Historically, neutrino physics is a field of continuous advancement: from the neutrino discovery, proposed by Pauli in order to balance the missing energy of the beta decay in the early 20th century, to the proof of the neutrino mass through the measurement of the solar neutrino oscillations by the end of it. The beginning of the 21st century opened the intensity frontier with the development of near mega-watt accelerator machines. These machines, competing with the mega-watt reactors as sources of neutrinos, have been pushing the frontier back at a rate of events per second. So, such experiments are now comparing sensitivity reach in units of “MWatt-kton-years.” Even from the first decade of this century, we have already the success of the neutrino oscillation mixing parameters going from unknown quantities to the best measured values in the field.

The sensitivity frontier is characterized by enormous detector systems that lead precision studies from laboratory or astrophysical neutrino sources. Relying on the detection sensitivity of these systems, there exist various works describing in high precision the fluxes, rates, distributions, and directions of reactor, beam, and supernova neutrinos. They review a wide range of subjects in accelerator neutrino oscillations at the GeV range, solar and astroneutrinos in sub-MeV to 10 MeV, neutrino nuclear interactions in the 10 MeV to GeV region, double beta decays, tritium beta decays, and interactions with complex nuclei. The phenomena are well known, but their absolute cross-sections need to be understood at a high precision in order to fix the strong part of the radiative corrections and be able to independently check the standard model, particularly in the light of the new boson discoveries. The frontiers have generated multinational collaborations tallying great numbers of scientists and engineers having built or designed multimillion projects.

Neutrino oscillation data come from a variety of solar (Super-K, SNO, BOREXINO, etc.), atmospheric (mainly Super-K), reactor (KamLAND, Double Chooz, RENO, and Daya Bay), and short- and long-baseline accelerator experiments (MINOS, MiniBooNE, MINERVA, OPERA, ICAROS, T2K, and NOvA) [1, 2]. They are fed by intense beams from advanced machines at JPARC, CERN, FERMILAB, and ORNL, along with several power plant nuclear reactors around the world. To describe them the simplest unitary form for the lepton mixing matrix is assumed and the state-of-the-art solar and atmospheric neutrino calculations are used. In this special issue, various papers are devoted to the latest research related to existing experiments as well as to the sensitivity estimations of future ones like Hyper-Kamiokande, MicroBOONE, DUNE (Deep Underground Neutrino Experiment, previously named LBNE), COHERENT, JUNO, LENA, and others [1, 3].

In the low-energy and intermediate-energy neutrino range, the charged-current (CC) and neutral-current (NC) neutrino-nucleus reactions provide crucial understanding of the underlying physics of fundamental electroweak interactions within and beyond the standard model. Coherent scattering of neutrinos on nuclei was proposed long ago as an excellent probe of neutral-current -nucleus processes for a plethora of conventional neutrino physics applications and new-physics open issues, but it was not yet measured experimentally [4]. However, a great number of events are expected to be recorded in the going experiments (e.g., COHERENT, TEXONO, and GEMMA). On the theoretical side, the neutrino-nucleus cross-sections calculations (with nuclear methods like the shell model, quasiparticle random-phase approximation, QRPA, shell-model Monte Carlo, etc.) predict quite reliably the nuclear transitions for neutrino energies  MeV. Simulated signatures of neutrino interactions on various isotopes (Ti, Ge, Cd, Xe, etc.) can, subsequently, be derived for several low- and intermediate-energy neutrino distributions of astrophysical neutrino sources, like the solar, supernova, and Earth neutrinos, as well as the laboratory neutrinos, the reactor neutrinos, the pion-muon stopped neutrinos, and the beta beam neutrinos [57]. In view of the operation of extremely intensive neutrino fluxes (at the SNS, PSI, JPARC, Fermilab, etc.), the sensitivity to search for new physics will be largely increased, and, therefore, through coherent neutrino-nucleus scattering cross-section measurements, several open questions involving nonstandard neutrino interactions, neutrino magnetic moment, sterile neutrino searches, and others may be answered.

Neutrinoless double beta decay () is suitable for high-sensitivity studies for Majorana masses and possible new particles beyond the standard model. The () transitions are extremely rare processes of the order of  per sec. There are several existing () experiments in progress as well as R&D on future ones such as MAJORANA, GERDA, MOON, Super-NEMO, CUORE, SNO+, EXO, KamLAND, COBRA, and NEXT [8]. The present volume includes two () papers. One is an interesting description of cryogenic multiton scale detectors with scintillation light read-outs. The other is a () study used to search for heavy and SUSY particles as a complementary probe to the energy frontier searches [8, 9].

During the last few years, there is growing interest in high-energy neutrino astronomy including high-energy -ray and neutrino astronomy with large neutrino telescopes under design or construction. New findings have been reported by ambitious projects in various stages of construction including ANTARES, NEMO, NESTOR, IceCube, AUGER, and AMANDA. Novel ideas for neutrino detection using acoustic and radio waves continue to receive serious attention. The relevant topics included in this volume refer to solar neutrinos, atmospheric neutrinos, results in high-energy neutrino astronomy, and plans in high-energy neutrino astronomy and dark matter research. Current experiments in the field of high-energy neutrino astronomy include AMANDA-II and IceCube at the South Pole. Astonishing developments in new telescopes and detector facilities are described for the CTA Cerenkov gamma-ray array, the Fermi orbital telescope, and the aforementioned IceCube neutrino detector. High-energy neutrinos are assumed to be produced in galactic XRB that include a stellar mass compact object with a companion (donor) star still in the main sequence that is away from the final stages of its evolution [10]. Such a binary system may emit in many different wavelengths, from radio and IR to high-energy gamma rays and neutrinos. In the present volume simulations of neutrino emissions from relativistic galactic astrophysical jets are modelled. These neutrinos have very high energies (in the order of 100 GeV) and specialized instruments are in operation in order to detect them, both on Earth and in space. Furthermore, some next generation instruments are in the process of design and construction.

Neutrino mass may be closely connected to the dark matter of the Universe. Thus, neutrino oscillations, along with neutrino mass generation schemes, suggest dark matter candidates with properties relevant to direct or indirect detection prospects. These issues may be closely interlinked and are several examples of neutrino-motivated dark matter candidates. Though all of them demonstrate cold dark matter properties, as far as the cosmic microwave background (CMB) is concerned, some of them may even behave as warm dark matter regarding its structure formation. When it comes to detection, some are ideal for direct detection, while others are ideal for indirect one through their decay products [11, 12]. These searches are presently negative, but they continue for other phenomena, such as weakly interacting massive particles or candidates of cold dark matter. Some of such developments are in this volume as well.

The knowledge about neutrinos continues to grow using atmospheric and solar neutrinos, though it is now concentrating on quantitative and not simply qualitative features of understanding the mixing parameters. It is expected that further understanding of the nature of neutrino will come from accelerator neutrinos, using off-axis beams, and reactor neutrinos, using multiple detectors underground. The sensitivity of the current and proposed detecting systems has spurred the detailed study of neutrino sources, be it man-made or astrophysical. Relying on the detection sensitivity of the intensity and sensitivity frontier systems there are various works of high precision estimation of fluxes, rates, and distributions of reactor, beam, and supernova neutrinos. The papers in this special issue are a sample of the world effort to push the frontiers even further back towards the neutrino mass hierarchy and the CP violation of the lepton sector. The road to the coveted new physics is now open, but perhaps this will be a subject for another special issue.

Theocharis Kosmas
Hiro Ejiri
Athanasios Hatzikoutelis