lepton flavour violation is found. Limits are derived on the mass and the couplings of leptoquarks inducing lepton flavour violation in an extension of the Buchmüller–Rückl–Wyler effective model. Leptoquarks produced in ep collisions with a coupling strength of λ=0.3 and decaying with the same coupling strength to a muon–quark pair or a tau–quark pair are excluded at 95% confidence level up to masses of 459 GeV and 379 GeV, respectively.     

Charged-lepton flavour physics

This write-up on a talk at the 2011 Lepton?CPhoton symposium in Mumbai, India, summarizes recent results in the charged-lepton flavour sector. Searches for charged-lepton flavour violation, lepton electric dipole moments and flavour-conserving CP violation are reviewed here. Recent progress in ??-lepton physics and in the Standard Model prediction of the muon anomalous magnetic moment is also discussed.     

Oscillation properties of active and sterile neutrinos and neutrino anomalies at short distances

A generalized phenomenological (3 + 2 + 1) model featuring three active and three sterile neutrinos that is intended for calculating oscillation properties of neutrinos for the case of a normal activeneutrino mass hierarchy and a large splitting between the mass of one sterile neutrino and the masses of the other two sterile neutrinos is considered. A new parametrization and a specific form of the general mixing matrix are proposed for active and sterile neutrinos with allowance for possible CP violation in the lepton sector, and test values are chosen for the neutrino masses and mixing parameters. The probabilities for the transitions between different neutrino flavors are calculated, and graphs representing the probabilities for the disappearance of muon neutrinos/antineutrinos and the appearance of electron neutrinos/antineutrinos in a beam of muon neutrinos/antineutrinos versus the distance from the neutrino source for various values of admissible model parameters at neutrino energies not higher than 50 MeV, as well as versus the ratio of this distance to the neutrino energy, are plotted. It is shown that the short-distance accelerator anomaly in neutrino data (LNSD anomaly) can be explained in the case of a specific mixing matrix for active and sterile neutrinos (which belongs to the a₂ type) at the chosen parameter values. The same applies to the short-distance reactor and gallium anomalies. The theoretical results obtained in the present study can be used to interpret and predict the results of ground-based neutrino experiments aimed at searches for sterile neutrinos, as well as to analyze some astrophysical observational data.     

Lepton mixing and neutrino oscillations

The present article is a review of phenomena connected with neutrino oscillations. Mixing of two neutrinos (Majorana as well as Dirac) with masses m₁ and m₂ is considered in detail. It is shown that the hypothesis of lepton mixing is not in contradiction with the existing data if |m₁²?m₂²| ? 1 (eV)². Possible experiments designed to reveal neutrino oscillations at reactor, meson factory and high energy accelerator facilities are considered. In such experiments oscillation might be found if $\text{|m}{}_1{}^2\text{?m}{}_2{}^2\text{| ? 0.01}\text{(eV)}{}^2$. The possibilities of searching for oscillations by experiments on cosmic ray neutrinos and especially on solar neutrinos are discussed in detail. The last experiments have an incredible high sensitivity from the point of view of testing the lepton mixing hypothesis (oscillation effects might be observable if $\text{|m}{}_1{}^2\text{?m}{}_2{}^2\text{| ? 10}{}^{\mathrm{?12}}\text{(eV)}{}^2$). The “solar neutrino puzzle” is also discussed from the point of view of lepton mixing. Neutrino oscillations are considered then in the case where in nature there exist N ? 2 neutrino types.In conclusion the case of heavy lepton mixing is considered. It is shown that in a concrete scheme with right-handed currents, the probabilities of such processes as μ → eγ, μ → 3e etc. can be close to existing experimental upper limits, provided the heavy lepton masses are of an order of a few GeV, whereas the probabilities of the above processes are entirely negligible if only neutrinos are mixed.     

Realistic neutrinogenesis with radiative vertex correction

We propose a new model for naturally realizing light Dirac neutrinos and explaining the baryon asymmetry of the universe through neutrinogenesis. To achieve these, we present a minimal construction which extends the Standard Model with a real singlet scalar, a heavy singlet Dirac fermion and a heavy doublet scalar besides three right-handed neutrinos, respecting lepton number conservation and a Z₂$Z_2$ symmetry. The neutrinos acquire small Dirac masses due to the suppression of weak scale over a heavy mass scale. As a key feature of our construction, once the heavy Dirac fermion and doublet scalar go out of equilibrium, their decays induce the CP asymmetry from the interference of tree-level processes with the radiative vertex corrections (rather than the self-energy corrections). Although there is no lepton number violation, an equal and opposite amount of CP asymmetry is generated in the left-handed and the right-handed neutrinos. The left-handed lepton asymmetry would then be converted to the baryon asymmetry in the presence of the sphalerons, while the right-handed lepton asymmetry remains unaffected.     

Phenomenology of the standard model under conditions of spontaneously broken mirror symmetry

Spontaneously broken mirror symmetry is able to reproduce observed qualitative properties of weak mixing for quark and leptons. Under conditions of broken mirror symmetry, the phenomenology of leptons—that is, small neutrino masses and a mixing character other than that in the case of quarks—requires the Dirac character of the neutrinos and the existence of processes violating the total lepton number. Such processes involve heavy mirror neutrinos; that is, they proceed at very high energies. Here, CP violation implies that a P-even mirror-symmetric Lagrangian must simultaneously be T-odd and, according to the CPT theorem, C-odd. All these properties create preconditions for the occurrence of leptogenesis, which is a mechanism of the emergence of the baryon–lepton asymmetry of the universe in models featuring broken mirror symmetry.     

Neutrino masses as a second-order effect in Higgs coupling

It is suggested that in the usual type of gauge theory all fermions, including neutrinos, have right-handed components. The smallness or vanishing of the observed neutrino masses is explained by the fact that the appropriate neutral Higgs boson does not develop a non-zero vacuum expectation value. In the case when the neutrino masses do not vanish they are finite, of order G_Fm³, where m is the mass of the charged lepton. Non-conservation of lepton flavor gives rise to an instability of all neutrinos except v_e and to μ→e+γ decay, but at a very low level.     

Lepton flavour symmetry and the neutrino magnetic moment

With the standard model gauge group and the three standard left-handed Weyl neutrinos, two minimal scenarios are investigated where an arbitrary non-Abelian lepton flavour symmetry groupG _H is responsible for a light neutrino with a large magnetic moment. In the first case, with scalar fields carrying lepton flavour, some finetuning is necessary to get a small enough neutrino mass for _v =O(10)^–11 _B. In the second scenario, the introduction of heavy charged gauge singlet fermions with lepton flavour allows for a strictly massless neutrino to one-loop order. In both cases, the interference mechanism for smallm and large _v is unique, independently ofG _H . In explicit realizations of the two scenarios, the horizontal groups are found to be non-Abelian extensions of a Zeldovich-Konopinski-Mahmoud lepton number symmetry. Only a discrete part ofG _H is spontaneously broken leading to a light Dirac neutrino with a large magnetic moment.     

Theory of neutrinoless double beta decay—A brief review

Neutrinoless double decay (0νββ-decay) is a unique probe for lepton number conservation and neutrino properties. This is a process with long and interesting history with important implications for particle physics and cosmology, but its observation is still elusive. The search for the 0νββ-decay represents the new frontiers of neutrino physics, allowing to determine the Majorana nature of neutrinos and to fix the neutrino mass scale and possible CP violation effects, which could explain the matter-antimatter asymmetry in the Universe. At present a complete theory is missing and, thus, to motivate and guide the experiments the mechanism mediated by light neutrinos is mostly considered. The subject of interest is an effective mass of Majorana neutrinos, which can be deduced from the measured half-life, once this process is definitely observed. The accuracy of the determination of this quantity is mainly determined by our knowledge of the nuclear matrix elements. There is a request to evaluate them with high precision, accuracy and reliability. Recently, there is an increased interest to the resonant neutrinoless double electron capture, which may also establish the Majorana nature of neutrinos. This possibility is considered as alternative and complementary to searches for the 0νββ-decay.     

Photon-neutrino interactions

We discuss the interaction of photons with neutrinos including two lepton loops. The parity violation in the γ ν → γν channel due to two lepton loops is substantially enhanced relative to the one lepton loop contribution. However, there is no corresponding enhancement in the parity conserving amplitude in either the direct or the cross channel \(\gamma \gamma \to v\bar v\).     

Experimental test of the Pauli Exclusion Principle

A short review is given of three experimental works on tests of the Pauli Exclusion Principle (PEP) in which the author has been involved during the last 10 years. In the first work a search for anomalous carbon atoms was done and a limit on the existence of such atoms was determined, \(^{12}\tilde{\mathrm{C}}\)/¹²C <2.5×10^?12. In the second work PEP was tested with the NEMO-2 detector and the limits on the violation of PEP for p-shell nucleons in ¹²C were obtained. Specifically, transitions to the fully occupied 1s _1/2-shell yielded a limit of 4.2×10²⁴ y for the process with the emission of a γ-quantum. Similarly limits of 3.1×10²⁴ y for β ^? and 2.6×10²⁴ y for β ⁺ Pauli-forbidded transition of ¹²C → ¹²Ñ(\(^{12}\tilde{\mathrm{B}}\)) are reported. In the third work it was assumed that PEP is violated for neutrinos, and thus, neutrinos obey at least partly the Bose-Einstein statistics. Consequences of the violation of the exclusion principle for double beta decays were considered. This violation strongly changes the rates of the decays and modifies the energy and angular distributions of the emitted electrons. It was shown that pure bosonic neutrinos are excluded by the present experimental data. In the case of partly bosonic neutrinos the analysis of the existing data allows one to put an upper bound for sin?² χ<0.6. The sensitivity of future measurements is also evaluated.     

Anomalous interactions at theZ 0-pole

We study, with an effective lagrangian technique, the possible deviations from the standard model ine ⁺ e ^? collisions at theZ ⁰-pole. We consider dimension 6 anomalous interactions satisfying the fullSU(2)×U(1) gauge invariance, baryon and lepton number conservation, as well as full chiral and flavour symmetry. In this scheme, for reactions not involving the Higgs, the effects are all parametrized by modifications ofZ ⁰ partial widths into fermions. Significant anomalies, including CP violation, could only come out inZ ⁰→H ⁰γ and \(Z^0 \to H^0 f\bar f\) .     

Sources of tetramuon production by neutrinos

We evaluate four sources of tetramuon production by neutrinos: (a) heavy quark cascade, (b) heavy lepton cascade, (c) charm production with radiative pair, (d) charm production plus an associated charm pair. Rates relative to ν→μ^? of order (a) 10^?6, (b) 10^?5, (c) 10^?7, (d) 10^?9, can be obtained. Invariant mass and azimuthal correlation distributions in 4μ events are presented. Relative multimuon rates differ strikingly in the models considered.