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

In the context of three-neutrino oscillations, we study the possibility of using antineutrinos from nuclear reactors to explore the 10(-4) eV(2) < Delta m(.)(2) less than or similar to 8 x 10(-4) eV(2) region of the LMA MSW solution of the solar neutrino problem and measure Deltam(.)(2) with high precision. The KamLAND experiment is not expected to determine Deltam(.)(2) 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 (nu) over bar (e) oscillations in vacuum, we show that the optimal baseline of such an experiment is L similar to (20-25) km. Furthermore, for 10(-4) eV(2) < Delta m(.)(2) less than or similar to 5 x 10(-4) eV(2), 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-Deltam(2) driven oscillations; for larger values of Deltam(.)(2) not exceeding 8.0 x 10(-4) eV(2), a shorter baseline, L congruent to 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 Deltam(2), a sufficiently large sin(2) theta and a non-maximal sin(2) 2theta., where theta and theta. 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 (nu) over bar (e) + p --> e(+) + n. (C) 2002 Elsevier Science B.V. All rights reserved.