Precision measurement of the (7)Be solar neutrino interaction rate in Borexino

The rate of neutrino-electron elastic scattering interactions from 862 keV (7)Be solar neutrinos in Borexino is determined to be 46.0±1.5(stat)(-1.6)(+1.5)(syst)?counts/(day·100 ton). This corresponds to a ν(e)-equivalent (7)Be solar neutrino flux of (3.10±0.15)×10(9) cm(-2)?s(-1) and, under the assumption of ν(e) transition to other active neutrino flavours, yields an electron neutrino survival probability of 0.51±0.07 at 862 keV. The no flavor change hypothesis is ruled out at 5.0?σ. A global solar neutrino analysis with free fluxes determines Φ(pp)=6.06(-0.06)(+0.02)×10(10) cm(-2)?s(-1) and Φ(CNO)<1.3×10(9) cm(-2)?s(-1) (95% C.L.). These results significantly improve the precision with which the Mikheyev-Smirnov-Wolfenstein large mixing angle neutrino oscillation model is experimentally tested at low energy.     

Weak and electromagnetic nuclear decay signatures for neutrino reactions in Super-Kamiokande

We suggest the study of events in the Super-Kamiokande neutrino data due to charged- and neutral-current neutrino reactions followed by weak and/or electromagnetic decays of struck nuclei and fragments thereof. This study could improve the prospects of obtaining evidence for tau production from nu(mu)-->nu(tau) oscillations and could augment the data sample used to disfavor nu(mu)-->nu(sterile) oscillations.     

Status of the Sudbury Neutrino Observatory

Solar neutrinos from the decay of ⁸B have been detected at the Sudbury Neutrino Observatory via the charged-current (CC) and neutral-current (NC) reactions on deuterium and by the elastic scattering (ES) of electrons. The CCr eaction is sensitive exclusively to electron neutrinos, the NCr eaction is sensitive to all neutrino species, and the ES reaction also has a small sensitivity to muon and tau neutrinos. These measurements provided strong evidence that neutrinos change flavor as they propagate from the center of the Sun to the Earth at the 5.3σ level. It will also be shown that a global solar neutrino analysis of matter-enhanced neutrino oscillations of two active flavors strongly favors the large mixing angle solution.     

Global analysis of solar and atmospheric neutrino data

A global analysis of solar (including the recent SNO result), atmospheric, and reactor neutrino data is presented in terms of three-and four-neutrino oscillations. We first present the allowed regions of solar and atmospheric oscillation parameters assuming three-neutrino families, showing that in this framework it is possible to reconcile the two anomalies and providing an unified fit of all the observables at a time. Then, we consider scenarios where a sterile neutrino is added to the three standard ones and the mass spectra present two separate doublets. We evaluate the allowed active-sterile admixture in both solar and atmospheric oscillations, showing that, although the Super-Kamiokande data disfavor both the pure ν_μ→ν _s atmospheric channel and, in combination with SNO, the pure ν _e →ν _s solar channel, the result from the combined analysis still favors close-to-pure active and sterile oscillations and disfavors oscillations into a near-maximal active-sterile admixture.     

Independent measurement of the total active 8B solar neutrino flux using an array of 3He proportional counters at the Sudbury Neutrino Observatory

The Sudbury Neutrino Observatory (SNO) used an array of 3He proportional counters to measure the rate of neutral-current interactions in heavy water and precisely determined the total active (nu_x) 8B solar neutrino flux. This technique is independent of previous methods employed by SNO. The total flux is found to be 5.54_-0.31;+0.33(stat)-0.34+0.36(syst)x10(6) cm(-2) s(-1), in agreement with previous measurements and standard solar models. A global analysis of solar and reactor neutrino results yields Deltam2=7.59_-0.21;+0.19x10(-5) eV2 and theta=34.4_-1.2;+1.3 degrees. The uncertainty on the mixing angle has been reduced from SNO's previous results.     

Present status of the opera experiment for the direct observation of neutrino oscillations in the νμ → ν τ channel

The OPERA experiment (Oscillation Project with an Emulsion-Tracking Apparatus) for the direct observation of neutrino oscillations in the ν_μ → ν _τ channel has been underway at the underground laboratory in Gran Sasso since 2007. In the course of data collection, more than 3000 ν_μ interactions have been registered in emulsion detectors. Twenty charmed neutrino interaction candidates have been found. Multidimensional criteria have been developed in order to select ν _τ interactions against νμ interactions accompanied by charmed hadron production. The first results from automated PAVIKOM emulsion scanning have been obtained. We discuss the measurements of the muon charge ratio of μ⁺ to μ^?.     

Methods for neutral-current neutrino detection in the Sudbury neutrino observatory

The Sudbury Neutrino Observatory will permit a comparison of the rate of neutral-current neutrino interactions to charged-current interactions, which is a test of neutrino mixing and mass. The neutral-current process can be measured by: a) comparison of the rate for electron elastic scattering to the pure charged-current rate on deuterium, b) neutron capture on Cl ions added to the heavy water, and, c) neutron capture in an array of ³He-filled proportional counters. The design and construction of suitable proportional counters are described.     

Results from the NOMAD experiment

The NOMAD experiment has sought ν_μ ? ν_τ oscillations by looking for the emergence of τ^? in events from the CERN SPS neutrino beam. With some improvements in the techniques of analysis in relation to the results published previously and with the inclusion of data from the 1998 run, no evidence for the oscillations has been found, which results in an updated limit on the oscillation probability [P(ν_μ → ν_τ) < 0.5 × 10^?3 at a 90% C.L.]. The corresponding limit on the oscillation mixing angle is given by sin²2θ_μτ < 1.0 × 10^?3 for large Δm ². By using a 1% contamination of ν_e in the neutrino beam, we can also rule out ν _e ? ν_τ oscillations and constrain the probability of the relevant transition as P(ν _e → ν_τ) < 3 × 10^?2 at a 90% C.L. (sin²2θ _eτ < 6 × 10^?2 at large Δm ²).     

Reactor neutrinos—present and future

The main characteristics of experimental searches for neutrino oscillations at nuclear reactors are reviewed. We then describe the Chooz and Palo Verde experiments and report their results to date. We also describe the KamLAND experiment, presently under construction in the Kamioka laboratory in Japan, which is designed to extend the sensitivity to ν_e ? ν_X oscillations by two orders of magnitude in Δm ². In particular, this experiment expects to be able to carry out a terrestial test of the large-angle MSW solution to the solar neutrino problem.     

Neutral currents and neutrino identity

The possibility is considered that in the neutral current reactions ν+ target → ν′ + …, the final neutrino ν′ may be distinct from the initial neutrino ν. A test of neutrino identity is proposed based on the scattering of low energy (reactor) antineutrinos $\text{ν}{}_{\mathrm{e}}$ from an electron target.     

Nuclear corrections in neutrino-nucleus deep inelastic scattering and their compatibility with global nuclear parton-distribution-function analyses

We perform a global χ2 analysis of nuclear parton distribution functions using data from charged current neutrino-nucleus (νA) deep-inelastic scattering (DIS), charged-lepton-nucleus (?(±)A) DIS, and the Drell-Yan (DY) process. We show that the nuclear corrections in νA DIS are not compatible with the predictions derived from ?(±)A DIS and DY data. We quantify this result using a hypothesis-testing criterion based on the χ2 distribution which we apply to the total χ2 as well as to the χ2 of the individual data sets. We find that it is not possible to accommodate the data from νA and ?(±)A DIS by an acceptable combined fit. Our result has strong implications for the extraction of both nuclear and proton parton distribution functions using combined neutrino and charged-lepton data sets.     

First evidence of pep solar neutrinos by direct detection in Borexino

We observed, for the first time, solar neutrinos in the 1.0-1.5 MeV energy range. We determined the rate of pep solar neutrino interactions in Borexino to be 3.1±0.6{stat}±0.3{syst} counts/(day·100 ton). Assuming the pep neutrino flux predicted by the standard solar model, we obtained a constraint on the CNO solar neutrino interaction rate of <7.9 counts/(day·100 ton) (95% C.L.). The absence of the solar neutrino signal is disfavored at 99.97% C.L., while the absence of the pep signal is disfavored at 98% C.L. The necessary sensitivity was achieved by adopting data analysis techniques for the rejection of cosmogenic {11}C, the dominant background in the 1-2 MeV region. Assuming the Mikheyev-Smirnov-Wolfenstein large mixing angle solution to solar neutrino oscillations, these values correspond to solar neutrino fluxes of (1.6±0.3)×10{8} cm{-2}?s^{-1} and <7.7×10{8} cm{-2}?s{-1} (95% C.L.), respectively, in agreement with both the high and low metallicity standard solar models. These results represent the first direct evidence of the pep neutrino signal and the strongest constraint of the CNO solar neutrino flux to date.     

Analysis of γ-ray production in neutral-current neutrino-oxygen interactions at energies above 200 MeV

It has long been recognized that the observation of γ rays originating from nuclear deexcitation can be exploited to identify neutral-current neutrino-nucleus interactions in water-Cherenkov detectors. We report the results of a calculation of the neutrino- and antineutrino-induced γ-ray production cross section for the oxygen target. Our analysis is focused on the kinematical region of neutrino energy larger than ～200 MeV, in which a single-nucleon knockout is known to be the dominant reaction mechanism. The numerical results have been obtained using for the first time a realistic model of the target spectral function, extensively tested against electron-nucleus scattering data. We find that at a neutrino energy of 600?MeV the fraction of neutral-current interactions leading to emission of γ?rays of energy larger than 6?MeV is ～41%, and that the contribution of the p_{3/2} state is overwhelming.     

Measurement of the total active 8B solar neutrino flux at the Sudbury Neutrino Observatory with enhanced neutral current sensitivity

The Sudbury Neutrino Observatory has precisely determined the total active (nu(x)) 8B solar neutrino flux without assumptions about the energy dependence of the nu(e) survival probability. The measurements were made with dissolved NaCl in heavy water to enhance the sensitivity and signature for neutral-current interactions. The flux is found to be 5.21 +/- 0.27(stat)+/-0.38(syst) x 10(6) cm(-2) s(-1), in agreement with previous measurements and standard solar models. A global analysis of these and other solar and reactor neutrino results yields Deltam(2)=7.1(+1.2)(-0.6) x 10(-5) eV(2) and theta=32.5(+2.4)(-2.3) degrees. Maximal mixing is rejected at the equivalent of 5.4 standard deviations.     

Neutrino oscillations and the early universe

The observational and theoretical status of neutrino oscillations in connection with solar and atmospheric neutrino anomalies is presented briefly. The effect of neutrino oscillations on the evolution of the early Universe is discussed in detail. A short review is given of the standard Big Bang Nucleosynthesis (BBN) and the influence of resonant and non-resonant neutrino oscillations on active neutrinos and on primordial synthesis of He-4. BBN cosmological constraints on neutrino oscillation parameters are discussed.     

The NOνA experiment

NOνA is a long-baseline neutrino experiment designed to study ν _μ →ν _e and $\overline \nu_{\mu} \to \overline \nu_e $ oscillations. It will measure the neutrino mixing angles θ ₁₃ and θ ₂₃ with high precision, probe the neutrino mass hierarchy, and search for CP violation in neutrino oscillations. The experiment consists of two detectors. The Near Detector will be located at Fermilab close to the source of the neutrino beam. The Far Detector is being built at Ash River in northern Minnesota. It is positioned 14 mrad off the neutrino beam axis where the neutrinos have an energy distribution with a narrow peak around 2 GeV, and where the transition probability of ν _μ →ν _e is close to its maximum.     

ν<Subscript>μ</Subscript> → ν<Subscript> <Emphasis Type="Italic">e</Emphasis> </Subscript> oscillations search in the OPERA experiment

The tracking capabilities of the OPERA detector allow to reconstruct τ-leptons and electrons. It gives a possibility to observe ν_μ → ν_τ oscillations in the appearance mode and to study ν_μ → ν_e oscillations in the ν_μ CNGS beam. Current results on ν_μ → ν_e channel in the three-flavour mixing model are presented. The same data allow to constrain the presence of additional sterile neutrino states. The analysis of the full 2008–2012 OPERA data set and work on its improvement are going on. Details of the achievements are presented.     

New MACRO results on atmospheric neutrino oscillations

The final results of the MACRO experiment on atmospheric neutrino oscillations are presented and discussed. The data concern different event topologies with average neutrino energies of ～3 and ～50 GeV. Multiple Coulomb scattering of the high-energy muons in absorbers was used to estimate the neutrino energy of each event. The angular distributions, the L/E_ν distribution, the particle ratios, and the absolute fluxes all favor ν_π?ν_τ oscillations with maximal mixing and Δm²=0.0023 eV². A discussion is made on the Monte Carlos used for the atmospheric neutrino flux. Some results on neutrino astrophysics are also briefly discussed.     

Improved search for Muon-neutrino to electron-neutrino oscillations in MINOS

We report the results of a search for ν(e) appearance in a ν(μ) beam in the MINOS long-baseline neutrino experiment. With an improved analysis and an increased exposure of 8.2 × 10(20) protons on the NuMI target at Fermilab, we find that 2 sin(2) (θ(23))sin(2)(2θ(13))<0.12(0.20) at 90% confidence level for δ = 0 and the normal (inverted) neutrino mass hierarchy, with a best-fit of 2sin(2) (θ(23))sin(2)(2θ(13)) = 0.041(-0.031)(+0.047) (0.079(-0.053) (+0.071)). The θ(13) = 0 hypothesis is disfavored by the MINOS data at the 89% confidence level.     

Effects of quantum space time foam in the neutrino sector

We discuss violations of CPT and quantum mechanics due to interactions of neutrinos with space-time quantum foam. Neutrinoless double beta decay and oscillations of neutrinos from astrophysical sources (supernovae, active galactic nuclei) are analysed. It is found that the propagation distance is the crucial quantity entering any bounds on EHNS parameters. Thus, while the bounds from neutrinoless double beta decay are not significant, the data of the supernova 1987a imply a bound being several orders of magnitude more stringent than the ones known from the literature. Even more stringent limits may be obtained from the investigation of neutrino oscillations from active galactic nuclei sources, which have an impressive potential for the search of quantum foam interactions in the neutrino sector. Received: 5 June 2000 / Accepted: 12 July 2000