Heavy Neutrino Mass Research Articles

Leptogenesis via Collisions: Leaking Lepton Number to the Hidden Sector

We propose a leptobaryogenesis mechanism in which the nonzero B-L of the Universe is produced in out-of-equilibrium, lepton number and CP-violating scattering processes that convert ordinary particles into particles of some hidden sector. In particular, we consider the processes l phi-->l(')phi('),l macro(')phi(') mediated by the heavy Majorana neutrinos N of the seesaw mechanism, where l and phi are ordinary lepton and Higgs doublets and l('), phi(') are their hidden counterparts. Such a leptogenesis mechanism is effective even if the reheat temperature is much smaller than the heavy neutrino masses. In particular, it can be as low as 10(9) GeV.

Dirac and Majorana heavy neutrinos at LEP II

The possibility of detecting single heavy Dirac and/or Majorana neutrinos at LEP II is investigated for heavy neutrino masses in the range \(M_N=(s^{1/2}/2,s^{1/2})\). We study the process \(e^+e^- \longrightarrow \nu e^\pm q_i \bar q_j\) as a clear signature for heavy neutrinos. Numerical estimates for cross sections and distributions for the signal and the background are calculated and a Monte Carlo reconstruction of final state particles after hadronization is presented.

CP violation, neutrino mixing and the baryon asymmetry

We study neutrino masses and mixings based on the simplest SO(10) mass relations and the seesaw mechanism. We find that the requirement of large neutrino mixings determines the relative magnitude of the heavy Majorana neutrino masses in terms of the known quark mass hierarchy. This leads to specific predictions for the structure of the neutrino mixing matrix, the light neutrino masses, CP violation in neutrino oscillations, neutrinoless double b-decay and the baryon asymmetry.

Erratum: “Neutrinoless double beta decay” at a neutrino factory [Phys. Rev. D63, 054031 (2001)

We examine the prospects of detecting an analogous process of neutrinoless double beta decay at a neutrino factory from a high energy muon storage ring. Limits from LEP experiments, neutrinoless double beta decay as well as from global fits have to be incorporated and severely restrict the results. We investigate what limits on light and heavy effective Majorana neutrino masses can be obtained and compare them with existing ones. Discussed are also contributions from right-handed neutrinos and purely right-handed interactions. Other ``new physics'' contributions to the same final state might produce large event numbers.

“Neutrinoless double beta decay” at a neutrino factory

We examine the prospects of detecting an analogous process of neutrinoless double beta decay at a neutrino factory from a high energy muon storage ring. Limits from LEP experiments, neutrinoless double beta decay as well as from global fits have to be incorporated and severely restrict the results. We investigate what limits on light and heavy effective Majorana neutrino masses can be obtained and compare them with existing ones. Discussed are also contributions from right-handed neutrinos and purely right-handed interactions. Other ``new physics'' contributions to the same final state might produce large event numbers.

Predictive minimal model for neutrino masses and mixings

A model is considered in which the scale of the heavy singlet neutrinos is a few orders of magnitude below the grand unification scale and where right-handed vector bosons still play a negligible role. In a basis with diagonal up-quark and Dirac-neutrino mass matrices it is assumed that the heavy neutrino mass matrix has only zero elements in its diagonal, in analogy with the light neutrino mass matrix in the Zee model. Connecting then the remaining matrix elements with the small parameter describing the hierarchy of quark masses and mixings and by assuming commutativity of the charged lepton with the down-quark mass matrix, the calculation of all neutrino properties can be performed in terms of the two mass differences relevant for atmospheric and solar neutrino oscillations. $\mathrm{CP}$ violation is directly related to $\mathrm{CP}$ violation in the quark sector.

The very low energy solar flux of electron and heavy-flavor neutrinos and antineutrinos

We calculate the thermal flux of low-energy solar neutrinos and antineutrinos of all flavors arising from a variety of neutrino pair processes: Compton production (including plasmon-pole diagrams), neutral current decay of thermally populated nuclear states, plasmon decay, and electron transitions from free to atomic bound states. The resulting flux density per flavor is significant (10 8–10 9/cm 2/sec/MeV) below ∼ 5 keV, and the distributions fill much of the valley between the high-energy edge of the cosmic background neutrino spectrum and the low energy tails of the pp-chain electron neutrino and terrestrial electron antineutrino spectra. Thermal neutrinos carry information on the solar core temperature distribution and on heavy flavor neutrino masses for m ν μ or m ν τ ≳ 1 keV. The detection of these neutrinos is a daunting but interesting challenge.

The effect of weak magnetism and induced pseudoscalar coupling in neutrinoless double-beta decay

In calculating the amplitude of the majorana neutrino-mass mechanism of neutrinoless double-beta decay (0νββ-decay), several approximations of the nucleon current have been done. for example, effects from induced current such as weak magnetism and pseudoscalar coupling have been neglected. we shall show in this work that, although such terms do not contribute significantly to the 2νββ-decay amplitude, they are important in the case of 0νββ decay. performing calculations within the renormalized quasiparticle random phase approximation (pn-rqrpa) for all nuclei undergoing double-beta decay in the region a=76 to a=150, we have found that these additional contributions of the nucleon current reduce considerably the matrix elements in all cases for the light neutrino as well as for the heavy neutrino mass. in the light-neutrino mass, we find reductions up to thirty percent, while in the heavy-neutrino mass, up to almost a factor of five. these reductions make the limits on the lepton-number-violating parameters 〈mν〉 and ηN less stringent.

HEAVY MAJORANA NEUTRINOS IN HIGH-ENERGY e-e- COLLISIONS COMPARED WITH THEIR EFFECTS INDUCING NEUTRINOLESS DOUBLE BETA DECAY

We discuss the potential of e-e- collisions at energies comparable to the masses of heavy Majorana neutrinos to reveal their mass and mixing parameters. This potential is compared with the low-energy environment in neutrinoless double beta decay of complex nuclei. We give estimates for the short-range repulsion between any pair of valence quarks bound inside two different nucleons giving rise to suppression factors in the range of 50–100 in amplitude for decays mediated by heavy flavors.

Neutrino masses and mixings in a seesaw framework

Assuming the seesaw mechanism for hierarchical neutrino masses, we calculate the heavy neutrino masses under the hypotheses that the mixing in the Dirac leptonic sector is similar to the quark mixing ($V_D \sim V_{CKM}$) and that $M_{\nu} \sim M_u$ or $M_e$, where $M_{\nu}$ is the Dirac mass matrix of neutrinos. As a result we find that for $M_{\nu} \sim M_u$ the vacuum oscillation solution of the solar neutrino problem leads to a scale for the heavy neutrino mass well above the unification scale, while for the MSW solutions there is agreement with this scale. For $M_{\nu} \sim M_e$ the vacuum solution is consistent with the unification scale, and the MSW solutions with an intermediate scale. The mass of the lightest heavy neutrino can be as small as $10^5$ GeV.

Seesaw mechanism and quark-lepton symmetry

A previous short analysis of the seesaw mechanism, based on quark-lepton symmetry, experimental data and hierarchical neutrino spectrum, is enlarged to include small but not zero U e3 and inverted mass hierarchy. The structure of the heavy neutrino mass matrix obtained in several cases is discussed. We find two leading forms for this matrix. One is diagonal and stands at the unification scale or above. The other is off-diagonal and stands at the intermediate scale.

Leptogenesis and the small-angle MSW solution

The lepton asymmetry created in the out-of-equilibrium decay of a heavy Majorana neutrino can generate the cosmological baryon asymmetry Y B when processed through fast anomalous electroweak reactions. In this work I examine this process under the following assumptions: (1) maximal ν μ −− ν τ mixing (2) hierarchical mass spectrum m 3≃5×10 −2 eV≫m 2 (3) small-angle MSW solution to the solar neutrino deficit. Working in a basis where the charged lepton and heavy neutrino mass matrices are diagonal, I find the following bounds on the heavy Majorana masses M i : ( a) for a symmetric Dirac neutrino mass matrix (no other constraints), a Y B compatible with BBN constraints can be obtained for min(M 2,M 3)>10 11 GeV ( b) if any of the Dirac matrix elements vanishes, successful baryogenesis can be effected for a choice of min( M 2, M 3) as low as a few ×10 9 GeV . The latter is compatible with reheat requirements for supersymmetric cosmologies with sub-TeV gravitino masses.

Heavy neutrino mass scale and quark–lepton symmetry

Assuming hierarchical neutrino masses we calculate the heavy neutrino mass scale in the seesaw mechanism from experimental data on oscillations of solar and atmospheric neutrinos and quark–lepton symmetry. The resulting scale is around or above the unification scale, unless the two lightest neutrinos have masses of opposite sign, in which case the resulting scale can be intermediate.

Leptogenesis and Yukawa textures

We study a set of textures giving rise to the correct masses and mixings of the charged fermions in the context of leptogenesis. The Dirac neutrino texture pattern is assumed to be identical with the up quark texture. The heavy Majorana neutrino mass matrix is obtained by inverting the type-I seesaw formula and using the neutrino masses and mixings required by the solar and atmospheric neutrino oscillation experiments as input. After making a feasibility study of the generated lepton asymmetry via the decay of the heavy right handed neutrino, we compute the generated baryon asymmetry by numerically solving the supersymmetric Boltzmann equations. We find for these models that both the hierarchy of the texture as well as the placement of the texture zeros are important to the viability of leptogenesis as the source of the observed baryon asymmetry of the universe.

Search for heavy isosinglet neutrinos in e +e − annihilation at [formula omitted] GeV

A search for heavy neutrinos that are isosinglets under the standard SU(2) L gauge group is made at center-of-mass energies 130< s <189 GeV with the L3 detector at LEP. Such heavy neutrinos are expected in many extensions of the Standard Model. The search is performed for the first generation heavy singlet neutrino, N e, through the decay mode N e→e+W. We set upper limits on the mixing parameter between the heavy and light neutrino for the heavy neutrino mass range from 80 GeV to 185 GeV.

Lepton flavor violation in the standard model extended by heavy singlet neutrinos

Some properties of extensions of standard model (SM) by heavy neutrinos with large lepton flavor violation (LFV) effects are given. The possible low energy processes and methods, which could show deviation from the SM in the neutrino sector, are mentioned. The low energy neutrinos LFV decays are listed. The amplitudes contributing to LFV decays and behaviour of total amplitudes in the large mass limit are given. Some interesting ratios of branching ratios are mentioned. The perturbative unitarity limit for heavy neutrino masses is discussed. The theoretical upper limits of branching ratios (BRs) are compared with the current experimental upper bounds and several choices of model parameters to fit the experimental results are evaluated. Astrophysical constraints on mixing angles are discussed.

Textures for neutrino mass matrices in gauge theories

We review previous work on neutrino mass textures in gauge theories. Such textures may arise as a result of flavour symmetries. In a given theory, it is possible to give a classification of heavy Majorana neutrino mass matrices, assuming the Dirac masses of the neutrinos to be of the same form as the ones of the upquarks. Heavy Majorana neutrino mass matrices leading to three degenerate neutrinos, are tabulated as an example.

Neutrinoless double β decay with composite neutrinos

We study in detail the contribution of heavy composite Majorana neutrinos to neutrinoless double beta decay (0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}). Our analysis confirms the result of a previous estimate by two of the authors. Excited neutrinos couple to the electroweak gauge bosons through a magnetic-type effective Lagrangian. The relevant nuclear matrix element is related to matrix elements available in the literature and current bounds on the half-life of 0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta} are converted into bounds on the compositeness scale and/or the heavy neutrino mass. Our bounds are of the same order of magnitude as those available from accelerator experiments.

Heavy neutrino mixing and single production at linear collider

We study the single production of heavy neutrinos via the processes $e^-e^+ \to \nu N$ and $e^-\gamma \to W^- N$ at future linear colliders. As a base of our considerations we take a wide class of models, both with vanishing and non-vanishing left-handed Majorana neutrino mass matrix $m_L$. We perform a model independent analyses of the existing experimental data and find connections between the characteristic of heavy neutrinos (masses, mixings, CP eigenvalues) and the $m_L$ parameters. We show that with the present experimental constraints heavy neutrino masses almost up to the collision energy can be tested in the future experiments.

Non-decoupling and lepton number violation in left-right models

We argue that large non-decoupling effects of heavy neutrinos can appear naturally in manifestly left-right symmetric models due to the minimization conditions of the scalar potential and the structure of vev's imposed by phenomenology. We derive constraints on off-diagonal light-heavy and heavy-heavy neutrino mixings from the searches for lepton violating decays μ → eγ, μ → ee − e + and μ − e conversion in nuclei. The most stringent limits come from the latter process because its amplitude shows a quadratic non-decoupling dependence on the heavy neutrino mass. Due to the suppression of right-handed currents by the large W R mass the present experiment are not sensitive to the intergenerational mixings between heavy neutrinos if M W R > 200 TeV.