Short-Baseline Electron Neutrino Disappearance, Tritium Beta Decay and Neutrinoless Double-Beta Decay

TL;DR

This work tests whether short-baseline electron neutrino disappearance in a 3+1 sterile-neutrino framework can explain the MiniBooNE low-energy anomaly and Gallium source anomaly, and connects these hints to reactor and Tritium β-decay constraints. A two-flavor-like survival probability with a large mass-squared difference $\Delta m^2$ around a few eV$^2$ is used to fit data from MB, Gallium, reactor, and Tritium experiments, yielding a consistent region near $\Delta m^2 \approx 2$ eV$^2$ and $\sin^2 2\vartheta$ of order $10^{-2}$–$10^{-1}$ for different data combinations; however, a tension arises between neutrino and antineutrino datasets, which the authors quantify as a mixing-angle asymmetry $A_{\sin^22\vartheta}$ with best-fit around 0.23 (2.6σ). The predicted heavy-state contributions to the effective masses in $\beta$-decay ($m_β$) and neutrinoless double-beta decay ($m_{2β}$) lie in sub-eV ranges, potentially accessible to KATRIN and next-generation 0νββ experiments, offering a concrete experimental avenue to test the 3+1 scenario. Overall, the work highlights a viable but tension-prone sterile-neutrino interpretation of short-baseline anomalies and motivates a suite of upcoming experiments to resolve the issue.

Abstract

We consider the interpretation of the MiniBooNE low-energy anomaly and the Gallium radioactive source experiments anomaly in terms of short-baseline electron neutrino disappearance in the framework of 3+1 four-neutrino mixing schemes. The separate fits of MiniBooNE and Gallium data are highly compatible, with close best-fit values of the effective oscillation parameters Delta m^2 and sin^2 2 theta. The combined fit gives Delta m^2 >~ 0.1 eV^2 and 0.11 < sin^2 2 theta < 0.48 at 2 sigma. We consider also the data of the Bugey and Chooz reactor antineutrino oscillation experiments and the limits on the effective electron antineutrino mass in beta-decay obtained in the Mainz and Troitsk Tritium experiments. The fit of the data of these experiments limits the value of sin^2 2 theta below 0.10 at 2 sigma. Considering the tension between the neutrino MiniBooNE and Gallium data and the antineutrino reactor and Tritium data as a statistical fluctuation, we perform a combined fit which gives Delta m^2 \simeq 2 eV and 0.01 < sin^2 2 theta < 0.13 at 2 sigma. Assuming a hierarchy of masses m_1, m_2, m_3 << m_4, the predicted contributions of m_4 to the effective neutrino masses in beta-decay and neutrinoless double-beta-decay are, respectively, between about 0.06 and 0.49 and between about 0.003 and 0.07 eV at 2 sigma. We also consider the possibility of reconciling the tension between the neutrino MiniBooNE and Gallium data and the antineutrino reactor and Tritium data with different mixings in the neutrino and antineutrino sectors. We find a 2.6 sigma indication of a mixing angle asymmetry.

Figures (12)

• Figure 1: Expected number of $\nu_{e}$ events compared with MiniBooNE data, represented by the black points. The energy bins are numbered with the index $j$. The uncertainty is represented by the vertical error bars, which represent the sum of statistical and uncorrelated systematic uncertainties. (a) Expected number of $\nu_{e}$-like events $N_{\nu_{e},j}^{\text{cal}}$ calculated by the MiniBooNE collaboration. $N_{\nu_{e},j}^{\text{cal}}$ is given by the sum of the $\nu_{e}$-induced events ($N_{\nu_{e},j}^{\nu_{e},\text{cal}}$) and the misidentified $\nu_{\mu}$-induced events ($N_{\nu_{e},j}^{\nu_{\mu},\text{cal}}$). (b) Best-fit value of the number of $\nu_{e}$-like events $N_{\nu_{e},j}^{\text{the}}$ obtained with the hypothesis of $\nu_{e}$ disappearance. $N_{\nu_{e},j}^{\text{the}}$ is given by the sum of $N_{\nu_{e},j}^{\nu_{e},\text{the}} = f_{\nu} P_{\nu_{e}\to\nu_{e}}^{(j)} N_{\nu_{e},j}^{\nu_{e},\text{cal}}$ and $N_{\nu_{e},j}^{\nu_{\mu},\text{the}} = f_{\nu} N_{\nu_{\mu}}^{\text{cal}}$. The best-fit values of $f_{\nu}$, $\sin^2 2\vartheta$ and $\Delta{m}^2$ are those in the first column of Tab. \ref{['017']} (MB$\nu$).
• 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 fit of MiniBooNE neutrino data. The best-fit point is indicated by a cross.
• Figure 3: 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 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 combined fit of the results of MiniBooNE neutrino data and the data of the gallium radioactive source experiments. The best-fit point corresponding to $\chi^2_{\text{min}}$ is indicated by a cross.
• Figure 5: $\Delta\chi^2$ as a function of $m_{\beta}$. The horizontal lines correspond to the indicated value of confidence level. The dashed and dotted lines have been obtained, respectively, from the results in Eqs. (\ref{['030']}) and (\ref{['031']}) of the Mainz and Troitsk Tritium $\beta$-decay experiments. The solid line is the result of the combined fit.