Resolving the octant of theta23 with T2K and NOvA

TL;DR

This paper evaluates how current and future neutrino experiments T2K and NOvA can determine whether the mixing angle theta23 is in the lower or higher octant, considering degeneracies with hierarchy and CP phase.

Contribution

It provides a combined sensitivity analysis of T2K and NOvA for resolving the octant of theta23, accounting for degeneracies and experimental configurations.

Findings

NOvA can exclude higher octant for sin^2θ23=0.41 at 2σ

Adding T2K data improves octant resolution

Equal neutrino and anti-neutrino runs enable 2σ resolution for certain sin^2θ23 ranges

Abstract

Preliminary results of MINOS experiment indicate that theta23 is not maximal. Global fits to world neutrino data suggest two nearly degenerate solutions for theta23: one in the lower octant (LO: theta23 < 45 degree) and the other in the higher octant (HO: theta23 > 45 degree). numu to nue oscillations in superbeam experiments are sensitive to the octant and are capable of resolving this degeneracy. We study the prospects of this resolution by the current T2K and upcoming NOvA experiments. Because of the hierarchy-deltacp degeneracy and the octant-deltacp degeneracy, the impact of hierarchy on octant resolution has to be taken into account. As in the case of hierarchy determination, there exist favorable (unfavorable) values of deltacp for which octant resolution is easy (challenging). However, for octant resolution the unfavorable deltacp values of the neutrino data are favorable for the anti-neutrino data and vice-verse. This is in contrast to the case of hierarchy determination. In this paper, we compute the combined sensitivity of T2K and NOvA to resolve the octant ambiguity. If sin^2\theta23 =0.41, then NOvA can rule out all the values of theta23 in HO at 2 sigma C.L., irrespective of the hierarchy and deltacp. Addition of T2K data improves the octant sensitivity. If T2K were to have equal neutrino and anti-neutrino runs of 2.5 years each, a 2 sigma resolution of the octant becomes possible provided sin^2\theta23 \leq 0.43 or \geq 0.58 for any value of deltacp.