arXiv subjects

E. Kh. Akhmedov

Publications and source records attributed to E. Kh. Akhmedov.

At least 37 records · Page 2Linked to original sources

arXiv2000

Neutrino masses and mixing with seesaw mechanism and universal breaking of extended democracy

In the framework of a minimal extension of the SM, where the only additional fields are three right-handed neutrinos, we suggest that the charged lepton, the Dirac neutrino and the right-handed Majorana neutrino mass matrices are all, to leading approximation, proportional to the democratic matrix. With the further assumption that the breaking of this extended democracy is universal for all leptonic mass matrices, a large mixing in the 2-3 sector can be obtained and is linked to the seesaw mechanism, together with the existence of a strong hierarchy in the masses of right-handed neutrinos. The structure of the resulting effective mass matrix of light neutrinos is stable against the RGE evolution, and a good fit to all solar and atmospheric neutrino data is obtained.

hep-ph↗

arXiv2000

Floquet theory of neutrino oscillations in the earth

We review the Floquet theory of linear differential equations with periodic coefficients and discuss its applications to neutrino oscillations in matter of periodically varying density. In particular, we consider parametric resonance in neutrino oscillations which can occur in such media, and discuss implications for oscillations of neutrinos traversing the earth and passing through the earth's core.

hep-ph↗

arXiv2000

Reply to ``Is $U_{e3}$ really related to the solar neutrino solutions?''

In a recent paper [1] we showed that, assuming no fine tuning between certain elements of the neutrino mass matrix, one can link the element U_{e3} of the lepton mixing matrix to solar and atmospheric neutrino oscillation parameters. This result has been recently criticized by Haba and Suzuki [2]. In the present note we show that their criticism is not valid and just reflects their failure to understand the content of our paper.

hep-ph↗

arXiv2000

SNO and the neutrino magnetic moment solution of the solar neutrino problem

Assuming that the solar neutrino deficit observed in the Homestake, SAGE, Gallex, Kamiokande and Super-Kamiokande experiments is due to the interaction of the neutrino transition magnetic moment with the solar magnetic field, we calculate the expected values of a number of observables to be measured by SNO. For three model solar magnetic field profiles that produce the best fits of the previous data we calculate the charged current event rate, the (neutral current)/(charged current) event ratio and the charged current electron spectrum as well as its first and second moments. We study the dependence of the calculated observables on the choice of the magnetic field profile and on the value of the solar $hep$ neutrino flux. We also compare our results with those obtained assuming that the solar neutrino problem is due to neutrino oscillations in vacuum or in matter. We show that there is an overlap or partial overlap between our predictions and those found for each of the oscillation solutions (SMA, LMA, LOW and VO). Given the uncertainties in the calculations and the expected uncertainties in the experimental results, the unambiguous discrimination between the two types of solutions to the solar neutrino problem (neutrino oscillations and magnetic moments) on the basis of the average rates and electron spectrum distortions appears to be difficult. The possible time dependence of the charged current signal and spectrum distortion in the case of the magnetic moment solution therefore remains the best hope for such a discrimination. For a hybrid solution (neutrino magnetic moment plus flavour mixing) the smoking gun signature would be an observation of $\barν_e$'s from the sun.

hep-ph↗

arXiv2000

Neutrino physics

In the present lectures the following topics are considered: general properties of neutrinos, neutrino mass phenomenology (Dirac and Majorana masses), neutrino masses in the simplest extensions of the standard model (including the seesaw mechanism), neutrino oscillations in vacuum, neutrino oscillations in matter (the MSW effect) in 2- and 3-flavour schemes, implications of CP, T and CPT symmetries for neutrino oscillations, double beta decay, solar neutrino oscillations and the solar neutrino problem, and atmospheric neutrinos. We also give a short overview of the results of the accelerator and reactor neutrino experiments and of future projects. Finally, we discuss how the available experimental data on neutrino masses and lepton mixing can be summarized in the phenomenologically allowed forms of the neutrino mass matrix.

hep-ph↗

arXiv2000

Seesaw mechanism and the neutrino mass matrix

The seesaw mechanism of neutrino mass generation is analysed under the following assumptions: (1) minimal seesaw with no Higgs triplets, (2) hierarchical Dirac masses of neutrinos, (3) large lepton mixing primarily or solely due to the mixing in the right-handed neutrino sector, and (4) unrelated Dirac and Majorana sectors of neutrino masses. It is shown that large mixing governing the dominant channel of the atmospheric neutrino oscillations can be naturally obtained and that this constrained seesaw mechanism favours the normal mass hierarchy for the light neutrinos leading to a small $U_{e3}$ entry of the lepton mixing matrix and a mass scale of the lightest right handed neutrino $M\simeq 10^{10} - 10^{11}$ GeV. Any of the three main neutrino oscillation solutions to the solar neutrino problem can be accommodated. The inverted mass hierarchy and quasi-degeneracy of neutrinos are disfavoured in our scheme.

hep-ph↗

arXiv1999

Linking solar and long baseline terrestrial neutrino experiments

We show that in the framework of three light neutrino species with hierarchical masses and assuming no fine tuning between the entries of the neutrino mass matrix, one can use the solar neutrino data to obtain information on the element $U_{e3}$ of the lepton mixing matrix. Conversely, a measurement of $U_{e3}$ in atmospheric or long baseline accelerator or reactor neutrino experiments would help discriminate between possible oscillation solutions of the solar neutrino problem.

hep-ph↗

arXiv1999

Seesaw mechanism and structure of neutrino mass matrix

We consider the seesaw mechanism of neutrino mass generation in the light of our present knowledge of the neutrino masses and mixing. We analyse the seesaw mechanism constrained by the following assumptions: (1) minimal seesaw with no Higgs triplets, (2) hierarchical Dirac masses of neutrinos, (3) large lepton mixing primarily or solely due to the mixing in the right-handed neutrino sector, and (4) unrelated Dirac and Majorana sectors of neutrino masses. We show that large mixing governing the dominant channel of the atmospheric neutrino oscillations can be naturally obtained and point out that this constrained seesaw mechanism favours the normal mass hierarchy for the light neutrinos leading to a small $V_{e3}$ entry of the lepton mixing matrix and a mass scale of the lightest right handed neutrino $M\simeq 10^{10} - 10^{11}$ GeV. Any of the three main neutrino oscillation solutions to the solar neutrino problem can be accommodated. The inverted mass hierarchy and quasi-degeneracy of neutrinos are disfavoured in our scheme.

hep-ph↗

arXiv1999

Comment on "New conditions for a total neutrino conversion in a medium"

We show that the conditions for total neutrino conversion found in [1] are equivalent to the conditions of maximal depth (parametric resonance) and ($π/2 + πk$) - phase of parametric oscillations. Therefore the effects considered in [1] are a particular case of the parametric resonance in neutrino oscillations. The existence of strong enhancement peaks in transition probability P rather than the condition P=1 is of physical relevance. We comment on possible realizations and implications of the parametric enhancement of neutrino oscillations.

hep-ph↗

arXiv1999

Resonance spin flavour precession and solar neutrinos

We examine the prospects for the resonance spin flavour precession as a solution to the solar neutrino problem. We study seven different realistic solar magnetic field profiles and, by numerically integrating the evolution equations, perform a fit of the event rates for the three types of solar neutrino experiments (Ga, Cl and SuperKamiokande) and a fit of the energy spectrum of the recoil electrons in SuperKamiokande. A $χ^2$ analysis shows that the quality of the rate fits is excellent for two of the field profiles and good for all others with $χ^2 /d.o.f.$ always well below unity. Regarding the fits for the energy spectrum, their quality is better than that for the small mixing angle MSW solution of the solar neutrino problem, at the same level as that for the large mixing angle MSW solution but worse than that for the vacuum oscillations one. The experimental data on the spectrum are however largely uncertain especially in the high energy sector, so that it is too early yet to draw any clear conclusions on the likeliest type of particle physics solution to the solar neutrino problem.

hep-ph↗

arXiv1999

Small entries of neutrino mass matrices

We consider phenomenologically allowed structures of the neutrino mass matrix in the case of three light neutrino species. Constraints from the solar, atmospheric and reactor neutrino experiments as well as those from the neutrinoless double beta decay are taken into account. Both hierarchical and quasi-degenerate neutrino mass cases are studied. Assuming maximal $ν_μ-ν_τ$ mixing we derive simple approximate expressions giving the values of the neutrino masses and remaining lepton mixing angles in terms of the entries of the neutrino mass matrix. Special attention is paid to the small entries which are usually not specified in discussions of the neutrino mass matrix textures. We specifically discuss the stability of neutrino masses and lepton mixing angles with respect to the choice and variations of these small entries.

hep-ph↗

arXiv1999

Parametric resonance in neutrino oscillations in matter

Neutrino oscillations in matter can exhibit a specific resonance enhancement -- parametric resonance, which is different from the MSW resonance. Oscillations of atmospheric and solar neutrinos inside the earth can undergo parametric enhancement when neutrino trajectories cross the core of the earth. In this paper we review the parametric resonance of neutrino oscillations in matter. In particular, physical interpretation of the effect and the prospects of its experimental observation in oscillations of solar and atmospheric neutrinos in the earth are discussed.

hep-ph↗

arXiv1999

Remarks on parametric resonance of neutrino oscillations in the earth

Neutrino oscillations in matter can exhibit a specific resonance enhancement -- parametric resonance, which is different from the MSW resonance. Recently it has been shown that the oscillations of atmospheric and solar neutrinos inside the earth can undergo parametric enhancement when neutrino trajectories cross the core of the earth. In this paper we continue the study of the parametric resonance of neutrino oscillations in the earth. The following issues are discussed: stability of the resonance with respect to the variations of the zenith angle of the neutrino source; higher-order resonances; prospects of the experimental observation of the parametric resonance of neutrino oscillations. We also comment on a recent controversy regarding the physical nature of the resonance enhancement of the oscillations of the core crossing neutrinos in the earth.

hep-ph↗

arXiv1998

Parametric resonance of neutrino oscillations and passage of solar and atmospheric neutrinos through the earth

We present an exact analytic solution to the neutrino evolution equation in matter with periodic step-function density profile and discuss in detail the parametric resonance of neutrino oscillations that can occur in such a system. Solar and atmospheric neutrinos traversing the earth pass through layers of alternating density and can therefore experience parametric resonance of their oscillations. Atmospheric neutrinos can undergo parametrically enhanced oscillations in the earth when their trajectories deviate from the vertical by about $26^\circ - 32^\circ$. Solar neutrinos traversing the earth can experience a strong parametric resonance of their oscillations in a wide range of zenith angles. If the small mixing angle MSW effect is the solution of the solar neutrino problem, the oscillations of solar neutrinos crossing the core of the earth {\em must} undergo strong parametric resonance; this phenomenon should facilitate significantly the observation of the day-night effect in oscillations of solar neutrinos. If observed, the enhanced day-night effect for core crossing neutrinos would therefore confirm both the MSW solution of the solar neutrino problem and the parametric resonance of neutrino oscillations.

hep-ph↗

arXiv1998

Atmospheric neutrinos at Super-Kamiokande and parametric resonance in neutrino oscillations

We consider the oscillations of atmospheric neutrinos in the earth in the three-neutrino scheme with a $Δm^2$ hierarchy and a small admixture of the electron neutrino in the heavy mass eigenstate characterized by the mixing angle $θ_{13}$. We show that for $Δm^2 \simeq (0.5 - 3) \times 10^{-3}$ eV$^2$ indicated by the Super-Kamiokande data and $\sin^2 2θ_{13} \aprle 0.2$, the oscillations of multi-GeV neutrinos in the subdominant $ν_μ \leftrightarrow ν_e$ mode are enhanced by the MSW and parametric resonances. The parametric resonance, which occurs when the neutrinos cross the core of the earth, dominates for $Δm^2 \simeq (1 - 2)\times 10^{-3}$ eV$^2$, $\sin^2 2θ_{13} \aprle 0.06$. The resonance matter effects lead to an observable excess of the e-like events with a specific zenith angle dependence even for small $θ_{13}$. The up-down asymmetry of the multi-GeV e-like events can reach 15% for $|\cos Θ_e| > 0.2$ and up to 30% for $|\cos Θ_e| > 0.6$, where $Θ_e$ is the zenith angle of the electron. The resonance matter effects are relevant for the interpretation of the Super-Kamiokande data.

hep-ph↗

arXiv1998

Baryogenesis via neutrino oscillations

We propose a new mechanism of leptogenesis in which the asymmetries in lepton numbers are produced through the CP-violating oscillations of ``sterile'' (electroweak singlet) neutrinos. The asymmetry is communicated from singlet neutrinos to ordinary leptons through their Yukawa couplings. The lepton asymmetry is then reprocessed into baryon asymmetry by electroweak sphalerons. We show that the observed value of baryon asymmetry can be generated in this way, and the masses of ordinary neutrinos induced by the seesaw mechanism are in the astrophysically and cosmologically interesting range. Except for singlet neutrinos, no physics beyond the Standard Model is required.

hep-ph↗

arXiv1998

Resonant spin-flavour precession of neutrinos and pulsar velocities

Young pulsars are known to exhibit large space velocities, up to $10^3$ km/s. We propose a new mechanism for the generation of these large velocities based on an asymmetric emission of neutrinos during the supernova explosion. The mechanism involves the resonant spin-flavour precession of neutrinos with a transition magnetic moment in the magnetic field of the supernova. The asymmetric emission of neutrinos is due to the distortion of the resonance surface by matter polarisation effects in the supernova magnetic field. The requisite values of the field strengths and neutrino parameters are estimated for various neutrino conversions caused by their Dirac or Majorana-type transition magnetic moments.

hep-ph↗

arXiv1998

The neutrino magnetic moment and time variations of the solar neutrino flux

The present status of the neutrino magnetic moment solutions of the solar neutrino problem is summarized. After a brief review of the basics of the neutrino spin and spin-flavor precession I discuss the experimental data and show how the neutrino resonant spin-flavor precession (RSFP) mechanism can naturally account for sizeable time variations in the Homestake signal and no observable time variations in the Kamiokande and gallium experiments. Fits of the existing data and predictions for the forthcoming solar neutrino experiments are also discussed. In the last section I summarize the objections to the RSFP mechanism that are frequently put forward and comment on them.

hep-ph↗