Inverted Mass Hierarchy Research Articles

Current Status and Future Prospects of the SNO+ Experiment

SNO+ is a large liquid scintillator-based experiment located 2 km underground at SNOLAB, Sudbury, Canada. It reuses the Sudbury Neutrino Observatory detector, consisting of a 12 m diameter acrylic vessel which will be filled with about 780 tonnes of ultra-pure liquid scintillator. Designed as a multipurpose neutrino experiment, the primary goal of SNO+ is a search for the neutrinoless double-beta decay (0νββ) of130Te. In Phase I, the detector will be loaded with 0.3% natural tellurium, corresponding to nearly 800 kg of130Te, with an expected effective Majorana neutrino mass sensitivity in the region of 55–133 meV, just above the inverted mass hierarchy. Recently, the possibility of deploying up to ten times more natural tellurium has been investigated, which would enable SNO+ to achieve sensitivity deep into the parameter space for the inverted neutrino mass hierarchy in the future. Additionally, SNO+ aims to measure reactor antineutrino oscillations, low energy solar neutrinos, and geoneutrinos, to be sensitive to supernova neutrinos, and to search for exotic physics. A first phase with the detector filled with water will begin soon, with the scintillator phase expected to start after a few months of water data taking. The0νββPhase I is foreseen for 2017.

Recent results from the T2K experiment

T2K is a long-baseline neutrino oscillation experiment based in Japan. It was the first experiment to observe ν e appearance in a νμ beam, and has more recently produced the highest precision measurement of the oscillation parameter θ 23 and has begun to probe the charge-parity violation phase δ . Using a νμ beam disappearance sample, T2K measures sin2 θ 23 = 0.514+0.055 -0.056 and Δm 2 32 = (2.51 ± 0.10)×10−3 eV2 /c4 assuming the normal mass hierarchy, and sin2 θ 23 = 0.511±0.055 and Δm 2 13 = (2.48 ± 0.10)×10−3 eV2 /c4 assuming the inverted mass hierarchy. A joint fit between νμ beam appearance and disappearance samples produces consistent results for sin2 θ 23 and Δm 2 32 whilst also excluding at 90% confidence level the regions δ = [0.15,0.83]π for the normal hierarchy and δ = [− 0.08,1.09]π . Using a beam disappearance sample, and , consistent with previous measurements and the T2K ν μ sample.

Testing nonradiative neutrino decay scenarios with IceCube data

We test the hypothesis of non-radiative neutrinos decay using the latest IceCube data. Namely, we calculate the track-to-shower ratio expected in IceCube for the normal and inverted neutrino mass hierarchy taking into account the uncertainties in neutrino oscillation parameters. We show that the subset of data with energy above 60 TeV actually excludes the possibility of a neutrinos decay at the 1 sigma level of significance for both neutrino mass hierarchies.

Distinguishing neutrino mass hierarchies using dark matter annihilation signals at IceCube

We explore the possibility of distinguishing neutrino mass hierarchies through the neutrino signal from dark matter annihilation at neutrino telescopes. We consider a simple extension of the standard model where the neutrino masses and mixing angles are obtained via the type-II seesaw mechanism as an explicit example. We show that future extensions of IceCube neutrino telescope may detect the neutrino signal from DM annihilation at the Galactic Center and inside the Sun, and differentiate between the normal and inverted mass hierarchies, in this model.

Testing the minimal S 4 model of neutrinos with the Dirac and Majorana phases

We propose two new simple lepton flavor models in the framework of the $S_4$ flavor symmetry. The neutrino mass matrices, which are given by two complex parameters, lead to the inverted mass hierarchy. The charged lepton mass matrix has the 1-2 lepton flavor mixing, which gives the non-vanishing reactor angle $\theta_{13}$. These models predict the Dirac phase and the Majorana phases, which are testable in the future experiments. The predicted magnitudes of the effective neutrino mass for the neutrino-less double beta decay are in the regions as $32~\text{meV}\lesssim |m_{ee}|\lesssim 49~\text{meV}$ and $34~\text{meV}\lesssim |m_{ee}|\lesssim 59~\text{meV}$, respectively. These values are close to the expected reaches of the coming experiments. The total sum of the neutrino masses are predicted in both models as $0.0952~\text{eV}\lesssim \sum m_i\lesssim 0.101~\text{eV}$ and $0.150~\text{eV}\lesssim \sum m_i\lesssim 0.160~\text{eV}$, respectively.

Neutrino Oscillation Physics

We present a global analysis of neutrino oscillation data, including in particular the high-precision measurements of the last SM mixing angle θ13 at reactors, which have confirmed previous indications in favor of θ13>0. We focus on the correlations between θ13 and the mixing angle θ23, as well as between θ13 and the neutrino CP-violation phase δ. Assuming normal hierarchy, we find possible hints about the other two unknowns, namely: a slight preference for the first θ23 octant, and a possible indication for non-zero CP violation (with sin δ<0), although at the level below 2σ for both the two cases. Note that the second hint appears also in inverted hierarchy, but with even lower statistical significance. No palpable difference between normal and inverted mass hierarchy emerges from the data.

Damping of self-induced neutrino flavor transitions due to inhomogeneities

Self-induced neutrino flavor dynamics has been typically characterized assuming that either the time (in the core-collapse supernova environment) or space (in the early universe) homogeneity in the initial conditions is preserved through the evolution. In this talk we show that small deviations from an initial postulated homogeneity can be amplified by the interacting neutrino gas, leading to a new flavor instability. To this end, we consider a simple two flavor isotropic neutrino gas evolving in time, and initially composed by only νe and ν‾e with equal densities. In the homogeneous case, this system shows a bimodal instability in the inverted mass hierarchy scheme, leading to the well studied flavor pendulum behavior. To break space homogeneity, we introduce small amplitude space-dependent perturbations in the matter potential. We find that even for arbitrarily tiny inhomogeneities, the system evolution runs away from the stable pendulum behavior and the space-averaged ensemble evolves towards flavor equilibrium.

Summary of the Latest Results and Future Prospects from the T2K Experiment

The T2K long-baseline experiment is located in Japan and is designed to study oscillations of muon neutrinos. T2K obtains a beam of muon neutrinos peaked at 0.6 GeV that are produced at J-PARC accelerator complex by converting a beam of 30-GeV protons hitting a graphite target. Upon traveling 295 km, neutrinos are detected by the Super-Kamiokande (SK) water Cherenkov detector. Located at 280 m from the target, the near detector complex (ND280) provides information about un-oscillated neutrino flux, beam stability and interaction cross-sections. The T2K experiment observed electron neutrino appearance at SK with the significance of 7.3σ and measured the associated oscillation parameter θ13 for both normal and inverted mass hierarchies. In addition, by looking at muon neutrino disappearance T2K provided improved measurements of the θ23 and Δm232 parameters. The results of these measurements are presented as well as a brief summary of the selected neutrino cross-section measurements. Future prospects of the T2K experiment are discussed.

Flavour dependent gauged radiative neutrino mass model

We propose a one-loop induced radiative neutrino mass model with anomaly free flavour dependent gauge symmetry: $\mu$ minus $\tau$ symmetry $U(1)_{\mu-\tau}$. A neutrino mass matrix satisfying current experimental data can be obtained by introducing a weak isospin singlet scalar boson that breaks $U(1)_{\mu-\tau}$ symmetry, an inert doublet scalar field, and three right-handed neutrinos in addition to the fields in the standard model. We find that a characteristic structure appears in the neutrino mass matrix: two-zero texture form which predicts three non-zero neutrino masses and three non-zero CP-phases from five well measured experimental inputs of two squared mass differences and three mixing angles. Furthermore, it is clarified that only the inverted mass hierarchy is allowed in our model. In a favored parameter set from the neutrino sector, the discrepancy in the muon anomalous magnetic moment between the experimental data and the the standard model prediction can be explained by the additional neutral gauge boson loop contribution with mass of order 100 MeV and new gauge coupling of order $10^{-3}$.

Recent Highlights From The T2K Experiment

Recent results from the T2K experiment on neutrino oscillation and neutrino interaction physics are presented. Details of the project, the current status and future plans are discussed. From a sample of 28νe appearance events T2K reports: sin2⁡2θ13=0.140−0.032+0.038(0.170−0.037+0.045) for normal (inverted) mass hierarchy and from a study of νμ disappearance events the following world-best value is extracted: sin2⁡(θ23)=0.514−0.056+0.055(sin2⁡(θ23)=0.511±0.055) for normal (inverted) mass hierarchy. Preliminary results from joint appearance/disappearance data fits are discussed which are begining to show sensitivity to allowed regions of δCP phase space.

Probing CP violation at LBNE with reactor experiments

In this work, we have explored the possibilities of improving CP violation (CPV) discovery potential of newly planned Long-Baseline Neutrino Experiment (LBNE), USA, by combining with data from reactors. The third mixing angle θ13 is now very precisely measured and this precise measurement of θ13 helps in the measurement of CPV. Here, CPV is studied with and without data from reactors. The impact of placing a neutrino data (ND) is also studied. It is found that CPV discovery potential of LBNE with ND increases when combined with data from reactors. With a far detector of 35 kt, it is possible to obtain 5σ sensitivity of CPV when run for 5 years in ν and 5 years in [Formula: see text] mode. When normal hierarchy is assumed, CPV sensitivity is maximum. CPV discovery is possible by combining 5 years neutrino data from LBNE with 3 years anti-neutrino data from reactors. This study reveals that CPV can also be discovered at 5σ cl in inverted mass hierarchy (IH) mode when appearance measurement of LBNE is combined with reactors.

Phenomenology of harmless family gauge bosons to $K^0-\bar{K}^0$ mixing

When we try to consider family gauge bosons with a lower energy scale, a major obstacle is constraints from the observed [Formula: see text] mixings [Formula: see text]. Against such a conventional view, we point out that, in a U(3) family gauge boson model, the bosons are harmless to any [Formula: see text] mixings independently of explicit values of the family mixings, if masses Mij of the gauge bosons [Formula: see text] (i, j are family indexes) satisfy a relation [Formula: see text]. If such the case can be realized together with an inverted mass hierarchy [Formula: see text], we can consider family gauge bosons with a considerably lower scale, so that we can expect rich signs for family gauge bosons in a TeV scale.

Barium Tagging from nEXO Using Resonance Ionization Spectroscopy

nEXO is a 5-ton liquid enriched-xenon time projection chamber (TPC) to search for neutrinoless double-beta decay, designed to have the sensitivity to completely probe the inverted mass hierarchy of Majorana neutrinos. The detector will accommodate—as a background reduction technique—a system to recover and identify the barium decay product. This upgrade will allow a background-free measurement of neutrinoless double-beta decay and increase the half-life sensitivity of the experiment by at least one order of magnitude. Ongoing research and development includes a system to test barium extraction from liquid xenon using surface adsorption and Resonance Ionization Spectroscopy (RIS).

Enhancing sensitivity to neutrino parameters at INO combining muon and hadron information

The proposed ICAL experiment at INO aims to identify the neutrino mass hierarchy from observations of atmospheric neutrinos, and help improve the precision on the atmospheric neutrino mixing parameters. While the design of ICAL is primarily optimized to measure muon momentum, it is also capable of measuring the hadron energy in each event. Although the hadron energy is measured with relatively lower resolution, it nevertheless contains crucial information on the event, which may be extracted when taken concomitant with the muon data. We demonstrate that by adding the hadron energy information to the muon energy and muon direction in each event, the sensitivity of ICAL to the neutrino parameters can be improved significantly. Using the realistic detector response for ICAL, we present its enhanced reach for determining the neutrino mass hierarchy, the atmospheric mass squared difference and the mixing angle theta23, including its octant. In particular, we show that the analysis that uses hadron energy information can distinguish the normal and inverted mass hierarchies with Deltachi^2 approx 9 with 10 years exposure at the 50 kt ICAL, which corresponds to about 40% improvement over the muon-only analysis.

Exploring the neutrinoless double beta decay in the inverted neutrino hierarchy with bolometric detectors

Neutrinoless double beta decay (0nubb) is one of the most sensitive probes for physics beyond the Standard Model, providing unique information on the nature of neutrinos. In this paper we review the status and outlook for bolometric 0nubb decay searches. We summarize recent advances in background suppression demonstrated using bolometers with simultaneous readout of heat and light signals. We simulate several configurations of a future CUORE-like bolometer array which would utilize these improvements and present the sensitivity reach of a hypothetical next-generation bolometric 0nubb experiment. We demonstrate that a bolometric experiment with the isotope mass of about 1 ton is capable of reaching the sensitivity to the effective Majorana neutrino mass (|mee|) of order 10-20 meV, thus completely exploring the so-called inverted neutrino mass hierarchy region. We highlight the main challenges and identify priorities for an R&D program addressing them.

The flavour of natural SUSY

An inverted mass hierarchy in the squark sector, as in so-called "natural supersymmetry", requires non-universal boundary conditions at the mediation scale of supersymmetry breaking. We propose a formalism to define such boundary conditions in a basis-independent manner and apply it to generic scenarios where the third-generation squarks are light, while the first two generation squarks are heavy and near-degenerate. We show that not only is our formalism particularly well-suited to study such hierarchical squark mass patterns, but in addition the resulting soft terms at the TeV scale are manifestly compatible with the principle of minimal flavour violation, and thus automatically obey constraints from flavour physics.

Parallel lepton mass matrices with texture and cofactor zeros

In this paper we investigate the parallel texture structures containing texture zeros in charged lepton mass matrix $M_{l}$ and cofactor zeros in neutrino mass matrix $M_{\nu}$. These textures are interesting since they are related to the $Z_{n}$ flavor symmetries. Using the weak basis permutation transformation, the 15 parallel textures are grouped as 4 classes (class I,II,III and IV) with the matrices in each class sharing the same physical implications. Under the current experimental data, the class I, III with inverted mass hierarchy and class II with normal mass hierarchy are phenomenologically acceptable. The correlations between some important physical variables are presented, which are essential for the model selection and can be text by future experiments. The model realization is illustrated by means of $Z_{4}\times Z_{2}$ flavor symmetry.

Impact of sterile neutrinos on the early time flux from a galactic supernova

We study the impact of the existence of an eV-mass scale sterile neutrino---with parameters in the ballpark of what is required to fit the laboratory anomalies---on the early time profile of the electron neutrino and antineutrino fluxes associated to a core-collapse supernova (SN). In particular, we focus on the universal feature of neutronization burst expected in the first tens of ms of the signal: Provided that a detector with sufficient sensitivity is available, it is well known that in the three-neutrino framework the detection of the neutronization burst in neutrino channel would signal inverted mass hierarchy. This conclusion is dramatically altered in the presence of a sterile neutrino: We study here, both analytically and numerically, the region in parameter space where this characteristic signal disappears, mimicking normal hierarchy expectations. Conversely, the detection of a peak consistent with expectations for inverted mass hierarchy would exclude the existence of a sterile state over a much wider parameter space than what is required by laboratory anomaly fits, or is even probed by detectors coming on-line in the near future. Additionally, we show the peculiar alteration in the energy-time double differential flux, with a delayed peak appearing for kinematical reasons, which might offer a remarkable signature in the case of favorable parameters and for a high statistics detection of a Galactic SN. We also comment on additional potentially interesting effects in the electron antineutrino channel, if more than one angle in the active-sterile sector is nonvanishing. As an ancillary result that we derived in the technical resolution of the equations, in an Appendix we report the Cayley-Hamilton formalism for the evolution of a four-neutrino system in matter, generalizing existing results in the literature.

Hint of family gauge bosons with an inverted mass hierarchy from the observed tau decays

The present data show a deviation from the e-m universality ε≡εμ−εe=0.0020±0.0016. If we consider that the deviation originates in a mass difference between family gauge bosons A32 and A31, the sign of the observed deviation suggests εμ>εe, i.e. mass relation M(A32)<M(A31). A possibility of a family gauge boson model with an inverted mass hierarchy is discussed. It is concluded that, contrary to a conventional expectation, it is possible to take a considerably lower mass M(A33)∼1 TeV, even the constraint from K0−K¯0 mixing is taken into consideration.

A predictive model of Dirac neutrinos

Assuming lepton number conservation, hermiticity of the neutrino mass matrix and νμ–ντ exchange symmetry, we show that we can determine the neutrino mass matrix completely from the existing data. Comparing with the existing data, our model predicts an inverted mass hierarchy (close to a degenerate pattern) with the three neutrino mass values, 9.16×10−2 eV, 9.21×10−2 eV and 7.80×10−2 eV, a large value for the CP violating phase, δ=109.63°, and of course, the absence of neutrinoless ββ decay. All of these predictions can be tested in the forthcoming or future precision neutrino experiments.