Proposed Search for a Fourth Neutrino with a PBq Antineutrino Source

Several observed anomalies in neutrino oscillation data can be explained by a hypothetical fourth neutrino separated from the three standard neutrinos by a squared mass difference of a few ${\mathrm{eV}}^2$. We show that this hypothesis can be tested with a PBq (ten kilocurie scale) ${}^{144}\mathrm{Ce}$ or ${}^{106}\mathrm{Ru}$ antineutrino beta source deployed at the center of a large low background liquid scintillator detector. In particular, the compact size of such a source could yield an energy-dependent oscillating pattern in event spatial distribution that would unambiguously determine neutrino mass differences and mixing angles.

Figure 1

Simplified decay scheme of the pair ${}^{144}\mathrm{Ce}\mathrm{\text{−}}{}^{144}\mathrm{Pr}$.

Figure 2

Advantage of ${\overline{\nu }}_e$ sources providing both $R$ and $E_{\mathrm{vis}}$ oscillation patterns. IBD rate for a 50 kCi ${}^{144}\mathrm{Ce}$ source deployed at a center of a LLSD, in 10 cm radius bins and 100 keV bins of visible energy, $E_{\mathrm{vis}}=E_e+2{\mathrm{m}}_e$. In one year, 38 000 ${\overline{\nu }}_e$ interact between 1.5 and 6 m radius, for $\Delta m_{\mathrm{new}}^2=2{\mathrm{eV}}^2$ and ${sin}^2(2{\theta }_{\mathrm{new}})=0.1$.

Figure 3

Signal rate of a 50 kCi ${}^{144}\mathrm{Ce}$ deployed for one year at a center of a LLSD (plain line), compared to the sum of all identified background rates (dashed line), as a function of the detector radius in 10 cm concentric bins. A shield made of 33 cm of W and 2 cm of Cu attenuates the backgrounds, dominated primarily by ${}^{144}\mathrm{Ce}$ $\gamma$ lines, then by external bremsstrahlung of the ${}^{144}\mathrm{Ce}$ $\beta$-decay electrons slowing down in the cerium material.

Figure 4

95% C.L. exclusion limit of the $50 \mathrm{kCi}·\mathrm{y}$ ${}^{144}\mathrm{Ce}$ experiment obtained in the $\Delta m_{\mathrm{new}}^2$ and ${sin}^2(2{\theta }_{\mathrm{new}})$ plane (2 dof). Our result (plain and dashed lines with and without knowledge of source activity) is compared to the 90% and 95% C.L. inclusion domains given by the combination of reactor neutrino experiments, Gallex and Sage calibration sources experiments, MiniBooNE as described in Fig. 8 of 5 (gray areas).