Limits on nu_e and anti-nu_e disappearance from Gallium and reactor experiments

TL;DR

This paper examines potential electron-neutrino and electron-antineutrino disappearance signals from Gallium source experiments and short-baseline reactor data within a two-neutrino mixing framework for $\Delta m^{2}$ much larger than solar/atmospheric values. The Gallium data exhibit a deficit compatible with oscillations, yielding a best-fit region around $\sin^{2}2\vartheta\sim0.22$ and $\Delta m^{2}\sim2\ \text{eV}^{2}$, while the Bugey reactor results suggest a smaller, compatible hint near $\Delta m^{2}\sim1.8\ \text{eV}^{2}$ with $\sin^{2}2\vartheta\sim0.05$. Chooz provides complementary constraints at higher $\Delta m^{2}$, though Gallium-Chooz combinations show tension at the 2σ level, and the full Bugey+Chooz+Gallium analysis finds compatibility with no oscillations as well as the short-baseline hint. The collective findings point to the intriguing possibility of a light sterile neutrino and underscore the need for further dedicated short-baseline experiments to decisively confirm or refute this signal.

Abstract

The deficit observed in the Gallium radioactive source experiments is interpreted as a possible indication of the disappearance of electron neutrinos. In the effective framework of two-neutrino mixing we obtain $\sin^{2}2\vartheta \gtrsim 0.03$ and $Δ{m}^{2} \gtrsim 0.1 \text{eV}^{2}$. The compatibility of this result with the data of the Bugey and Chooz reactor short-baseline antineutrino disappearance experiments is studied. It is found that the Bugey data present a hint of neutrino oscillations with $0.02 \lesssim \sin^{2}2\vartheta \lesssim 0.08$ and $Δ{m}^{2} \approx 1.8 \text{eV}^{2}$, which is compatible with the Gallium allowed region of the mixing parameters. This hint persists in the combined analyses of Bugey and Chooz data, of Gallium and Bugey data, and of Gallium, Bugey, and Chooz data.

Figures (9)

• Figure 1: Allowed regions in the $\sin^{2}2\vartheta$--$\Delta{m}^{2}$ plane obtained from the fits of the results of the two GALLEX ${}^{51}\text{Cr}$ radioactive source experiments, Cr1 and Cr2, and the SAGE ${}^{51}\text{Cr}$ and ${}^{37}\text{Ar}$ radioactive source experiments. The curves in the GALLEX Cr1 and SAGE ${}^{51}\text{Cr}$ plots exclude the region on the right. In the GALLEX Cr2 and SAGE ${}^{37}\text{Ar}$ plots, the pairs of $1\sigma$ and $2\sigma$ curves delimit allowed regions, whereas the $3\sigma$ curves exclude the region on the right.
• Figure 2: Allowed regions in the $\sin^{2}2\vartheta$--$\Delta{m}^{2}$ plane and marginal $\Delta\chi^{2}$'s for $\sin^{2}2\vartheta$ and $\Delta{m}^{2}$ obtained from the combined fit of the results of the two GALLEX ${}^{51}\text{Cr}$ radioactive source experiments and the SAGE ${}^{51}\text{Cr}$ and ${}^{37}\text{Ar}$ radioactive source experiments. The best-fit point corresponding to $\chi^2_{\text{min}}$ is indicated by a cross.
• Figure 3: 90% C.L. exclusion curves in the $\sin^{2}2\vartheta$--$\Delta{m}^{2}$ plane obtained from a raster-scan analysis of Bugey data (solid line) and from a standard global least-squares fit (dashed line).
• Figure 4: Allowed regions in the $\sin^{2}2\vartheta$--$\Delta{m}^{2}$ plane and marginal $\Delta\chi^{2}$'s for $\sin^{2}2\vartheta$ and $\Delta{m}^{2}$ obtained from the least-squares analysis of Bugey data. The best-fit point corresponding to $\chi^2_{\text{min}}$ is indicated by a cross.
• Figure 5: Best fit of Bugey data (points with error bars Declais:1995su). The three panels show the ratio $R$ of observed and expected (in the case of no oscillation) event rates at the three source-detector distances in the Bugey experiment as functions of the measured positron kinetic energy $E$ (see Eq. (\ref{['016']})). In each panel, the solid and dashed histograms correspond, respectively, to the best-fit values of $\left( A a_{j} + b \left( E_{ji} - E_{0} \right) \right) R_{ji}^{\text{the}}$ and $R_{ji}^{\text{the}}$ (see Eq. (\ref{['015']})).