The LMA MSW solution of the solar neutrino problem, inverted neutrino mass hierarchy and reactor neutrino experiments

Abstract

In the context of three-neutrino oscillations, we study the possibility of using antineutrinos from nuclear reactors to explore the 10−4 eV2<Δm2⊙≲8×10−4 eV2 region of the LMA MSW solution of the solar neutrino problem and measure Δm2⊙ with high precision. The KamLAND experiment is not expected to determine Δm2⊙ if the latter happens to lie in the indicated region. By analysing both the total event rate suppression and the energy spectrum distortion caused by ν̄e oscillations in vacuum, we show that the optimal baseline of such an experiment is L∼(20–25) km. Furthermore, for 10−4 eV2<Δm2⊙≲5×10−4 eV2, the same experiment might be used to try to distinguish between the two possible types of neutrino mass spectrum—with normal or with inverted hierarchy, by exploring the effect of interference between the atmospheric- and solar-Δm2 driven oscillations; for larger values of Δm2⊙ not exceeding 8.0×10−4 eV2, a shorter baseline, L≅10 km, would be needed for the purpose. The indicated interference effect modifies in a characteristic way the energy spectrum of detected events. Distinguishing between the two types of neutrino mass spectrum requires, however, a high precision determination of the atmospheric Δm2, a sufficiently large sin2θ and a non-maximal sin22θ⊙, where θ and θ⊙ are the mixing angles, respectively, limited by the CHOOZ and Palo Verde data and characterizing the solar neutrino oscillations. It also requires a relatively high precision measurement of the positron spectrum in the reaction ν̄e+p→e++n.