Neutrino oscillations in Gallium and reactor experiments and cosmological effects of a light sterile neutrino

Abstract : Neutrino oscillations is a very well studied phenomenon and the observations from Solar, very-long-baseline Reactor, Atmospheric and Accelerator neutrino oscillation experiments give very robust evidence of three-neutrino mixing. On the other hand, some experimental data have shown anomalies that could be interpreted as indication of exotic neutrino physics beyond three-neutrino mixing. Furthermore, from a cosmological point of view, the possibility of extra light species contributing as a subdominant hot (or warm) component of the Universe is still interesting. In the first part of this Thesis, we focused on the anomaly observed in the Gallium radioactive source experiments. These experiments were done to test the Gallium solar neutrino detectors GALLEX and SAGE, by measuring the electron neutrino flux produced by intense artificial radioactive sources placed inside the detectors. The measured number of events was smaller than the expected one. We interpreted this anomaly as a possible indication of the disappearance of electron neutrinos and, in the effective framework of two-neutrino mixing, we obtained sin2 2Theta > 0.03 and Deltam2 > 0.1 eV2. We also studied the compatibility of this result with the data of the Bugey and Chooz reactor antineutrino disappearance experiments. We found that the Bugey data present a hint of neutrino oscillations with 0.02 < sin2 2Theta < 0.07 and Deltam2 ≈ 1.95 eV2, which is compatible with the Gallium allowed region of the mixing parameters. Then, combining the data of Bugey and Chooz, the data of Gallium and Bugey, and the data of Gallium, Bugey and Chooz, we found that this hint persists, with an acceptable compatibility of the experimental data. Furthermore, we analyzed the experimental data of the I.L.L., S.R.S, and Gösgen nuclear Reactor experiments. We obtained a good fit of the I.L.L. data, showing 1 and 2Sigma allowed regions in the oscillation parameters space. However, the combination of I.L.L. data with the Bugey data showed a very low compatibility, so we did not use the I.L.L. data for additional analyses. Our fit of the S.R.S. experiment gave very small values of the goodness-of-fit, indicating that the data are incompatible with the oscillations hypothesis, as well as with the no oscillations hypothesis. We do not have any explanation for this result. From the analysis of the G¨osgen experiment, we obtained upper limits for the mixing parameters, excluding the region with sin2 2Theta ≥ 0.3 and Deltam ≥ 0.05 eV2 at 3Sigma C.L.. With the combination of these data with those of Gallium, Bugey and Chooz, we found that the hint of neutrino oscillations persists with 0.03 < sin2 2Theta < 0.07 and Deltam2 ≈ 1.93 eV2, with a good compatibility of the data. However, the no oscillations hypothesis cannot be excluded. Motivated by these results, in the second part of this work we studied cosmological constrains on a light non-thermal sterile neutrino. We fitted up-to-date cosmological data with an extended LCDM model, including light relics with a mass in the range 0.1-10 eV. We obtained constrains on the current density and velocity dispersion of those relics, as well as constrains on their mass, assuming that they consist either of early decoupled thermal relics, or of non-resonantly produced sterile neutrinos. Our results are useful to constrain particle-motivated models with three active neutrinos and one extra light species. We got, for instance, that at the 3Sigma confidence level, a sterile neutrino with mass ms = 2 eV can be accommodated with the data provided that it is thermally distributed with Ts/Tid . 0.8 (with Tid the temperature of neutrinos in the instantaneous decoupling limit), or is non-resonantly produced with DeltaNeff . 0.5. The bounds become dramatically tighter when the mass increases. For ms . 0.9 eV and at the same confidence level, the data is still compatible with a standard thermalized neutrino.
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Mario Acero O. Neutrino oscillations in Gallium and reactor experiments and cosmological effects of a light sterile neutrino. High Energy Physics - Phenomenology [hep-ph]. Università degli studi di Torino; Université de Savoie, 2009. English. ⟨tel-01045108⟩

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