Neutrino Oscillation Hypothesis Research Articles

Proposal to look for an up/down asymmetry in atmospheric neutrinos beyond the multi-GeV region with existing experimental data

We discuss a possible test of the neutrino oscillation hypothesis by proposing a combined analysis of high energy atmospheric neutrino induced muon events that have been detected around the horizontal direction in the Kolar Gold Field (KGF) underground site and below the horizontal direction by many large detectors such as Super-Kamiokande and MACRO. The up/down asymmetry obtained using contained events recorded by the detectors at the Kamioka site probes the low energy region of the atmospheric neutrino whereas the suggested method probes high energy neutrinos. It mainly depends on the observations and it is free of uncertainties in the neutrino flux, the interaction cross section, etc. Here we demonstrate that the method is sensitive to the region of oscillation parameter space that explains all the features of the atmospheric neutrino data in the Super-Kamiokande detector, the limiting factor being the statistical strength of the KGF observations. This method provides the only way to study the up/down asymmetry beyond the multi-GeV region which is yet to be measured experimentally.

Neutrino-induced upward stopping muons in Super-Kamiokande

A total of 137 upward stopping muons of minimum energy 1.6 GeV are observed by Super-Kamiokande during 516 detector live days. The measured muon flux is 0.39±0.04 (stat.)±0.02 (syst.)×10 −13 cm −2s −1sr −1 compared to an expected flux of 0.73±0.16 (theo.)×10 −13 cm −2s −1sr −1 . Using our previously-published measurement of the upward through-going muon flux, we calculate the stopping/through-going flux ratio R , which has less theoretical uncertainty. The measured value of R=0.22±0.02 (stat.)±0.01 (syst.) is significantly smaller than the value 0.37 +0.05 −0.04(theo.) expected using the best theoretical information (the probability that the measured R is a statistical fluctuation below the expected value is 0.39%). A simultaneous fitting to zenith angle distributions of upward stopping and through-going muons gives a result which is consistent with the hypothesis of neutrino oscillations with the parameters sin 22 θ>0.7 and 1.5×10 −3< Δm 2<1.5×10 −2 eV 2 at 90% confidence level, providing a confirmation of the observation of neutrino oscillations by Super-Kamiokande using the contained atmospheric neutrino events.

Atmospheric Neutrino Observations and Flavor Changing Interactions

Flavor changing (FC) neutrino-matter interactions can account for the zenith-angle-dependent deficit of atmospheric neutrinos observed in the SuperKamiokande experiment, without directly invoking either neutrino mass or mixing. We find that FC ${\ensuremath{\nu}}_{\ensuremath{\mu}}$-matter interactions provide a good fit to the observed zenith angle distributions, comparable in quality to the neutrino oscillation hypothesis. The required FC interactions arise naturally in many attractive extensions of the standard model.

Solar neutrinos at super-kamiokande

A huge ring imaging water Cherenkov detector, super-Kamiokande, has started data accumulation on April 1st, 1996 as promised. This experiment is expected to give a definite answer to the neutrino oscillation solutions of the long standing solar neutrino problem through high statistics and high precision spectrum and day/night flux measurement. Super-Kamiokande is accumulating 8B solar neutrino data very quickly and preliminary results obtained from 374 days of data are presented here, instead of 306 days of data presented at the conference. No significant day/night variation nor seasonal variation are found. Systematic errors of energy scale are largely reduced by the LINAC calibrations at various positions. And the experiment is getting closer to the level of the systematic errors where we can definitely discuss about the spectrum distortion. Also implications of those preliminary results are discussed within two neutrino oscillation hypothesis.

Is the solar neutrino deficit energy-dependent?

All existing measurements of the solar neutrino flux are compared with the predictions of the most recent solar model by Bahcall and Pinsonneault, modified by introducing the hypothesis of neutrino oscillations with mass differences large enough to render energy-independent any quantity observable on earth. It is concluded that the data are consistent with this hypothesis and that, at least for the time being, any energy-dependence of the solar neutrino deficit should not be regarded as a compelling reality.

Measurement of the atmospheric neutrino-induced upgoing muon flux using MACRO

We present a measurement of the flux of neutrino-induced upgoing muons (<E ν >∼ 100 GeV) using the MACRO detector. The ratio of the number of observed to expected events integrated over all zenith angles is 0.74 ±0.036 (stat) ±0.046 (systematic) ±0.13 (theoretical). The observed zenith distribution for −1.0≤cos θ≤−0.1 does not fit well with the no oscillation expectation, giving a maximum probability for χ 2 of 0.1%. The acceptance of the detector has been extensively studied using downgoing muons, independent analyses and Monte Carlo simulations. The other systematic uncertainties cannot be the source of the discrepancies between the data and expectations. We have investigated whether the observed number of events and the shape of the zenith distribution can be explained by a neutrino oscillation hypothesis. Fitting either the flux or zenith distribution independently yields mixing parameters of sin 22 θ=1.0 and Δm 2 of a few times 10 −3 eV 2. However, the observed zenith distribution does not fit well with any expectations, giving a maximum probability for χ 2 of 5% for the best oscillation hypothesis, and the combined probability for the shape and number of events is 17%. We conclude that these data favor a neutrino oscillation hypothesis, but with unexplained structure in the zenith distribution not easily explained by either the statistics or systematics of the experiment.

Is the solar neutrino deficit energy-dependent?

All existing measurements of the solar neutrino flux are compared with the predictions of the most recent solar model by Bahcall and Pinsonneault, modified by introducing the hypothesis of neutrino oscillations with mass differences large enough to render energy-independent any quantity observable on earth. It is concluded that the data are consistent with this hypothesis and that, at least for the time being, any energy-dependence of the solar neutrino deficit must be regarded as just an attractive theoretical possibility, but not as a compelling reality.

Future solar neutrino experiments

In a few years, significant improvements in our knowledge of solar neutrino physics are expected. First of all, GALLEX is being exposed to an intense artificial source of neutrinos ( a similar test is also planned for SAGE); the results of this exposure will make us completely confident on GALLEX and, more in general, in radiochemical experiments. The construction of SNO and SUPERKAMIOKANDE is progressing as planned; these two detectors will collect large statistics and will allow model independent tests of the neutrino oscillation hypothesis. A very complete prototype of BOREXINO, an experiment aimed to measure 7Be neutrinos , is close to start the data taking; in less than one year from now, its feasibility will be demonstrated. In last years, also many new projects and ideas have been proposed; some of them will be shorlty discussed.

Experimental study of upward-going muons in Kamiokande.

Upward-going muons produced in the surrounding rock by high-energy neutrinos from astronomical objects have been searched for using the large underground water Cherenkov detector, Kamiokande. During a total of 1255 days observation, no significant signal was observed from the direction of eight astronomical objects. The 90%-confidence-level (-C.L.) upper limits on upward-going muon fluxes with energy greater than 1.7 GeV are 9.9×10−14 cm−2s−1 for Cygnus X-3 and 2.6×10−14 cm−2s−1 for LMC X-4. Our observed upward-going muon flux is compared with the calculation of the upward-going muon flux produced by the atmospheric neutrinos to examine the neutrino-oscillation hypothesis. The experimental average upward-going muon flux with energy greater than 1.7 GeV, (2.05±0.18)×10−13 cm−2s−1sr−1, is consistent with the theoretical expectation. If νμ⇆ντ vacuum oscillations and large mixing angle are assumed, Δm2≳10−2 eV2 is newly rejected. The 90%-C.L. upper limit on the Δm2 for the maximum mixing is found to be Δm2=0.03 eV2 and Δm2=0.0055 eV2, depending on assumptions.Received 29 September 1988DOI:https://doi.org/10.1103/PhysRevD.39.1481©1989 American Physical Society

The solubility problem of heavy elements in internal stellar plasma revisited

Theoretical predictions of the phase separation of heavy elements in internal stellar plasmas, based on the classical Debye-Huckel and mean spherical approximation, have offered a solution to the ‘solar neutrino puzzle’. Recent contributions, however, expressed some doubt about the correctness of these calculations. In the present paper, we challenge the latter conclusions by showing that the linearization of the Poisson-Boltzmann equation can be preformed to avoid the negativity of pair correlation functions, and that the importance of classical charge-charge effects is considerably greater than quantum effects in determining the internal energy of solar plasmas. Comparison between gravity and radiation pressure acting on phase-separated high-Z plasma ‘droplets’ supports the formation of a small ‘iron core’ in the center of the Sun confirming Rouse's suggestion that the frequencies of the non-radialg-modes and the five-min band of oscillations in the Sun can be explained only by the existence of such a core. The depletion of the Sun's interior of heavy elements results in a decreased opacity and, consequently a higher temperature which finally leads to a chlorine neutrino signal of about 2.5 solar neutrino unit in agreement with Davies's experimental result. Essentially the same high neutrino capture rate as given by the present standard Sun model is predicted for the future gallium experiment. This prediction is in contrast to the neutrino oscillation hypothesis in which a wide range of coupling parameters suppresses both chlorine and gallium signals.

Direct approach to resolve the solar-neutrino problem.

A direct approach to resolve the solar-neutrino problem would be to observe neutrinos by use of both neutral-current and charged-current reactions. Then, the total neutrino flux and the electron neutrino flux would be separately determined to provide independent tests of the neutrino-oscillation hypothesis and the standard solar model. A large heavy-water Cherenkov detector, sensitive to neutrinos from /sup 8/B decay via the neutral-current reaction ..nu..+d..--> nu..+p+n and the charged-current reaction ..nu../sub e/+d..-->..e/sup -/+p+p is suggested for this purpose.

New limits on oscillation parameters for electron antineutrinos

The positron spectrum from the reaction ν e + p→ e + + n has been measured at L=37.9 from the core of the Gösgen power reactor to search for neutrino oscillations of the type ν e ↔ x in the low mass parameter range. The results are consistent with the no neutrino oscillation hypothesis. Upper limits of δm 2 ⪕ 0.016 eV 2 (90% c.l..) for full mixing, and sin 22θ⪕0.17 (90% c.l.) in the limit of large mass parameters were determined. For the ratio of the integrated experimental yield to that predicted for the case of no oscillations a value of 1.05 ± 0.02 (statistical) ± 0.05 (systematic; 68% c.l.) was obtained.

Possible evidence for neutrino oscillations in the Brookhaven solar-neutrino experiment

Pomeranchuk has suggested that one might directly observe neutrino oscillations in a solar-neutrino experiment if the oscillation wavelength were comparable to the annual earth-sun-distance variation from perihelion to aphelion, $\ensuremath{\Delta}r=5\ifmmode\times\else\texttimes\fi{}{10}^{6}$km. We find that data from the Brookhaven solar-neutrino experiment can be interpreted to marginally favor the neutrino-oscillation hypothesis at the 2-standard-deviation level. If the effect is not a statistical fluctuation, the estimated value for $\ensuremath{\Delta}{m}^{2}=m_{{\ensuremath{\nu}}_{1}}^{}{}_{}{}^{2}\ensuremath{-}m_{{\ensuremath{\nu}}_{2}}^{}{}_{}{}^{2}\ensuremath{\sim}4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ e${\mathrm{V}}^{2}$ is such that terrestrial tests would seem to be unfeasible.