This volume follows the one published in 2014, on the occasion of the 100th anniversary of the birth of Bruno Pontecorvo.
The last two years have witnessed an impressive progress in our understanding of the phenomenon of neutrino oscillations.
The field of neutrino oscillations has moreover acquired worldwide resonance after the 2015 Physics Nobel Prize awarded to Takaaki Kajita and Arthur B. McDonald “for the discovery of neutrino oscillations, which shows that neutrinos have mass.”
In parallel, the construction of new detectors aimed at a direct measurement of the mass of the electron neutrino and of neutrinoless double beta decay has been completed or is approaching completion.
Reactor antineutrino experiments have improved by large amounts the precision in the determination of the oscillation parameters, notably those of and .
Thanks to a big increase in the sensitivity of both reactor and accelerator experiments, a determination of the CP violating parameter seems within reach now.
Recent, accurate measurements of the electron antineutrino flux from nuclear reactors have been reported by the experiments Daya Bay, Double Chooz, and RENO. These have confirmed the previous hints for an electron antineutrino flux lower than previously assumed, with a visible anomaly around 5 MeV. This will have implications not only for the determination of the neutrino oscillation parameters but also for the possible existence of the so-called sterile neutrinos.
It should moreover be recalled that in 2015-2016 the first observation has taken place of neutrinos of astrophysical origin, by the IceCube experiment. This experiment and its European counterpart, ANTARES, have provided a detailed measurement of the flux and energy distribution of atmospheric neutrinos, used to obtain independent information on neutrino oscillations, as shown in one of the contributions to this volume.
Following the discovery of neutrino oscillations that require neutrinos to be massive but can only provide information on neutrino mass differences, the efforts to measure the absolute neutrino masses have become even more topical. The paper “The Use of Low Temperature Detectors for Direct Measurements of the Mass of the Electron Neutrino” by A. Nucciotti describes extensively the progress in the use of low temperature detectors now being used and planned to reach sub-eV sensitivity. Progress in the energy resolution and scalability of these detectors has made this technique competitive with the more traditional tritium beta decay magnetic/electrostatic spectrometer method.
The article “T2K and Beyond” by M. G. Catanesi describes the detector and the results of the T2K experiment in Japan on the measurement of angle . The paper then goes on to summarize the future prospects of the project for the measurement of and of the CP phase with the already approved running and also following possible upgrades of the beam line and reduction of the measurement systematics which could lead to a first observation of a nonzero value of . The synergy of T2K with the NOvA experiment in the USA is also covered.
The European Spallation Source is currently being built in Sweden. The article “The Opportunity Offered by the ESSnuSB Project to Exploit the Larger Leptonic CP Violation Signal at the Second Oscillation Maximum and the Requirements of this Project on the ESS Accelerator Complex,” by E. Wildner and collaborators describes the possibility of using it for the production of a very intense 0.4 GeV neutrino beam. The paper then summarizes how this beam, when coupled with a large underground water Cerenkov detector placed at the second oscillation maximum 540 km from the neutrino source, could lead to the discovery of CP violation at level over a significant fraction of the range of the CP violation phase.
The article “Current Status and Future Prospects of the SNO+ Experiment” by S. Andringa et al. describes the many physics targets that can be reached by that experiment, ranging from neutrinoless double beta decay to neutrino oscillations and supernova neutrinos.
The experiment is located at a depth of about 5900 meters of water equivalent, at the site of SNOLAB (Sudbury, Canada). Neutrinoless double beta decay will be the main object of the experiment and certainly the one that will be tackled initially. To this purpose the detector will be filled with about 780 tons of ultrapure liquid scintillator, loaded with 800 kg of 130Te. The corresponding effective Majorana neutrino sensitivity is expected to fall in the range 55–133 meV.
SNO+ aims in addition to measure reactor antineutrino oscillations, with neutrinos coming from reactors located at various distances from the detector, both in Canada and in the USA.
SNO+ also aims at the detection of geoneutrinos and solar neutrinos and of supernova neutrinos.
The article “Measurement of atmospheric neutrino oscillations with very large volume neutrino telescopes” by J. P. Yanez and A. Kouchner discusses the recent results obtained by the IceCube and ANTARES experiments for atmospheric neutrino oscillations. These are mainly sensitive to oscillations and mainly to the large mass splittings. The article discusses in detail the geometry of the experiments and the event reconstruction and analysis.
The results obtained so far are still not competitive with those provided by accelerator experiments, but with the increase in statistics and detector improvements they will provide substantially more precise determinations.
The article also discusses the results that will be obtained by the forthcoming extensions of both experiments: PINGU at the South Pole and ORCA in the Mediterranean.
The article “The Antineutrino Energy Structure in Reactor Experiments” by P. Novella summarizes the current status of the 5 MeV excess (~4 sigma) seen in the visible energy spectrum of the electron antineutrinos in the three reactor neutrino experiments: Daya Bay, Double Chooz, and RENO. All three experiments showed a consistent result for the 5 MeV excess, which is not explained by the reactor neutrino model most widely used. The possible causes of the 5 MeV excess are studied but such excess is most likely due to an incomplete or inaccurate model and to the associated uncertainty on the spectrum as predicted by the model. This article suggests possible future directions in the study of the reactor antineutrino spectrum that could hopefully explain the excess.
The article “The Results of MINOS and the Future with MINOS+” by A. Timmons describes the achievements of the long baseline MINOS experiment. The experiment, using the Fermilab NuMI muon neutrino-antineutrino beam, having energies between 0 and 15 GeV and a near and a far detector located about 700 km away from each other, has measured the disappearance of muon neutrinos and antineutrinos, made a search for the appearance of electron neutrinos, and measured the neutral-current interaction rate. The latter has confirmed oscillations between only three neutrino flavours, thus placing limits on the so-called sterile neutrinos. MINOS will continue as MINOS+ in an upgraded beam with increased energy and intensity, allowing precision tests of the three-flavour neutrino oscillation picture, in particular a very sensitive search of sterile neutrinos.
The article “The Deep Underground Neutrino Experiment (DUNE),” by M. Goodman, discusses the status and perspectives of the next-generation long baseline neutrino experiment that will use a beam generated at Fermilab. The collaboration plans to build a staged 40 kt liquid argon detector at the Sanford Underground Research Facility in South Dakota, a high precision near detector, and a powerful neutrino beam line generated by a 1.2 MW proton beam produced by the PIP-II upgrade, evolving to a power of 2.3 MW by 2030. The paper describes the oscillation physics goals and the status of the collaboration.
The neutrinoless double beta decay process violates lepton number by two units and thus its observation would imply the Majorana nature of the neutrinos, underscoring the importance of this process. In the review article “Neutrinoless Double Beta Decay: 2015 Review” by S. Dell’Oro et al. a comprehensive review on the status of this process is presented.
The review discusses various aspects of the experiments searching for this rare process and the role of nuclear physics including salient features of the different models for calculating nuclear matrix element uncertainties as well as implications for neutrino mass models.
In the article “Quasi-Classical Gravity Effect on Neutrino Oscillations in a Gravitational Field of a Heavy Astrophysical Object” by J. Miller and R. Pasechnik, implications of quantum gravity effects on neutrino oscillations are explored.
The authors compare the decoherence in the neutrino propagation states due to quantum gravity effects with the one induced by the Earth matter effect. This enables them to propose a new way for detecting quantum gravity effects through the measurement of the flavour composition of astrophysical neutrinos.
The article “Partial Quark-Lepton Universality and Neutrino CP Violation” by J. Liao et al. presents a model with partial quark-lepton universality. Such models can naturally arise in the context of grand unified theories. The constraint on the model parameters in terms of the Dirac CP phase is discussed.
In the article “Constraints on the Nonstandard Interaction in Propagation from Atmospheric Neutrinos” by S. Fukasawa and O. Yasuda, the effect of nonstandard interactions on the propagation of atmospheric neutrinos is studied assuming only the electron and the tau neutrinos are affected by such interactions. Constraints on parameters characterizing this effect are obtained from SuperKamiokande data and predictions for the future HyperKamiokande experiment are also made.
Vincenzo Flaminio
Mauro Mezzetto
Leslie Camilleri
Srubabati Goswami
Seon-Hee Seo