Testing the very-short-baseline neutrino anomalies at the solar sector

TL;DR

This work investigates the impact of a light sterile neutrino in a 3+1 framework on solar-neutrino phenomenology, motivated by short-baseline anomalies. By adopting a tailored 4×4 mixing parameterization and solving the MSW problem in the solar environment, the authors derive vacuum and matter oscillation probabilities and perform a global fit to solar and KamLAND data with updated reactor fluxes. They find that the solar sector is sensitive to the mixing with the sterile state, with the amplitude $|U_{e4}|$ comparable to the standard $|U_{e3}|$ and a degeneracy between them such that current data cannot distinguish whether the admixture involves $\nu_3$ or a sterile state; the significance depends on the reactor flux model (about $1.3$–$1.8\sigma$). The results imply that in 3+s schemes, nonzero electron-neutrino mixing with distant mass eigenstates can mimic the $\theta_{13}$ signal, underscoring the need for complementary measurements to break degeneracies and to test sterile-neutrino hypotheses across different experimental arenas.

Abstract

Motivated by the accumulating hints of new sterile neutrino species at the eV scale, we explore the consequences of such an hypothesis on the solar sector phenomenology. After introducing the theoretical formalism needed to describe the MSW conversion of solar neutrinos in the presence of one (or more) sterile neutrino state(s) located "far" from the (nu_1,nu_2) "doublet", we perform a quantitative analysis of the available experimental results, focusing on the electron neutrino mixing. We find that the present data posses a sensitivity to the amplitude of the lepton mixing matrix element U_e4 --- encoding the admixture of the electron neutrino with a new mass eigenstate --- which is comparable to that achieved on the standard matrix element U_e3. In addition, and more importantly, our analysis evidences that, in a 4-flavor framework, the current preference for |U_e3|>0 is indistinguishable from that for |U_e4|>0, having both a similar statistical significance (which is ~ 1.3 sigma adopting the old reactor fluxes determinations, and ~ 1.8 sigma using their new estimates). We also point out that, differently from the standard 3-flavor case, in a 3+1 scheme the Dirac CP-violating phases cannot be eliminated from the description of solar neutrino conversions.

Figures (3)

• Figure 1: Region allowed in the [$\sin^2\theta_{12}, \sin^2\theta_{13}$] plane for $\theta_{14} =0$, after marginalization of $\Delta m^2_{12}$ as constrained by KamLAND, separately (left panel) by solar (S) and KamLAND (K) data and by their combination (right panel). In both panels it has been set $\theta_{24} = \theta_{34} =0$. The contours refer to $\Delta \chi^2 =1$ (dotted line) and $\Delta \chi^2 = 4$ (solid line).
• Figure 2: Region allowed in the [$\sin^2\theta_{12}, \sin^2\theta_{14}$] plane for $\theta_{13} =0$, after marginalization of $\Delta m^2_{12}$ as constrained by KamLAND, separately (left panel) by solar (S) and KamLAND (K) data and by their combination (right panel). In both panels it has been set $\theta_{24} = \theta_{34} =0$. The contours refer to $\Delta \chi^2 =1$ (dotted line) and $\Delta \chi^2 = 4$ (solid line).
• Figure 3: Region allowed, after marginalization of $\Delta m^2_{12}$ and $\theta_{12}$, by the combination of solar and KamLAND data (for $\theta_{24} = \theta_{34} =0$). The contours refer to $\Delta \chi^2 =1$ (dotted line) and $\Delta \chi^2 = 4$ (solid line).