Focus on Neutrino Physics 2014

This paper discusses recent results and near-term prospects of the long-baseline neutrino experiments MINOS, MINOS+, T2K and NOvA. The non-zero value of the third neutrino mixing angle θ₁₃ allows experimental analysis in a manner which explicitly exhibits appearance and disappearance dependencies on additional parameters associated with mass-hierarchy, CP violation, and any non-maximal θ₂₃. These current and near-future experiments begin the era of precision accelerator long-baseline measurements and lay the framework within which future experimental results will be interpreted.

We review the potential to probe new physics with neutrinoless double beta decay $(A,Z)\to (A,Z+2)+2{e}^{-}.$ Both the standard long-range light neutrino mechanism as well as non-standard long-range and short-range mechanisms mediated by heavy particles are discussed. We also stress aspects of the connection to lepton number violation at colliders and the implications for baryogenesis.

Neutrino oscillations have become well-known phenomena; the measurements of neutrino mixing angles and mass squared differences are continuously improving. Future oscillation experiments will eventually determine the remaining unknown neutrino parameters, namely, the mass ordering, normal or inverted, and the CP-violating phase. On the other hand, the absolute mass scale of neutrinos could be probed by cosmological observations, single beta decay as well as by neutrinoless double beta decay experiments. Furthermore, the last one may shed light on the nature of neutrinos, Dirac or Majorana, by measuring the effective Majorana mass of neutrinos. However, the neutrino mass generation mechanism remains unknown. A well-motivated phenomenological approach to search for new physics, in the neutrino sector, is that of non-standard interactions. In this short review, the current constraints in this picture, as well as the perspectives from future experiments, are discussed.

We review the collider phenomenology of neutrino physics and the synergetic aspects at energy, intensity and cosmic frontiers to test the new physics behind the neutrino mass mechanism. In particular, we focus on seesaw models within the minimal setup as well as with extended gauge and/or Higgs sectors, and on supersymmetric neutrino mass models with seesaw mechanism and with R-parity violation. In the simplest type-I seesaw scenario with sterile neutrinos, we summarize and update the current experimental constraints on the sterile neutrino mass and its mixing with the active neutrinos. We also discuss the future experimental prospects of testing the seesaw mechanism at colliders and in related low-energy searches for rare processes, such as lepton flavor violation and neutrinoless double beta decay. The implications of the discovery of lepton number violation at the Large Hadron Collider for leptogenesis are also studied.

The large range of energies and pathlengths spanned by atmospheric neutrinos have made them a useful tool in the discovery and subsequent study of neutrino oscillations. With the recent measurement of the ${{\theta }_{13}}$ mixing angle, though it is now known that all mixing angles and mass differences in the PMNS oscillation framework are non-zero, several open questions, including the nature of the neutrino mass hierarchy, the value of the ${{\theta }_{23}}$ octant, and whether or not neutrinos violate charge-parity symmetry, remain. As atmospheric neutrinos are capable of addressing these issues as well as those from more exotic models, the importance of their continued role in neutrino physics is clear. Accordingly, this work reviews recent progress in the study of atmospheric neutrinos, including oscillation and flux measurements from ongoing experiments, and reports on future prospects for next-generation detectors.

Nuclear reactors are the most intense man-made source of antineutrinos, providing a useful tool for the study of these particles. Oscillation due to the neutrino mixing angle ${{\theta }_{13}}$ is revealed by the disappearance of reactor ${{\bar{\nu }}_{e}}$ over ∼km distances. Use of additional identical detectors located near nuclear reactors reduce systematic uncertainties related to reactor ${{\bar{\nu }}_{e}}$ emission and detector efficiency, significantly improving the sensitivity of oscillation measurements. The Double Chooz, RENO, and Daya Bay experiments set out in search of ${{\theta }_{13}}$ using these techniques. All three experiments have recently observed reactor ${{\bar{\nu }}_{e}}$ disappearance, and have estimated values for ${{\theta }_{13}}$ of 9.3^◦ ± 2.1°, 9.2^◦ ± 0.9°, and 8.7^◦ ± 0.4° respectively. The energy-dependence of ${{\bar{\nu }}_{e}}$ disappearance has also allowed measurement of the effective neutrino mass difference, $\mid \Delta m_{ee}^{2}\mid$ ≈ $\mid \Delta m_{31}^{2}\mid$. Comparison with $\mid \Delta m_{\mu \mu }^{2}\mid$ ≈ $\mid \Delta m_{32}^{2}\mid$ from accelerator ${{\nu }_{\mu }}$ measurements supports the three-flavor model of neutrino oscillation. The current generation of reactor ${{\bar{\nu }}_{e}}$ experiments are expected to reach ∼3% precision in both ${{\theta }_{13}}$ and $\mid \Delta m_{ee}^{2}\mid$. Precise knowledge of these parameters aids interpretation of planned ${{\nu }_{\mu }}$ measurements, and allows future experiments to probe the neutrino mass hierarchy and possible CP-violation in neutrino oscillation. Absolute measurements of the energy spectra of ${{\bar{\nu }}_{e}}$ deviate from existing models of reactor emission, particularly in the range of 5–7 MeV.

The origin of neutrino masses and the nature of dark matter are two in most pressing open questions in modern astro-particle physics. We consider here the possibility that these two problems are related, and review some theoretical scenarios which offer common solutions. A simple possibility is that the dark matter particle emerges in minimal realizations of the seesaw mechanism, as in the majoron and sterile neutrino scenarios. We present the theoretical motivation for both models and discuss their phenomenology, confronting the predictions of these scenarios with cosmological and astrophysical observations. Finally, we discuss the possibility that the stability of dark matter originates from a flavor symmetry of the leptonic sector. We review a proposal based on an A₄ flavor symmetry.

New and more precise measurements of neutrino cross sections have renewed interest in a better understanding of electroweak interactions on nucleons and nuclei. This effort is crucial to achieving the precision goals of the neutrino oscillation program, making new discoveries, like the CP violation in the leptonic sector, possible. We review the recent progress in the physics of neutrino cross sections, putting emphasis on the open questions that arise in the comparison with new experimental data. Following an overview of recent neutrino experiments and future plans, we present some details about the theoretical development in the description of (anti)neutrino-induced quasielastic (QE) scattering and the role of multi-nucleon QE-like mechanisms. We cover not only pion production in nucleons and nuclei but also other inelastic channels including strangeness production and photon emission. Coherent reaction channels on nuclear targets are also discussed. Finally, we briefly describe some of the Monte Carlo event generators, which are at the core of all neutrino oscillation and cross-section measurements.

Relic neutrinos play an important role in the evolution of the Universe, modifying some of the cosmological observables. We summarize the main aspects of cosmological neutrinos and describe how the precision of present cosmological data can be used to learn about neutrino properties. In particular, we discuss how cosmology provides information on the absolute scale of neutrino masses, complementary to beta decay and neutrinoless double-beta decay experiments. We explain why the combination of Planck temperature data with measurements of the baryon acoustic oscillation angular scale provides a strong bound on the sum of neutrino masses, 0.23 eV at the 95% confidence level, while the lensing potential spectrum and the cluster mass function measured by Planck are compatible with larger values. We also review the constraints from current data on other neutrino properties. Finally, we describe the very good perspectives from future cosmological measurements, which are expected to be sensitive to neutrino masses close to the minimum values guaranteed by flavour oscillations.

The origin of fermion mass hierarchies and mixings is one of the unresolved and most difficult problems in high-energy physics. One possibility to address the flavour problems is by extending the standard model to include a family symmetry. In the recent years it has become very popular to use non-Abelian discrete flavour symmetries because of their power in the prediction of the large leptonic mixing angles relevant for neutrino oscillation experiments. Here we give an introduction to the flavour problem and to discrete groups that have been used to attempt a solution for it. We review the current status of models in light of the recent measurement of the reactor angle, and we consider different model-building directions taken. The use of the flavons or multi-Higgs scalars in model building is discussed as well as the direct versus indirect approaches. We also focus on the possibility of experimentally distinguishing flavour symmetry models by means of mixing sum rules and mass sum rules. In fact, we illustrate in this review the complete path from mathematics, via model building, to experiments, so that any reader interested in starting work in the field could use this text as a starting point in order to obtain a broad overview of the different subject areas.