Evidence For Neutrino Oscillations Research Articles

Feasibility and physics potential of detecting 8B solar neutrinos at JUNO * *This work was supported by the Chinese Academy of Sciences, the National Key R&D Program of China, the CAS Center for Excellence in Particle Physics, the Joint Large-Scale Scientific Facility Funds of the NSFC and CAS, Wuyi University, and the Tsung-Dao Lee Institute of Shanghai Jiao Tong University

The Jiangmen Underground Neutrino Observatory (JUNO) features a 20 kt multi-purpose underground liquid scintillator sphere as its main detector. Some of JUNO's features make it an excellent location for B solar neutrino measurements, such as its low-energy threshold, high energy resolution compared with water Cherenkov detectors, and much larger target mass compared with previous liquid scintillator detectors. In this paper, we present a comprehensive assessment of JUNO's potential for detecting B solar neutrinos via the neutrino-electron elastic scattering process. A reduced 2 MeV threshold for the recoil electron energy is found to be achievable, assuming that the intrinsic radioactive background U and Th in the liquid scintillator can be controlled to 10 g/g. With ten years of data acquisition, approximately 60,000 signal and 30,000 background events are expected. This large sample will enable an examination of the distortion of the recoil electron spectrum that is dominated by the neutrino flavor transformation in the dense solar matter, which will shed new light on the inconsistency between the measured electron spectra and the predictions of the standard three-flavor neutrino oscillation framework. If eV , JUNO can provide evidence of neutrino oscillation in the Earth at approximately the 3 (2 ) level by measuring the non-zero signal rate variation with respect to the solar zenith angle. Moreover, JUNO can simultaneously measure using B solar neutrinos to a precision of 20% or better, depending on the central value, and to sub-percent precision using reactor antineutrinos. A comparison of these two measurements from the same detector will help understand the current mild inconsistency between the value of reported by solar neutrino experiments and the KamLAND experiment.

Alignment methods for the OPERA drift tube detector

The goal of the OPERA experiment is to give the first direct evidence for neutrino oscillations in the channel νμ→ντ. The OPERA detector is designed to observe the appearance of tau neutrinos in the originally pure muon neutrino CNGS beam. An important part of the magnetic spectrometer is the Precision Tracker (PT), a drift tube detector consisting of 9504 drift tubes. Its main task is the determination of the muon charge and momentum. The alignment strategy for the PT consists of two parts: the hardware measurement by theodolite and the software alignment using long muon tracks. In this paper, the hardware and the software alignment are described, and the track-based alignment methods are explained in detail. Results of the software alignment are presented with a focus on the analysis of cosmic particles.

Neutrino masses

The various experiments on neutrino oscillation evidence that neutrinos have indeed non‐zero masses but cannot provide the absolute neutrino mass scale. This scale of neutrino masses is very important for understanding the evolution and the structure formation of the universe as well as for nuclear and particle physics beyond the present Standard Model. Complementary to deducing constraints on the sum of all neutrino masses from cosmological observations, two different methods to determine the neutrino mass scale in the laboratory are pursued: the search for neutrinoless double β‐decay and the direct neutrino mass search by investigating single β‐decays or electron captures. The former method is not only sensitive to neutrino masses but also probes the Majorana character of neutrinos and thus lepton number violation with high sensitivity. Currently quite a few experiments using different techniques are being constructed, commissioned, or are even running, which aim for a sensitivity on the neutrino mass of (100) meV. The principal methods and these experiments are discussed in this short review.

Double Beta Decay Experiments: Beginning of a New Era

Interest in neutrinoless double beta decay (0νββ) has seen a significant renewal in the recent 10 years after evidence for neutrino oscillations was obtained from the results of atmospheric, solar, reactor, and accelerator neutrino experiments. These results are impressive proof that neutrinos have a nonzero mass. The detection and study of 0νββ decay may clarify the following problems of neutrino physics: (i) lepton number non-conservation, (ii) neutrino nature: whether the neutrino is a Dirac or a Majorana particle, (iii) absolute neutrino mass scale, (iv) the type of neutrino mass hierarchy (normal, inverted, or quasidegenerate), and (v) CP violation in the lepton sector (measurement of the Majorana CP-violating phases).

LSND versus MiniBooNE: sterile neutrinos with energy dependent masses and mixing?

Standard active-sterile neutrino oscillations do not provide a satisfactory description of the LSND evidence for neutrino oscillations together with the constraints from MiniBooNE and other null-result short-baseline oscillation experiments. However, if the mass or the mixing of the sterile neutrino depends in an exotic way on its energy all data become consistent. I explore the phenomenological consequences of the assumption that either the mass or the mixing scales with the neutrino energy as 1/Eνr (r > 0). Since the neutrino energy in LSND is about 40 MeV, whereas MiniBooNE operates at around 1 GeV, oscillations get suppressed in MiniBooNE and the two results become fully compatible for r ≳ 0.2. Furthermore, also the global fit of all relevant data improves significantly by exploring the different energy regimes of the various experiments. The best fit χ2 decreases by 12.7 (14.1) units with respect to standard sterile neutrino oscillations if the mass (mixing) scales with energy.

Correlation between the charged current interactions of light and heavy Majorana neutrinos

The evidence for neutrino oscillations implies that three neutrino flavors (νe,νμ,ντ) must have different mass states (ν1,ν2,ν3). The most popular idea of generating tiny masses of νi is to introduce three heavy Majorana neutrinos Ni (for i=1,2,3) into the Standard Model and implement the seesaw mechanism. In this approach the neutrino mixing matrix V appearing in the charged current interactions of νi is not unitary, and the strength of unitarity violation of V is associated with the matrix R which describes the strength of charged current interactions of Ni. We present an explicit parametrization of the correlation between V and R in terms of nine rotation angles and nine phase angles, which can be measured or constrained in the precision neutrino oscillation experiments and by exploring possible signatures of Ni at the LHC and ILC. Two special but viable scenarios, the type-I seesaw model with two heavy Majorana neutrinos and the type-II seesaw model with one heavy Majorana neutrino and one Higgs triplet, are taken into account to illustrate the simplified V–R correlation. The implications of R≠0 on the low-energy neutrino phenomenology are also discussed. In particular, we demonstrate that the non-unitarity of V is possible to give rise to an appreciable CP-violating asymmetry between νμ→ντ and ν¯μ→ν¯τ oscillations with short or medium baselines.

New result contradicts troubling old evidence of neutrino oscillation at small distances

A decade ago, the findings of an accelerator experiment at Los Alamos seemed to require the existence of sterile neutrinos immune to the weak nuclear force.

Oscillations Do Not Distinguish Between Massive and Tachyonic Neutrinos

It is shown that the hypothesis of tachyonic neutrinos leads to the same oscillations effect as if they were usual massive particles. Therefore, the experimental evidence of neutrino oscillations does not distinguish between massive and tachyonic neutrinos. Thus, in view of the fact that the results of the direct neutrino mass measurements are not decisive, the question about the nature of neutrinos remains still open.

MiniBooNE

The physics motivations, design, and status of the Booster Neutrino Experiment at Fermilab, MiniBooNE, are briefly discussed. Particular emphasis is given on the ongoing preparatory work that is needed for the MiniBooNE muon neutrino to electron neutrino oscillation appearance search. This search aims to confirm or refute in a definitive and independent way the evidence for neutrino oscillations reported by the LSND experiment.

Neutrino masses, mixing and oscillations

Basics of neutrino oscillations is discussed. Importance of time-energy uncertainty relation is stressed. Neutrino oscillations in the leading approximation and evidence for neutrino oscillations are briefly summarized.

Update on MiniBooNE

MiniBooNE (Booster Neutrino Experiment) is searching for ν μ → ν e oscillations in the neutrino beam produced by the 8 GeV Booster synchrotron at Fermilab. The Booster has delivered 3.66 × 10 20 protons-on-target with over 380 thousand neutrino events recorded in the detector since September 2002. MiniBooNE is now accumulating enough data to achieve its goal of conclusively confirming or refuting the evidence for neutrino oscillations observed by the LSND experiment.

KamLAND: Updated results

KamLAND has accumulated 766.3 ton-year of data with extended period and enlarged fiducial volume. No oscillation predicts 365.2±23.7 reactor neutrino events and 17.8±7.3 background events in the data, while only 258 events were observed. This deficit is consistent with the previous evidence for reactor antineutrino disappearance and strengthened the confidence level to 99.998%. Increased statistics allowed us to examine spectral distortion and the observed spectrum was inconsistent with no-oscillation shape at 99.6% C.L. Combining these deficit and spectral distortion, no-oscillation is excluded at 99.999995% C.L. The observed distortion is fully consistent with the neutrino oscillation hypothesis with the oscillation parameter of the LMA solution for the solar neutrino problem and the distortion disagrees with the other flavor conversion hypothesis such as neutrino decay and decoherence. If one plots the survival probability as a function of L 0 / E where L 0 is chosen at typical reactor distance of 180 km, one sees clear oscillatory behavior. This is a stand-alone evidence of neutrino oscillation. This clear signal of oscillation provided precise determination of Δ m 2 to be 7.9 −0.5 +0.6 × 10 −5 eV 2 when analyzed in two neutrino oscillation scenario.

Evidence for neutrino oscillations - II: Atmospheric and accelerator neutrinos

The first evidence for neutrino oscillation was obtained from zenith-angle distributions of atmospheric neutrinos observed by Super-Kamiokande. Recently, Super-Kamiokande has succeeded in observing the oscillation pattern in the event distribution as a function of L/E. This is the behavior expected from vacuum neutrino oscillations. Also, the first accelerator-based long baseline neutrino oscillation experiment, K2K, confirmed the atmospheric neutrino oscillation with a probability of no oscillation being 0.01%. From these experiments as well as other atmospheric neutrino experiments, a consistent picture of ν μ ↔ ν τ oscillation has been established. Near-future accelerator-based neutrino oscillation experiments aim at observing sub-dominant ν μ ↔ ν e oscillation in order to measure the mixing angle θ 13 .

Evidence for Neutrino Oscillations I: Solar and Reactor Neutrinos

This paper discusses evidence for neutrino oscillations obtained from measurements with solar neutrinos and reactor neutrinos.

Super-Kamiokande Results on Neutrino Oscillations

Super-Kamiokande (SK) is able to measure neutrino interactions over the 5 decades of the energy from 5 MeV to a few hundreds GeV. In 1998, SK has obtained strong evidence of neutrino oscillations by observing a deficit of the atmospheric νμ flux coming upward to the detector. This discovery has revealed that neutrinos have finite masses. The mass difference is ~0.002 eV2 and a mixing angle is nearly maximal. The experimental evidence on the atmospheric neutrino oscillation has been strengthened by the observation of the oscillatory pattern as a function of L/E.The non-observation of the day-night flux differences and the non-observation of the spectrum distortions of solar neutrinos have placed a strong constraint on the oscillation parameters. In 2000, the so called small mixing angle solutions were rejected and it was shown that the solar neutrino oscillation should be large mixing.Conclusive evidence of the solar neutrino oscillation was obtained by comparing the precisely measured flux by the neutrino-electron scattering in SK and the flux obtained by the charged current interaction of electron neutrinos in SNO in 2001. The result of the fit by using all the solar neutrino experiments has selected a MSW Large Mixing Angle solution.

NEUTRINOS: A BRIEF REVIEW

The major steps in the development of our knowledge about neutrinos are reviewed. The basics of neutrino oscillation formalism is presented. Neutrino oscillations in the framework of three-neutrino mixing are considered. The evidence for neutrino oscillations is discussed. Neutrinoless double β-decay is briefly considered.

LOW ENERGY NEUTRINO PHYSICS AFTER SNO AND KamLAND

In the recent years important discoveries in the field of low energy neutrino physics (Eν in the ≈ MeV range) have been achieved. Results of the solar neutrino experiment SNO show clearly flavor transitions from νe to νμ,τ. In addition, the long standing solar neutrino problem is basically solved. With KamLAND, an experiment measuring neutrinos emitted from nuclear reactors at large distances, evidence for neutrino oscillations has been found. The values for the oscillation parameters, amplitude and phase, have been restricted. In this paper the potential of future projects in low energy neutrino physics is discussed. This encompasses future solar and reactor experiments as well as the direct search for neutrino masses. Finally the potential of a large liquid scintillator detector in an underground laboratory for supernova neutrino detection, solar neutrino detection, and the search for proton decay p→K+ν is discussed.

Neutrino mass parameters from Kamland, SNO, and other solar evidence

An updated analysis of all available neutrino oscillation evidence in Reactor (Kamland, first 145 days of data) and Solar experiments (SK day and night spectra, global rates from Homestake, SAGE and GALLEX) including the $SNO CC$ and $NC$ data is presented. In the framework of two active neutrino oscillations we determine the allowed regions in neutrino parameter space, we obtain, from the Kamland spectral shape and global signal, the following antineutrino best solution parameters, $ \Delta m^2_{kl}= 7.7\times 10^{-5} \eV^2, \tan^2\theta_{kl}= 0.98.$ The overall effect of the measured observed ratio $R\sim 0.6$ is that the LMA region remains the only one which is still favored. Combining Kamland and Solar data and assuming CPT invariance, i.e. the same mass matrix for neutrino and antineutrinos, we obtain the following antineutrino best solution parameters (LMAI solution), $ \Delta m^2= 7.1\times 10^{-5} \eV^2,\tan^2\theta= 0.47.$ A second solution (LMAII) appears for values $ \Delta m^2= 1.5\times 10^{-4} \eV^2, \tan^2\theta= 0.48.$ We determine additionally individual neutrino mixing parameters and their errors from fits to marginal likelihood distributions, the values are compatible with previous results. In both methods, $\chi^2$ minimization and marginal likelihood, the combined analysis of solar and Kamland data concludes that maximal mixing is not favored at the $\sim 3\sigma $ level at least.

The MiniBooNE experiment: Status and plans

The MiniBooNE experiment has recently started a search for ν μ → ν e oscillations at Fermilab. This will provide a definitive test of the LSND results which show evidence for neutrino oscillation. The MiniBooNE neutrino source and detector are both now operational and data taking has begun. We describe the experiment, report on the status, and show the expected sensitivity to neutrino oscillations.

Neutrino physics and oscillation studies at CERN

The CERN short baseline program, designed to search for ν μ → ν τ oscillations, was carried out by two experiments using very different approaches: CHORUS, aiming at the direct observation of the τ decay in emulsion, and NOMAD, based on the indirect kinematical selection with an electronic detector. A review of the two experiments and of their τ appearance search, giving no evidence for neutrino oscillations, is presented here. The preliminary results of a search for ν μ → ν e oscillations in the NOMAD experiment are also reported, excluding the parameter region of LSND in the range Δm 2 > 10 eV 2.