On Lepton asymmetry and BBN

The strongest available cosmological constraint on lepton asymmetry is L<0.01. We discuss in more detail a BBN model with late ν_e↔ν_s oscillations which is capable of measuring extremely small lepton asymmetry, L>10⁻⁸. This sensitivity is achieved through the influence of small L on the neutrino oscillations, suppressing or enhancing them, and thus decreasing or increasing the primordially produced ⁴He. The cases of asymmetry generated by late resonant electron-sterile oscillations and relic lepton asymmetry are considered. The influence of L on nucleons freezing in pre-BBN epoch is numerically analyzed in the full range of the oscillation parameters for the model and L≥10⁻¹⁰.     

Electroweak scale seesaw and heavy Dirac neutrino signals at LHC

Models of type I seesaw can be implemented at the electroweak scale in a natural way provided that the heavy neutrino singlets are quasi-Dirac particles. In such case, their contribution to light neutrino masses has the suppression of a small lepton number violating parameter, so that light neutrino masses can arise naturally even if the seesaw scale is low and the heavy neutrino mixing is large. We implement the same mechanism with fermionic triplets in type III seesaw, deriving the interactions of the new quasi-Dirac neutrinos and heavy charged leptons with the SM fermions. We then study the observability of heavy Dirac neutrino singlets (seesaw I) and triplets (seesaw III) at LHC. Contrarily to common wisdom, we find that heavy Dirac neutrino singlets with a mass around 100 GeV are observable at the 5σ level with a luminosity of 13 fb⁻¹. Indeed, in the final state with three charged leptons ?^±?^±?^?${?}^{\pm }{?}^{\pm }{?}^{?}$, not previously considered, Dirac neutrino signals can be relatively large and backgrounds are small. In the triplet case, heavy neutrinos can be discovered with a luminosity of 1.5 fb⁻¹ for a mass of 300 GeV in the same channel.     

Cherenkov effect in the weak interactions generated by the neutrinos and new approach for estimation of neutrino mass

It is shown that if weak interactions can generate masses and polarize matter, then the Cherenkov effect induced by these interactions at v _v > c/n appears. The effect of (resonance) enhancement of neutrino oscillations in matter (v _v < c/n) and the Cherenkov (v _v > c/n) effect are competitive processes and at definite neutrino energies the effect of (resonance) enhancement of neutrino oscillations in matter will change to the Cherenkov effect. Then neutrino vacuum oscillations will regenerate and we obtain an excellent possibility of estimating neutrino masses. And knowing estimation of mass for one (electron) neutrino we can obtain masses of the rest neutrinos by using the values for mass differences for neutrinos obtained in oscillation experiments.     

Experimental evidence of electron neutrino oscillations and validation of MSW-LMA model with Borexino

We report the real time measurements of ⁷Be and ⁸B solar neutrino fluxes performed with the Borexino experiment at the Laboratori Nazionali del Gran Sasso. The achievement of these measurements was possible thanks to the excellent levels of the radiopurity reached. The measurement of the ⁷Be in real time is the first direct measurements of the survival probability for solar electron neutrinos in the vacuum region. For ⁸B we reached a threshold energy of 3MeV which is the lowest achieved so far in real time. For the first time, the same apparatus can measure two different oscillation regions (vacuum-driven and matter-enhanced) predicted by the MSW-LMA model. Borexino also quotes the ratio between the survival probabilities, corresponding to 1.93 ± 0.75, and validates the presence of the transition region between the two oscillation regimes, according to the MSW-LMA solution.In addition, a preliminary result on the Day-Night Asymmetry (ADN) for the ⁷Be neutrino flux is presented and corresponds to 0.007 ± 0.073. This measurement makes Borexino able to give once more an independent confirmation of the MSW-LMA solution.     

Study ofD *+ and search forD **0 production by neutrinos in BEBC

Data from BEBC experiments are combined to provide large statistics for neutrino interactions. ChargedD ^* mesons are produced in (1.22±0.25)% of neutrino and (1.01±0.31)% of antineutrino charged current interactions. The mean fraction of the hadronic laboratory energy taken by theD ^*+ in these events is 0.59±0.03±0.08. Less than 18% of all chargedD ^* mesons from (anti)neutrino interactions are found to be daughters ofD ^**0 (at the 90% confidence level).     

Spontaneous R parity violation in supersymmetry: A model for solar neutrino oscillations

Spontaneous breaking of R parity in the standard supergravity model implies the existence of a majoron which is strongly constrained by astrophysics. This is suggested as the origin of the small parameter required for the resonant amplification of neutrino oscillations to take place in the sun. To a good approximation only one neutrino type (most likely the tau neutrino) acquires a mass. Therefore the form of the leptonic charged current is very much restricted: it is fully described by two mixing angles and there is no CP violation. As a result, matter can only amplify oscillations in one channel which, for small mixings, is v_ℓv_τ. Agreement with the chlorine data on the one hand and the nonobservation of charged supersymmetric states in e⁺e^- collisions on the other, tightly restricts the parameters of the model in a way that will be directly tested by collider experiments.     

Two-detector reactor neutrino oscillation experiment Kr2Det at Krasnoyarsk: Status report

We consider the status of the Kr2Det project aimed at sensitive searches for neutrino oscillations in the atmospheric neutrino mass parameter region around Δm² ～ 3×10^?3 eV² and at obtaining new information on the electron neutrino mass structure (U_e3).     

Seasonal and energy dependence of solar neutrino vacuum oscillations

We make a global vacuum neutrino oscillation analysis of solar neutrino data, including the seasonal and energy dependence of the recent Super-Kamiokande 708-day results. The best fit parameters for ν_e oscillations to an active neutrino are δm²=4.42×10⁻¹⁰ eV², sin²2θ=0.93. The allowed mixing angle region is consistent with bi-maximal mixing of three neutrinos. Oscillations to a sterile neutrino are disfavored. Allowing an enhanced hep neutrino flux does not significantly alter the oscillation parameters.     

The charge asymmetry in the μ± - nucleus collision as a test for the neutral weak current

The effect of the neutral weak current can be detected in the inelastic μ^±-nucleus collisions through the Q² dependence of the charge asymmetry in the cross sections. The asymmetry is estimated for the four-quark model of Weinberg by use of the experimental data on the muonless neutrino processes. The effect may be as large as 2 ～ 3% in the large Q² region accessible at NAL.     

Particle asymmetries in the early universe

The total lepton asymmetry l=∑_fl_f in our universe is only poorly constrained by theories and experiments. It might be orders of magnitudes larger than the observed baryon asymmetry b?O(10⁻¹⁰), |l|/b≤O(10⁹). We found that the dynamics of the cosmic QCD transition changes for large asymmetries. Predictions for asymmetries in a single flavour l_f allow even larger values. We find that asymmetries of l_f≤O(1) in single or two flavours may change the relic abundance of WIMPs. However, large lepton and large individual lepton flavour asymmetries influence significantly the dynamics of the early universe between the electroweak phase transition and the onset of neutrino oscillations.     

Neutrino oscillations in the field of a linearly polarized electromagnetic wave

Neutrino oscillations ν _iL ? ν _jR in the field of a linearly polarized electromagnetic wave are studied on the basis of a recently proposed effective Hamiltonian that describes the evolution of a spin in an arbitrary electromagnetic field. The condition of resonance amplification of the oscillations is analyzed in detail. A method is developed for qualitatively studying solutions to the equation of neutrino evolution in the resonance region. This method can be used to explore neutrino oscillations in fields of various configurations.     

The νMSM,inflation, and dark matter

We show how to enlarge the νMSM (the minimal extension of the Standard Model by three right-handed neutrinos) to incorporate inflation and provide a common source for electroweak symmetry breaking and for right-handed neutrino masses. In addition to inflation, the resulting theory can explain simultaneously dark matter and the baryon asymmetry of the Universe; it is consistent with experiments on neutrino oscillations and with all astrophysical and cosmological constraints on sterile neutrino as a dark matter candidate. The mass of inflaton can be much smaller than the electroweak scale.     

Testing the vacuum oscillation and the MSW solutions of the solar neutrino problem

Solar model independent tests of the vacuum oscillation and MSW solutions of the solar neutrino problem are considered. Detailed predictions for time (seasonal) variations of the signals due to neutrino oscillations in vacuum are given for the future solar neutrino detectors (SNO, Super-Kamiokande, BOREXINO, HELLAZ). Results on the distortions of the spectra of ⁸B neutrinos, and of e⁻ from the reaction ν + e⁻ → ν + e⁻ induced by ⁸B neutrinos, are presented in the cases of vacuum oscillations or MSW transitions for a large number of values of the relevant parameters. The possibilities to distinguish between the vacuum oscillation, the MSW adiabatic, and the MSW nonadiabatic transitions in the future solar neutrino experiments are discussed.     

Neutrino dispersion properties in an external magnetic field

The neutrino self-energy operator Σ(p) in a magnetic field is calculated for the case of high-energy neutrinos, this corresponding to the crossed field approximation. The probability of the neutrino decay ν → e ^? W ⁺ is found by using the imaginary part of the operator Σ(p). A simple analytic result is obtained in the parameter region that is the most interesting from the physical point of view and which was not considered earlier. The contribution of an external magnetic field to the neutrino magnetic moment is calculated. The result obtained here for this contribution corrects formulas available previously.     

On neutrino oscillations and the time-energy uncertainty relation

We consider neutrino oscillations as a nonstationary phenomenon based on the Schrödinger evolution equation and mixed neutrino states with definite flavor. We demonstrate that for such states, invariance under translations in time does not take place. We show that the time-energy uncertainty relation plays a crucial role in neutrino oscillations. We compare neutrino oscillations with K ⁰ ? -K ⁰, B _d ⁰ ? B _d ⁰ , and other oscillations.     

On vacuum oscillations solution of the solar neutrino problem

Experimental signatures of vacuum oscillations solution of the solar neutrino problem are considered. This solution predicts a strict correlation between a distortion of the neutrino energy spectrum and an amplitude of seasonal variations of the neutrino flux. The slope parameter which characterizes a distortion of the recoil electron energy spectrum in the Super-Kamiokande experiment and the seasonal asymmetry of the signal have been calculated in a wide range of oscillation parameters. The correlation of the slope and asymmetry gives crucial criteria for identification or exclusion of this solution. For the positive slope indicated by preliminary Super-Kamiokande data we predict (40 – 60) % enhancement of the seasonal variations.     

Searches for muon-to-electron (anti) neutrino flavor change

Employing an 800 MeV, high-intensity proton beam, the LSND experiment performed a sensitive search for neutrino oscillations and obtained evidence for flavor change. Although the KARMEN experiment observed no such evidence, a joint analysis of the two experiments shows that the data sets are compatible with neutrino oscillations occurring either in a band from 0.2 to 1 eV ² or in a region around 7 eV ². The MiniBooNE experiment at Fermilab was designed to test the LSND evidence for neutrino oscillations [C. Athanassopoulos et al., Phys. Rev. Lett. 75, 2650 (1995); 77, 3082 (1996); 81, 1774 (1998); A. Aguilar et al., Phys. Rev. D 64, 112007 (2001)]. The MiniBooNE oscillation result in neutrino mode [A. Aguilar-Arevalo et al., Phys. Rev. Lett. 98, 231801 (2007); A. Aguilar-Arevalo et al. arXiv:0812.2243] shows no significant excess of events at higher energies (), although a sizeable excess is observed at lower energies (). The lack of a significant excess at higher energies allows MiniBooNE to rule out simple 2−ν oscillations as an explanation of the LSND signal. However, the low-energy excess is presently unexplained. Additional antineutrino data and NuMI data may allow the collaboration to determine whether the excess is due, for example, to a neutrino neutral-current radiative interaction or to neutrino oscillations involving sterile neutrinos and whether the excess is related to the LSND signal. If the excess is consistent with being due to sterile neutrinos or other new physics, then future experiments at FNAL (MicroBooNE & BooNE) or ORNL (OscSNS) or with the Low-Energy Neutrino Spectrometer (LENS) detector could confirm their existence.     

MINOS experiment at Fermilab

MINOS is a two-detector experiment to study neutrino oscillations in the NuMI high-intensity neutrino beam at the Fermi National Accelerator Laboratory. Results on ν_μ disappearance, ν_e appearance, sterile neutrino mixing (ν neutral current ‘disappearance’), and disappearance are summarized for an exposure of ≈3×10²⁰ protons on target. Data from a 7×10²⁰ POT exposure already in hand are being analyzed.     

Lepton mixing under the lepton charge nonconservation, neutrino masses and oscillations and the “forbidden” decay µ− → e − + γ

The lepton-charge (L _e , L _μ , L _τ ) nonconserving interaction leads to the mixing of the electron, muon, and tau neutrinos, which manifests itself in spatial oscillations of a neutrino beam, and also to the mixing of the electron, negative muon, and tau lepton, which, in particular, may be the cause of the “forbidden” radiative decay of the negative muon into the electron and γ quantum. Under the assumption that the nondiagonal elements of the mass matrices for neutrinos and ordinary leptons, connected with the lepton charge nonconservation, are the same, and by performing the joint analysis of the experimental data on neutrino oscillations and experimental restriction for the probability of the decay µ^? → e ^? + γ per unit time, the following estimate for the lower bound of neutrino mass has been obtained: m ^(ν) > 1.5 eV/c ².     

Neutrino physics with cryogenic detectors

The recent results on neutrino oscillations and the consequent need to measure the value of the neutrino mass are briefly discussed. The operating principle of cryogenic detectors working at low temperatures, where the small heat capacity allows one to record and measure the temperature increase due to the tiny energy lost by a particle in form of heat is described. An application of these detectors is the measurement, or at least an upper constraint, of the neutrino mass in β decay. This approach is complementary and can, in the future, be competitive with experiments based on the spectrometric measurement of the electron energy. The search for neutrinoless double beta decay could reach a better sensitivity on the mass if a neutrino is a Majorana particle. A large cryogenic detector, named CUORICINO, on neutrinoless double beta decay (DBD) of ¹³⁰Te already yields the best constraint on the absolute value of the Majorana neutrino mass. A much larger detector, named CUORE, for Cryogenic Underground Observatory for Rare Events, is currently under construction. With its active mass of 750 kg of natural TeO₂ it aims to reach the sensitivity in the determination of the Majorana neutrino mass suggested by the results of neutrino oscillation under the inverse hierarchy hypothesis. The problem is closely connected with what I call “the second mystery of Ettore Majorana” who suggested a particle that would violate the lepton number.