The discovery of neutrino oscillations

The Nobel Prize for physics 2015 was awarded to Takaaki Kajita and Arthur McDonald for the discovery of neutrino oscillations, showing that neutrinos have a mass. This article describes the two areas of research namely the atmospheric neutrino anomaly and the problem of missing solar neutrinos which lead to these groundbreaking discoveries.     

Towards the detection of light and heavy relic neutrinos

The standard Big Bang cosmology predicts that the universe is abundantly populated with neutrinos. As expected there are at least 114 neutrinos per cubic centimeter averaged over the whole space. Like the cosmic background radiation the cosmic neutrinos at present posses a very small kinetic energy due to expansion of the universe. This prediction is one of the cornerstones of modern cosmology. On the other hand the existence of cosmic neutrinos has not yet been confirmed by direct detection experiments. For now we only have a lower limit on the total mass of this free floating ghostly gas of neutrinos, but even so it is roughly equivalent to the total mass of all the visible stars in universe. There could be many more neutrinos at Earth because of condensation of neutrinos, now moving slowly under the gravitational pull of our galaxy. Here we discuss the possibility of detection of relic neutrinos in KATRIN and MARE experiments via neutrino capture on tritium and rhenium, respectively. We also examine single and double relic neutrino capture on double β-decaying nuclei which might be relevant in the context of the new generation double beta decay experiments. Further we explore feasibility of experiments for detection of heavy sterile neutrinos with masses in MeV region, which may have important astrophysical and cosmological implications.     

Probing pseudo-Dirac neutrino through detection of neutrino-induced muons from gamma ray burst neutrinos

The possibility to verify the pseudo-Dirac nature of neutrinos is investigated here via the detection of ultra-high energy neutrinos from distant cosmological objects like γ-ray bursts (GRBs). The very long baseline and the energy range from ∼TeV to ∼EeV for such neutrinos invoke the likelihood to probe very small pseudo-Dirac splittings. The expected secondary muons from such neutrinos that can be detected by a kilometer scale detector such as ICECUBE is calculated and compared with the same in the case of mass-flavour oscillations and for no oscillation cases. The calculated muon yields indicate that to probe such small pseudo-Dirac splittings one needs to look for a nearby GRB (red shift z ∼ 0.03 or less) whereas for a distant GRB (z ∼ 1) the flux will be much depleted and such phenomenon cannot be distinguished. Also calculated are the muon-to-shower ratios.      

Three flavour oscillation interpretation of neutrino data

I consider the mixing of the three active neutrino flavours and obtain the constraints on the parameters of this mixing from the solar, atmospheric and reactor neutrino data.     

SINGAO: a very large telescope for neutrino gamma astronomy and cosmic-ray studies

Summary We discuss the feasibility of a telescope consisting in a sampling array for extensive air showers measure combined with a muon tracking device. The sampling array will extend over a surface of ≥10⁷ m² while the muon tracking device will cover ≥10⁴ m². The telescope should be done with resistive plates counters and would become a very powerful device to study high-energy neutrinos and gamma-ray astronomy as well as cosmicray physics up to the highest energy (≥10¹⁹ eV) region. The content of this paper has been elaborated with the following people interested in performing the experiment: M. De Palma, G. Iaselli, C. Maggi, S. Natali, S. Nuzzo, A. Ranieri, C. Raso, F. Romano, F. Ruggeri, G. Selvaggi, P. Tempesta, G. Zito; A. Rossi, G. Susinno; A. Grillo, F. Ronga, V. Valente; P. Bernardini, P. Pistilli; A. Watson, R. Reid, M. Lawrence; M. Ambrosio, G. Barbarino, B. Bartoli, V. Silvestrini; R. Buccheri, M. Carollo, O. Catalano, J. Linsley, L. Scarsi; G. Bressi, A. Lanza, M. Cambiaghi, S. Ratti; M. Bonori, G. D'Agostini; M. De Vincenzi, E. Lamanna, P. Lipari, G. Martellotti, F. Massa, M. Mattioli, A. Nigro, S. Petrera; R. Cardarelli, F. Rossi, R. Santonico; L. De Cesare, G. Grella, M. Guida, F. Mancini, G. Marini, G. Romano, G. Vitiello; C. Cappa, B. D'Ettore Piazzoli, P. Ghia, G. Gomez, P. Trivero. [Bari, Cosenza, Laboratori Nazionali di Frascati Lecce, Leeds, Napoli, Palermo, Pavia, Roma I, Roma II, Salerno, Torino, Istituto di Cosmogeofisica del CNR].     

Matter effects in active-sterile solar neutrino oscillations

We study the matter effects for solar neutrino oscillations in a general scheme, without any constraint on the number of sterile neutrinos and the mixing matrix elements, only assuming a realistic hierarchy of neutrino squared-mass differences in which the smallest squared-mass difference is effective in solar neutrino oscillations. The validity of the analytic results is illustrated with a numerical solution of the evolution equation in the simplest case of four-neutrino mixing with the realistic matter density profile inside the Sun.     

Solar neutrino problem in light of super-Kamiokande data

I discuss the status of the accepted solutions to the solar neutrino problem in light of the super-Kamiokande data.     

Extrapolation of NEMO Technique to Future Generation of 2β-Decay Experiments

Possibilities of NEMO technique for future neutrinoless double-beta-decay experiments are discussed. Main idea is to have a realistic project with planned sensitivity for half-life on the level (1-2) × 10²⁶ y (sensitivity to neutrino mass (0.05-0.1) eV). It is demonstrated that this can be achieved using improved NEMO technique to investigate 100 kg of ⁸²Se. Possible improvements of NEMO technique and background conditions are discussed. Scheme of future SUPERNEMO detector and main characteristics of experiment are presented. Such detector can be used also to investigate 0 decay in ¹⁰⁰Mo, ¹³⁰Te and ¹¹⁶Cd with sensitivity up to (5-10)×10²⁵ y (or with sensitivity to neutrino mass 0.1 eV).     

Solar flare neutrinos

Summary The theory of high-energy neutrinos generation in the solar atmosphere during the flares is developed. A possible effect in different neutrino detectors is discussed.     

Recent developments in high energy physics

Recent results from experiments with solar, atmospheric and accelerator neutrinos are presented. Some of the important results from the LEP and TEVATRON colliders are summarised.     

Possible violation of the spin-statistics relation for neutrinos: checking through future galactic supernova

We use the detection of neutrinos from a future galactic type-II supernova event in a water Cerenkov detector like Super-Kamiokande to constrain the possible violation of spin-statistics by neutrinos resulting in their obeying a mixed statistics instead of Fermi–Dirac.     

Discriminating neutrino mass models using type-II see-saw formula

An attempt has been made to discriminate theoretically the three possible patterns of neutrino mass models,viz., degenerate, inverted hierarchical and normal hierachical models, within the framework of Type-II see-saw formula. From detailed numerical analysis we are able to arrive at a conclusion that the inverted hierarchical model with the same CP phase (referred to as Type [IIA]), appears to be most favourable to survive in nature (and hence most stable), with the normal hierarchical model (Type [III]) and inverted hierarchical model with opposite CP phase (Type [IIB]), follow next. The degenerate models (Types [IA,IB,IC]) are found to be most unstable. The neutrino mass matrices which are obtained using the usual canonical see-saw formula (Type I), and which also give almost good predictions of neutrino masses and mixings consistent with the latest neutrino oscillation data, are re-examined in the presence of the left-handed Higgs triplet within the framework of non-canonical see-saw formula (Type II). We then estimate a parameter (the so-called discriminator) which may represent the minimum degree of suppression of the extra term arising from the presence of left-handed Higgs triplet, so as to restore the good predictions on neutrino masses and mixings already acquired in Type-I see-saw model. The neutrino mass model is said to be favourable and hence stable when its canonical see-saw term dominates over the non-canonical (perturbative) term, and this condition is used here as a criterion for discriminating neutrino mass models.     

Nuclear responses for neutrinos and neutrino studies by double beta decays and inverse beta decays

This is a brief report on recent studies of nuclear responses for neutrinos (v) by charge exchange reactions, v masses by double beta (ββ) decays and of solar and supernova v’s by inverse β decays. Subjects discussed include (1) v studies in nuclear micro-laboratories, (2) v masses studied by ββ decays of ¹⁰⁰Mo and nuclear responses for ββ — v, (3) solar and supernova v’s by inverse β decays and v responses for ⁷¹Ga and ¹⁰⁰Mo, and (4) MOON (molybdenum observatory of neutrinos) for spectroscopic studies of Majorana v masses with sensitivity of m _v ∼ 0.03 eV by ββ decays of ¹⁰⁰Mo and real-time studies of low energy solar and supernova v’s by inverse β decays of ¹⁰⁰Mo.     

A step toward CNO solar neutrino detection in liquid scintillators

The detection of CNO solar neutrinos in ultrapure liquid scintillator detectors is limited by the background produced by bismuth-210 nuclei that undergo β-decay to polonium-210 with a lifetime of ∼7 days. Polonium-210 nuclei are unstable and decay with a lifetime equal to ∼200 days emitting α particles that can be also detected. In this Letter, we show that the Bi-210 background can be determined by looking at the time evolution of α-decay rate of Po-210, provided that α particle detection efficiency is stable over the data acquisition period and external sources of Po-210 are negligible. A sufficient accuracy can be obtained in a relatively short time. As an example, if the initial Po-210 event rate is ∼2000 cpd/100 ton or lower, a Borexino-like detector could start discerning CNO neutrino signal from Bi-210 background in Δt∼1 yr.     

SNO and the new SNOLAB

A new international science laboratory, SNOLAB, has recently begun operation 2 km underground at the site of the Sudbury Neutrino Observatory (SNO), near Sudbury, Canada. The laboratory incorporates the SNO detector and includes several new experimental areas with a total excavated volume about three times larger than the original SNO cavity. The SNO detector has now completed operation with heavy water and data analysis is being completed with improved accuracy by combining all three phases of the project. A new project, SNO +, has recently been funded and will provide high sensitivity for neutrino-less double beta decay, low energy solar neutrinos, geoneutrinos, supernovae and other physics measurements. A number of dark matter measurements are in progress or preparing for deployment and a new supernova measurement, HALO is also being deployed. The status and physics objectives of the experimental program is outlined.     

Neutrino astronomy: An update

Detecting neutrinos associated with the still enigmatic sources of cosmic rays has reached a new watershed with the completion of IceCube, the first detector with sensitivity to the anticipated fluxes. In this review, we will briefly revisit the rationale for constructing kilometer-scale neutrino detectors and summarize the status of the field.     

Coherence and oscillations of cosmic neutrinos

For cosmic neutrinos we study the conditions and the effects of the coherence loss as well as coherent broadening of the spectrum. We evaluate the width of the neutrino wavepacket produced by charged particles under various circumstances: in an interaction-free environment, in a radiation-dominated medium (typical of the sources of the gamma ray bursts) and in the presence of a magnetic field. The effect of the magnetic field on the wavepacket size appears to be more important than the scattering. If the magnetic field at the source is larger than 10 Gauss, the coherence of neutrinos will be lost while traveling over cosmological distances. Various applications of these results have been considered. We find that for large magnetic fields (B>10⁹ Gauss) and high energies (E_ν>PeV), “coherent broadening” can modify the energy spectrum of neutrinos. In the coherent case, averaging out the oscillatory terms of the probabilities does not induce any statistical uncertainty beyond what expected in the absence of these terms. A deviation from the standard quantum mechanics that preserves average energy and unitarity cannot alter the picture.     

Fundamental Physics in Space: A Quantum-Gravity Perspective

I consider the possibility that space experiments be used to search for quantum properties of spacetime. On the basis of recent quantum-gravity results, I argue that insight on some quantum properties of spacetime can be obtained with experiments planned for the International Space Station, such as AMS and EUSO, and with satellite gamma-ray telescopes, such as GLAST.     

Neutrino mass ordering and μ-τ reflection symmetry breaking

If the neutrino mass spectrum turns out to be m_3 m_1 m_2, it may be relabeled as m_1 m_2 m_3 such that all the masses of fundamental fermions with the same electrical charges are in order. In this case the columns of the 3×3 lepton flavor mixing matrix U should be reordered accordingly, and the resulting pattern U may involve one or two large mixing angles in the standard parametrization or its variations. Since the Majorana neutrino mass matrix remains unchanged in such a mass relabeling, a possible μ-τ reflection symmetry is respected in this connection and its breaking effects are model-independently constrained at the 3σ level by using current experimental data.