Type Of Neutrino Mass Spectrum Research Articles

Observables in neutrino mass spectroscopy using atoms

The process of collective de-excitation of atoms in a metastable level into emission mode of a single photon plus a neutrino pair, called radiative emission of neutrino pair (RENP), is sensitive to the absolute neutrino mass scale, to the neutrino mass hierarchy and to the nature (Dirac or Majorana) of massive neutrinos. We investigate how the indicated neutrino mass and mixing observables can be determined from the measurement of the corresponding continuous photon spectrum taking the example of a transition between specific levels of the Yb atom. The possibility of determining the nature of massive neutrinos and, if neutrinos are Majorana fermions, of obtaining information about the Majorana phases in the neutrino mixing matrix, is analyzed in the cases of normal hierarchical, inverted hierarchical and quasi-degenerate types of neutrino mass spectrum. We find, in particular, that the sensitivity to the nature of massive neutrinos depends critically on the atomic level energy difference relevant in the RENP.

Neutrino mass hierarchy determination using reactor antineutrinos

Building on earlier studies, we investigate the possibility to determine the type of neutrino mass spectrum (i.e., "the neutrino mass hierarchy") in a high statistics reactor electron antineutrino experiment with a relatively large KamLAND-like detector and an optimal baseline of 60 Km. We analyze systematically the Fourier Sine and Cosine Transforms (FST and FCT) of simulated reactor antineutrino data with reference to their specific mass hierarchy-dependent features discussed earlier in the literature. We perform also a binned \chi^2 analysis of the sensitivity of simulated reactor electron antineutrino event spectrum data to the neutrino mass hierarchy, and determine, in particular, the characteristics of the detector and the experiment (energy resolution, visible energy threshold, exposure, systematic errors, binning of data, etc.), which would allow us to get significant information on, or even determine, the type of the neutrino mass spectrum. We find that if \sin^2 2\theta_{13} is sufficiently large, \sin^2 2\theta_{13} \gtap 0.02, the requirements on the set-up of interest are very challenging, but not impossible to realize.

Seesaw neutrino signals at the Large Hadron Collider

We discuss the scenario with gauge singlet fermions (right-handed neutrinos) accessible at the energy of the Large Hadron Collider. The singlet fermions generate tiny neutrino masses via the seesaw mechanism and also have sizable couplings to the standard-model particles. We demonstrate that these two facts, which are naively not satisfied simultaneously, are reconciled in the five-dimensional framework in various fashions, which make the seesaw mechanism observable. The collider signal of tri-lepton final states with transverse missing energy is investigated for two explicit examples of the observable seesaw, taking account of three types of neutrino mass spectrum and the constraint from lepton flavor violation.

Sterile neutrinos as the warm dark matter in the type II seesaw model

In the framework of type II seesaw mechanism we discuss the number of sterile right-handed Majorana neutrinos being the warm dark matter (WDM). When the type II seesaw mass term ${M}_{\ensuremath{\nu}}^{\mathrm{II}}$ is far less than the type I seesaw mass term ${M}_{\ensuremath{\nu}}^{\mathrm{I}}$, only one of three sterile neutrinos may be the WDM particle. On the contrary, the WDM particles may contain all sterile neutrinos. If ${M}_{\ensuremath{\nu}}^{\mathrm{II}}\ensuremath{\sim}{M}_{\ensuremath{\nu}}^{\mathrm{I}}$, the allowed number is not more than $\mathcal{N}\ensuremath{-}1$ for $\mathcal{N}$ sterile neutrinos. It is worthwhile to stress that three different types of neutrino mass spectrum are permitted when ${M}_{\ensuremath{\nu}}^{\mathrm{II}}\ensuremath{\gg}{M}_{\ensuremath{\nu}}^{\mathrm{I}}$ and ${M}_{\ensuremath{\nu}}^{II}\ensuremath{\sim}{M}_{\ensuremath{\nu}}^{I}$.

A three-parameter model for the neutrino mass matrix

Using the type-II seesaw mechanism with three Higgs doublets ϕα (α = e, μ, τ) and four Higgs triplets, we build a model for lepton mixing based on a 384-element horizontal symmetry group, generated by the permutation group S3 and by six transformations. The charged-lepton mass matrix is diagonal; the symmetries of the model would require all the three masses mα to be equal, but different vacuum expectation values of ϕα allow mα to split. The number of parameters in the Majorana neutrino mass matrix depends on two options: full breaking of the permutation group S3, or leaving a μ–τ interchange symmetry intact, and hard or spontaneous violation of CP. We discuss in detail the case with the minimal number of three parameters, wherein is real, symmetric under μ–τ interchange and has equal diagonal elements. In that case, CP is conserved in lepton mixing, atmospheric neutrino mixing is maximal and θ13 = 0; moreover, the type of neutrino mass spectrum and the absolute neutrino mass scale are sensitive functions of the solar mixing angle.

Determining the neutrino mass hierarchy with atmospheric neutrinos

The possibility to determine the type of neutrino mass hierarchy by studying atmospheric neutrino oscillations with a detector capable to distinguish between neutrino and antineutrino events, such as magnetized iron calorimeters, is considered. We discuss how the ability to distinguish between the neutrino mass spectrum with normal and inverted hierarchy depends on detector characteristics like neutrino energy and direction resolutions or charge miss-identification, and on the systematical uncertainties related to the atmospheric neutrino fluxes. We show also how the neutrino mass hierarchy determination depends on the true values of θ 13 and θ 23 , as well as on the type of the true hierarchy. We find that for μ-like events, an accurate reconstruction of the energy and direction of the neutrino greatly improves the sensitivity to the type of neutrino mass spectrum. For sin 2 2 θ 13 ≅ 0.1 and a precision of 5% in the reconstruction of the neutrino energy and 5° in the neutrino direction, the type of neutrino mass hierarchy can be identified at the 2 σ C.L. with approximately 200 events. For resolutions of 15% for the neutrino energy and 15° for the neutrino direction roughly one order of magnitude larger event numbers are required. For a detector capable to distinguish between ν e and ν ¯ e induced events the requirements on energy and direction resolutions are, in general, less demanding than for a detector with muon charge identification.

The absolute neutrino mass scale, neutrino mass spectrum, Majorana CP-violation and neutrinoless double-beta decay

Assuming 3-ν mixing, massive Majorana neutrinos and neutrinoless double-beta ((ββ)0ν-) decay generated only by the (V−A) charged current weak interaction via the exchange of the three Majorana neutrinos, we briefly review the predictions for the effective Majorana mass |〈m〉| in (ββ)0ν-decay and reanalyze the physics potential of future (ββ)0ν-decay experiments to provide information on the type of neutrino mass spectrum, the absolute scale of neutrino masses, and Majorana CP-violation in the lepton sector. Using as input the most recent experimental results on neutrino oscillation parameters and the prospective precision that can be achieved in future measurements of the latter, we perform a statistical analysis of a (ββ)0ν-decay half-life measurement taking into account experimental and theoretical errors, as well as the uncertainty implied by the imprecise knowledge of the corresponding nuclear matrix element (NME). We show, in particular, how the possibility to discriminate between the different types of neutrino mass spectra and the constraints on the absolute neutrino mass scale depend on the mean value and the experimental error of |〈m〉| and on the NME uncertainty. The constraints on Majorana CP-violation phases in the neutrino mixing matrix, which can be obtained from a measurement of |〈m〉| and additional data on the sum of neutrino masses, are also investigated in detail. We estimate the required experimental accuracies on both types of measurements, and the required precision in the NME permitting to address the issue of Majorana CP-violation in the lepton sector.

Three-neutrino oscillations of atmospheric neutrinos, [formula omitted], neutrino mass hierarchy and iron magnetized detectors

We derive predictions for the Nadir angle ( θ n ) dependence of the ratio N μ − / N μ + of the rates of the μ − and μ + multi-GeV events, and for the μ − – μ + event rate asymmetry, A μ − μ + = [ N ( μ − ) − N ( μ + ) ] / [ N ( μ − ) + N ( μ + ) ] , in iron-magnetized calorimeter detectors (MINOS, INO, etc.) in the case of 3-neutrino oscillations of the atmospheric ν μ and ν ¯ μ , driven by one neutrino mass squared difference, | Δ m 31 2 | ∼ ( 2.0 – 3.0 ) × 10 −3 eV 2 ≫ Δ m 21 2 . The asymmetry A μ − μ + (the ratio N μ − / N μ + ) is shown to be particularly sensitive to the Earth matter effects in the atmospheric neutrino oscillations, and thus to the values of sin 2 θ 13 and sin 2 θ 23 , θ 13 and θ 23 being the neutrino mixing angle limited by the CHOOZ and Palo Verde experiments and that responsible for the dominant atmospheric ν μ → ν τ ( ν ¯ μ → ν ¯ τ ) oscillations. It is also very sensitive to the type of neutrino mass spectrum which can be with normal ( Δ m 31 2 > 0 ) or with inverted ( Δ m 31 2 < 0 ) hierarchy. We find that for sin 2 θ 23 ≳ 0.50 , sin 2 2 θ 13 ≳ 0.06 and | Δ m 31 2 | = ( 2 – 3 ) × 10 −3 eV 2 , the Earth matter effects produce a relative difference between the integrated asymmetries A ¯ μ − μ + and A ¯ μ − μ + 2 ν in the mantle ( cos θ n = 0.30 – 0.84 ) and core ( cos θ n = 0.84 – 1.0 ) bins, which is bigger in absolute value than approximately ∼15%, can reach the values of ( 30 – 35 ) % , and thus can be sufficiently large to be observable. The sign of the indicated asymmetry difference is anticorrelated with the sign of Δ m 31 2 . An observation of the Earth matter effects in the Nadir angle distribution of the asymmetry A μ − μ + (ratio N μ − / N μ + ) would clearly indicate that sin 2 2 θ 13 ≳ 0.06 and sin 2 θ 23 ≳ 0.50 , and would lead to the determination of the sign of Δ m 31 2 .

Atmospheric neutrinos and ν-mass hierarchy

We discuss the possibility for matter effects in the three-neutrino oscillations of the atmospheric ν e ( ν ¯ e ) and ν μ ( ν ¯ μ ) , driven by one neutrino mass squared difference, | Δ m 31 2 | ≫ Δ m 21 2 , to be observable under appropriate conditions. We derive predictions for the Nadir angle (θn) dependence of the ratio Nμ/Ne of the rates of the μ–like and e–like multi-GeV events which is particularly sensitive to the Earth matter effects in the atmospheric neutrino oscillations, and thus to the values of sin 2 θ 13 and sin 2 θ 23 , and also to the type of neutrino mass spectrum.

Atmospheric neutrino oscillations, θ13 and neutrino mass hierarchy

We derive predictions for the Nadir angle ( θ n ) dependence of the ratio N μ / N e of the rates of the μ-like and e-like multi-GeV events measured in water-Čerenkov detectors in the case of 3-neutrino oscillations of the atmospheric ν e ( ν ̄ e ) and ν μ ( ν ̄ μ ), driven by one neutrino mass squared difference, |Δ m 2 31|∼(2.5–3.0)×10 −3 eV 2⪢Δ m 2 21. This ratio is particularly sensitive to the Earth matter effects in the atmospheric neutrino oscillations, and thus to the values of sin 2 θ 13 and sin 2 θ 23, θ 13 and θ 23 being the neutrino mixing angle limited by CHOOZ and Palo Verde experiments and that responsible for the dominant atmospheric ν μ → ν τ ( ν ̄ μ→ ν ̄ τ ) oscillations. It is also sensitive to the type of neutrino mass spectrum which can be with normal (Δ m 2 31>0) or with inverted (Δ m 2 31<0) hierarchy. We show that for sin 2 θ 13≳0.01, sin 2 θ 23≳0.5 and at cos θ n ≳0.4, the Earth matter effects modify substantially the θ n -dependence of the ratio N μ / N e and in a way which cannot be reproduced with sin 2 θ 13=0 and a different value of sin 2 θ 23. For normal hierarchy the effects can be as large as ∼25% for cos θ n ∼(0.5–0.8), can reach ∼35% in the Earth core bin cos θ n ∼(0.84–1.0), and might be observable. They are typically by ∼10% smaller in the inverted hierarchy case. An observation of the Earth matter effects in the Nadir angle distribution of the ratio N μ / N e would clearly indicate that sin 2 θ 13≳0.01 and sin 2 θ 23≳0.50.

On the neutrino mass spectrum and neutrinoless double-beta decay

Assuming 3-ν mixing, neutrino oscillation explanation of the solar and atmospheric neutrino data and of the first KamLAND results, massive Majorana neutrinos and neutrinoless double-beta ((ββ)0ν-) decay generated only by the (V–A) charged current weak interaction via the exchange of the three Majorana neutrinos, we analyze in detail the possibility of determining the type of the neutrino mass spectrum by measuring of the effective Majorana mass |〈m〉| in (ββ)0ν-decay. The three possible types of neutrino mass spectrum are considered: (i) normal hierarchical (NH) m1⪡m2⪡m3, (ii) inverted hierarchical (IH), m1⪡m2≅m3, and (iii) quasi-degenerate (QD), m1≅m2≅m3, m1,2,3≳0.20 eV. The uncertainty in the measured value of |〈m〉| due to the imprecise knowledge of the relevant nuclear matrix elements is taken into account in the analysis. We derive the ranges of values of tan2θ⊙, θ⊙ being the mixing angle which controls the solar neutrino oscillations, and of the nuclear matrix element uncertainty factor, for which the measurement of |〈m〉| would allow one to discriminate between the NH and IH, NH and QD, and IH and QD spectra.

The LMA MSW solution of the solar neutrino problem, inverted neutrino mass hierarchy and reactor neutrino experiments

In the context of three-neutrino oscillations, we study the possibility of using antineutrinos from nuclear reactors to explore the 10−4 eV2<Δm2⊙≲8×10−4 eV2 region of the LMA MSW solution of the solar neutrino problem and measure Δm2⊙ with high precision. The KamLAND experiment is not expected to determine Δm2⊙ if the latter happens to lie in the indicated region. By analysing both the total event rate suppression and the energy spectrum distortion caused by ν̄e oscillations in vacuum, we show that the optimal baseline of such an experiment is L∼(20–25) km. Furthermore, for 10−4 eV2<Δm2⊙≲5×10−4 eV2, the same experiment might be used to try to distinguish between the two possible types of neutrino mass spectrum—with normal or with inverted hierarchy, by exploring the effect of interference between the atmospheric- and solar-Δm2 driven oscillations; for larger values of Δm2⊙ not exceeding 8.0×10−4 eV2, a shorter baseline, L≅10 km, would be needed for the purpose. The indicated interference effect modifies in a characteristic way the energy spectrum of detected events. Distinguishing between the two types of neutrino mass spectrum requires, however, a high precision determination of the atmospheric Δm2, a sufficiently large sin2θ and a non-maximal sin22θ⊙, where θ and θ⊙ are the mixing angles, respectively, limited by the CHOOZ and Palo Verde data and characterizing the solar neutrino oscillations. It also requires a relatively high precision measurement of the positron spectrum in the reaction ν̄e+p→e++n.

Majorana neutrinos, CP violation, neutrinoless double beta and tritium beta decays

If the present or upcoming searches for neutrinoless double beta decay give a positive result, the Majorana nature of massive neutrinos will be established. From the determination of the value of the effective Majorana mass parameter |<m>|, it would be possible to obtain information on the type of neutrino mass spectrum. Assuming 3-neutrino mixing and massive Majorana neutrinos, we discuss the information a measurement of, or an upper bound on, |<m>| can provide on the value of the lightest neutrino mass m1. With additional data on the neutrino masses obtained in tritium beta decay experiments, it might be possible to establish whether the CP-symmetry is violated in the lepton sector. This would require very high precision measurements. If CP-invariance holds, the allowed patterns of the relative CP-parities of the massive Majorana neutrinos would be determined.