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Register a new userFirst Indication of Terrestrial Matter Effects on Solar Neutrino Oscillation
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We report an indication that the elastic scattering rate of solar $^8$B neutrinos with electrons in the Super-Kamiokande detector is larger when the neutrinos pass through the Earth during nighttime. We determine the day/night asymmetry, defined as the difference of the average day rate and average night rate divided by the average of those two rates, to be $(-3.2pm1.1(text{stat})pm0.5(text{syst}))%$, which deviates from zero by 2.7 $sigma$. Since the elastic scattering process is mostly sensitive to electron-flavored solar neutrinos, a non-zero day/night asymmetry implies that the flavor oscillations of solar neutrinos are affected by the presence of matter within the neutrinos flight path. Super-Kamiokandes day/night asymmetry is consistent with neutrino oscillations for $4times10^{-5}$eV$^2leqDelta m^2_{21}leq7times10^{-5}$eV$^2$ and large mixing values of $theta_{12}$, at the $68%$ C.L.
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In a previous paper [1], we presented a three-flavour oscillation analysis of the solar neutrino measurements and of the first data from the KamLAND experiment, in terms of the relevant mass-mixing parameters (delta m^2, theta_12, theta_13). The analysis, performed by including the terrestrial neutrino constraints coming from the CHOOZ (reactor), KEK-to-Kamioka (K2K, accelerator) and Super-Kamiokande (SK, atmospheric) experiments, provided a stringent upper limit on theta_13, namely, sin^2(theta_13)<0.05 at 3 sigma. We reexamine such upper bound in the light of a recent (although preliminary) reanalysis of atmospheric neutrino data performed by the SK collaboration, which seems to shift the preferred value of the largest neutrino square mass difference Delta m^2 downwards. By taking the results of the SK official reanalysis at face value, and by repeating the analysis in [1] with such a new input, we find that the upper bound on theta_{13} is somewhat relaxed: sin^2(theta_13)<0.067 at 3 sigma. Related phenomenological issues are briefly discussed.
We present the results of the Neutrino-4 experiment on search for a sterile neutrino. The experiment has been carried out on the SM-3 reactor having a compact active zone of $42times42times35textrm{cm}^3$ and operating on the highly enriched uranium-235 at 90 MW thermal power. We report the results of the Neutrino-4 experiment of measurements of reactor antineutrino flux and spectrum dependence on the distance in the range 6-12 meters from the center of the reactor core. Using the measured spectrum and the distance dependence of antineutrino flux, we performed the model independent analysis of restrictions on the oscillation parameters $Delta m^2_{14}$ and $sin^2 2theta_{14}$. The method of coherent addition of results of measurements is proposed. It allows us to directly observe the effect of oscillations. We observed the oscillation effect at CL $3.5sigma$ in the vicinity of $Delta m^2_{14} approx 7.26textrm{eV}^2$ and $sin^2 2theta_{14} approx 0.38$. Combining the result of the Neutrino-4 experiment and the result of the gallium anomaly effect we obtained value $sin^2 2theta_{14} approx 0.35 pm 0.07 (5sigma)$. The analysis of systematics effects is presented. Comparison with results of other experiments is presented. Future prospect of the experiment is discussed. It is necessary to notice that obtained values $sin^2 2theta_{14} approx 0.35 pm 0.07 (5sigma)$ and $Delta m^2_{14} approx (7.3 pm 0.7)textrm{eV}^2$ allow make assessment on the mass of a neutrino: $m_{beta} approx 0.8textrm{eV}$.
We explore the complementarity between terrestrial neutrino oscillation experiments and astrophysical/cosmological measurements in probing the existence of sterile neutrinos. We find that upcoming accelerator neutrino experiments will not improve on constraints by the time they are operational, but that reactor experiments can already probe parameter space beyond the reach of Planck. We emphasize the tension between cosmological experiments and reactor antineutrino experiments and enumerate several possibilities for resolving this tension.
A search for day-night variations in the solar neutrino flux resulting from neutrino oscillations has been carried out using the 504 day sample of solar neutrino data obtained at Super-Kamiokande. The absence of a significant day-night variation has set an absolute flux independent exclusion region in the two neutrino oscillation parameter space.
Upward-going stopping muons initiated by atmospheric umu and anumu interactions in the rock below the Soudan 2 detector have been isolated, together with a companion sample of neutrino-induced single muons, created within the detector, which travel downwards and exit. The downward-going sample is consistent with the atmospheric-neutrino flux prediction, but the upward-going sample exhibits a sizeable depletion. Both are consistent with previously reported Soudan-2 neutrino-oscillation results. Inclusion of the two samples in an all-event likelihood analysis, using recent 3D-atmospheric-neutrino-flux calculations, reduces both the allowed oscillation parameter region and the probability of the no-oscillation hypothesis.
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