The Palo Verde reactor neutrino experiment a test for long baseline neutrino oscillations

Our collaboration has installed a long baseline neutrino oscillation experiment at the Palo Verde Nuclear Generating Station in Arizona. 12 tons of Gd loaded liquid scintillator, in a segmented detector, are used to search for oscillations at 740 m distance to three reactors. The anti-neutrino capture on the proton serves as detection reaction. The experiment is expected to reach a sensitivity of Δm² > 1.3 · 10⁻³ eV² and sin²2Θ > 0.1. Our range of sensitivity is tuned to test the ν_μ ↔ ν_e solution of the atmospheric neutrino anomaly.     

KARMEN: Neutrino oscillation limits and new results with the upgrade

The neutrino experiment KARMEN is situated at the beam stop neutrino source ISIS which provides ν_μ's, ν_e's and from the π⁺−μ⁺-decay at rest. The oscillation channels ν_{μ → νe} and are investigated with a 56 t liquid scintillation calorimeter. No evidence for oscillations could be found with KARMEN, resulting in 90% CL exclusion limits of sin²(2Θ) < 8.5 · 10⁻³ ( ) and sin²(2Θ) < 4.0 · 10⁻² (ν_μ → ν_e) for Δm² > 100 eV². In 1996, the experiment has been upgraded by an additional veto counting system with a total coverage of 300 m². The new system allows the identification of cosmic muons in the vicinity of the detector. Vetoing these muons suppresses energetic neutrons from deep inelastic scattering of muons as well as from μ-capture by a factor of 40. Up to 1996, these neutrons represented the main background for oscillation search. The experimental sensitivity for will be significantly enhanced towards sin²(2Θ) 1.0 · 10⁻³ after a further measuring period of 2–3 years.     

Neutrino mixing and see-saw mechanism

Models of neutrino masses are discussed capable of explaining in a natural way the maximal mixing between ν_μ and ν_τ observed by the Super-Kamiokande Collaboration. For three generations of leptons two classes of such models are found implying: a) Δm₂₃²Δm₁₂²≈Δm₁₃² and a small mixing between ν_e and the other two neutrinos, b) Δm₁₂²Δm₁₃²≈Δm₂₃² and a nearly maximal mixing for solar neutrino oscillations in vacuum.     

Final results on the search for νμ→νe oscillations in the NOMAD experiment

The results of the search for ν _μ →ν _e oscillations in the NOMAD experiment at CERN are presented. The experiment looked for the appearance of ν _e in a predominantly ν _μ wideband neutrino beam at the CERN SPS. No evidence for oscillations was found. The 90% confidence limits obtained are Δm ²<0.4 eV² for maximal mixing and sin²(2θ)<1.4×10⁻³ for large Δm ². This result excludes the LSND allowed region of oscillation parameters with Δm ²≳10 eV². From Yadernaya Fizika, Vol. 67, No. 11, 2004, pp. 1967–1972. Original English Text Copyright ? 2004 by Popov. This article was submitted by the author in English. The author represents the NOMAD Collaboration     

New experimental limits on νμ→νe oscillations

The results of an experiment which searched for ν_e events arising from oscillations of a low-energy ν_μ beam are presented. The BEBC heavy liquid bubble chamber, placed at a distance of 825 m from an external proton target at the CERN PS, was used as a neutrino detector. The appearance of ν_e CC interactions provides a sensitive indication of ν_μ→ν_e oscillations. 470 ν_μ CC events and 4 ν_e events, with an estimated background of 3 ν_c CC events, have been observed. The resulting limits on the oscillation parameters are: δm²⩽0.09 eV² (for maximal mixing) and sin²θ⩽0.013 for δm²=2.2 eV², at the 90% confidence level.     

Solar neutrino oscillations in the quasi-vacuum regime

Motivated by recent experimental data, we study solar neutrino oscillations in the range δm²/E ε [10⁻¹⁰, 10⁻⁷] eV²/MeV. In this range vacuum oscillations become increasingly affected by (solar and terrestrial) matter effects for increasing δm², smoothly reaching the MSW regime. A numerical study of matter effects in such “quasi-vacuum” regime is performed. The results are applied to the analysis of the recent solar neutrino phenomenology.     

Seasonal and energy dependence of solar neutrino vacuum oscillations

We make a global vacuum neutrino oscillation analysis of solar neutrino data, including the seasonal and energy dependence of the recent Super-Kamiokande 708-day results. The best fit parameters for ν_e oscillations to an active neutrino are δm²=4.42×10⁻¹⁰ eV², sin²2θ=0.93. The allowed mixing angle region is consistent with bi-maximal mixing of three neutrinos. Oscillations to a sterile neutrino are disfavored. Allowing an enhanced hep neutrino flux does not significantly alter the oscillation parameters.     

A study ofv e −v μ universality and search for neutrino oscillations

In the SKAT neutrino bubble chamber experiment at the Serpukhov accelerator we observe 83 events with electrons and no further lepton in the final state. Comparing the ratio of electron to muon charged current events at energies 3<E _v <30 GeV with that expected fromv _e -beam background assuming lepton universality we find \(R = (N_{e^ - } /N_{\mu ^ - } )_{\exp } /(N_{e^ - } /N_{\mu ^ - } )_{calc} = 0.98 \pm 0.15\) . Furthermore we compare experimental observations and theoretical expectations for some differential distributions of the electron-neutrino charged current reaction. No significant deviation from lepton universality is found. Looking in the charged current reaction for oscillations of muon to electron type neutrinos we find no effect down to the 90% confidence limits Δm ² =1.3 eV² at sin²2θ=1 and sin²2θ=2.5·10^?3 at Δm ²=60 eV².     

Final results from the ντ appearance search in the NOMAD experiment

An updated analysis of the full NOMAD data corresponding to 1.35 × 10⁶ charged current interactions has been performed to search for neutrino oscillations through ν_τ appearance. This document updates the recently published results on the ν_μ → ν_τ and ν_μ → ν_e oscillations search in NOMAD [1] with a unified analysis of the hadronic channels.     

A search for neutrino oscillations

We present here the final results of experiments searching for neutrino oscillations, carried out by the CHARM Collaboration. The data — taking took place in 1983. The first experiment was performed by exposing two detectors simultaneously to the CERN PS low energyv _µ beam. In the second experiment the full CHARM detector was exposed to the wide-band horn-focusedv _µ beam of the CERN SPS. Complete details of the experiments and data reduction are presented. No statistically significant signals for neutrino oscillations were observed. Our 90% CL limits in the appearance experiment (v _µ→v _e ) exclude Δm ²≧0.19 eV² for complete mixing (sin²2θ=1), and sin²2θ≧0.008 for the region Δm ²≧30 eV². These results, and the limits observed for (v _µ→v _x ) (disappearance of (v _µ), are in agreement with those of most other experiments but exclude part of the region previously reported as a possible indication ofv _µ→v _e oscillations.     

Matter effects for solar neutrino oscillations

Possible solar neutrino oscillations are reviewed in the two-neutrino case taking into account the effect of coherent forward scattering when neutrinos travel through the sun and earth. As recently pointed out by Mikheyev and Smirnov this effect can induce a large suppression of the solar ν_e flux for values of Δm ² around 10^?4–10^?8 eV² even for small values of the mixing angle. It also may cause substantial modifications of the solar neutrino spectrum shape. All this may be used for determining Δm ² and sin² 2θ in a large domain from the experimental results of the chlorine, gallium, indium and heavy water detectors.     

Two-detector reactor neutrino oscillation experiment Kr2Det at Krasnoyarsk: Status report

We consider the status of the Kr2Det project aimed at sensitive searches for neutrino oscillations in the atmospheric neutrino mass parameter region around Δm² ～ 3×10^?3 eV² and at obtaining new information on the electron neutrino mass structure (U_e3).     

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).     

The NE detector for a long baseline neutrino oscillation experiment

The project of a large underground experiment, N^OE (Neutrino Oscillation Experiment), composed by modules of scintillating fiber calorimeter interleaved with TRD modules, total weight 6.7 ktons, is presented. This apparatus has been optimized for long baseline neutrino oscillation studies, in particular to be sensitive in the region of sin² 2θ and Δm² suggested by the atmospheric neutrino anomaly (fig. 3).     

Future atmospheric neutrino data from Super-Kamiokande

Expected sensitivity of future atmospheric neutrino data from Super-Kamiokande on neutrino oscillation physics is discussed. We expect that the accuracy of the sin ²θ₂₃ measurement will be improved with (exposure time) . By analyzing high energy fully contained events, it could be possible to statistically demonstrate the existence of charged current ν_τ interactions at the 3 standard deviation level with a few more years of data. Subdominant ν_μ → ν_e oscillations could be observed if θ₁₃ is near the present limit. However, significantly more data will be required to observe a 3 standard deviation effect.     

Limits on neutrino oscillations derived from the ratio of neutral- to charged-current cross sections of neutrinos measured at the CERN SPS NBB

We present an analysis, in terms of neutrino oscillations, of the recent measurements of the ratio R of neutral-current to charged-current neutrino cross sections performed by the CDHS and CHARM Collaborations at the CERN Super Proton Synchroton narrow-band beam. The measurementswere aimed at the determination of sin² θ_w. Our analysis is based on the observation that muon-neutrino oscillations would decrease the number of interactions with a muon in the final state. The effect of oscillations on R can be studied, comparing the actual measurements with the value of R predicted by an independent measurement of sin² θ_w. Limits on ν_μ−ν_e, ν_μ−ν_τ, and ν_μ−ν_x oscillations are presented. They refer to a region of high mass squared difference of the two oscillating neutrinos (Δm² > 3 eV²). For high Δm² the limits on the mixing angle θ are very restrictive. In particular, for the transition ν_μ−ν_x (x≠e, τ) they represent the best available results for large intervals in Δm².     

Further evidence for neutrino oscillations from LSND: the νμ → νe decay-in-flight channel

A search for ν_μ → ν_e oscillations has been conducted at the Los Alamos Meson Physics Facility (LAMPF) using ν_μ from π⁺ decay in flight. An excess in the number of beam-related events from the ν_e C → e⁻ X inclusive reaction is observed. The excess is too large to be explained by normal ν_e contamination in the beam at a confidence level greater than 99%. If interpreted as an oscillation signal, the observed oscillation probability of (2.6 ± 1.3 ± 0.5) × 10⁻³ is consistent with the previously reported oscillation evidence from LSND.     

Limits on the neutrino magnetic moment from solar neutrino experiments

If neutrinos have non-vanishing mass and non-vanishing magnetic moments, then electron neutrinos emitted in nuclear reactions in the solar interior may undergo flavour oscillations, spin precession or resonant spin-flavour precession. Assuming equal values for the magnetic moments of all neutrino flavours and using the data from Homestake and SuperKamiokande we obtain an upper limit on the neutrino magnetic moment and find μ_νe ≤ (2.2 − 2.3) × 10⁻¹⁰μ_B, within four standard solar models. We also point out that this limit may be further reduced if the detector threshold energy for the ν_e,χe⁻ scattering is decreased.     

Status of the CHORUS neutrino oscillation experiment

The CHORUS experiment is designed to search for ν_μ → ν_τ oscillation with a hybrid detector system containing 800 kg nuclear emulsions as target and vertex detector. Run I (320 000 recorded ν_μCC in 1994/5) and more than half of the run II (460 000 ν_μCC in 1996/7) data taking have been successfully completed. Approximately 80 000 events have been analyzed so far, searching for and τ⁻ → h⁻ (nπ⁰) ν_τ decays. No candidate has been found, leading to a limit sin² 2θ_μτ ≤ 4.5 10⁻³ for large Δm².     

Statistical analysis of solar neutrino data

We calculate with Monte Carlo the goodness of fit and the confidence level of the standard allowed regions for the neutrino oscillation parameters obtained from the fit of the total rates measured in solar neutrino experiments. We show that they are significantly overestimated in the standard method. We also calculate exact allowed regions with correct frequentist coverage. We show that the exact VO, LMA and LOW regions are much larger than the standard ones and merge together giving an allowed band at large mixing angles for all Δm² 10⁻¹⁰ eV².