New look at the degeneracies in the neutrino oscillation parameters and their resolution by T2K, NOνA and ICAL

Abstract

The three major unknown neutrino oscillation parameters at the present juncture are the mass hierarchy, the octant of the mixing angle θ₂₃ and the CP phase δ_CP. It is well known that the presence of hierarchy-δ_CP and octant degeneracies affects the unambiguous determination of these parameters. In this paper, we show that a comprehensive way to study the remaining parameter degeneracies is in the form of a generalized hierarchy-θ₂₃−δ_CP degeneracy. This is best depicted as contours in the test (θ23−δ_CP) plane for different representative true values of parameters. We show that the wrong-hierarchy and/or wrong-octant solutions can be further classified into eight different solutions depending on whether they occur with the wrong or right value of δ_CP. These eight solutions are different from the original eightfold degenerate solutions and can exist, in principle, even if θ₁₃ is known. These multiple solutions, apart from affecting the determination of the true hierarchy and octant, also affect the accurate estimation of δ_CP. We identify which of these eight different degenerate solutions can occur in the test (θ23−δ_CP) parameter space, taking the long-baseline experiment NOνA running in the neutrino mode as an example. The inclusion of the NOνA antineutrino run removes the wrong-octant solutions appearing with both right and wrong hierarchy. Adding T2K data to this resolves the wrong hierarchy-right octant solutions to a large extent. The remaining wrong-hierarchy solutions can be removed by combining NOνA+T2K with atmospheric neutrino data. We demonstrate this using ICAL@INO as the prototype atmospheric neutrino detector. We find that the degeneracies can be resolved at the 2σ level by the combined data set, for the true parameter space considered in the study.