Octant sensitivity for large θ ₁₃ in atmospheric and long-baseline neutrino experiments

Abstract

One of the unknown parameters in neutrino oscillation studies is the octant of the atmospheric neutrino mixing angle θ ₂₃. In this paper, we discuss the possibility of determining the octant of θ ₂₃ in the long-baseline experiments T2K and NOνA in conjunction with future atmospheric neutrino detectors, in the light of non-zero value of θ ₁₃ measured by reactor experiments. We consider two detector technologies for atmospheric neutrinos — magnetized iron calorimeter and non-magnetized Liquid Argon Time Projection Chamber. We present the octant sensitivity for T2K/NOνA and atmospheric neutrino experiments separately as well as the combined sensitivity. For the long-baseline experiments, a precise measurement of θ ₁₃, which can exclude degenerate solutions in the wrong octant, increases the sensitivity drastically. For θ ₂₃ = 39° and sin2 2θ ₁₃ = 0.1, at least ~ 2σ sensitivity can be achieved by T2K + NOνA for all values of δ CP for both normal and inverted hierarchy. For atmospheric neutrinos, the moderately large value of θ ₁₃ measured in the reactor experiments is conducive to octant sensitivity because of enhanced matter effects. A magnetized iron detector can give a 2σ octant sensitivity for 500 kT yr exposure for θ ₂₃ = 39°, δ CP = 0 and normal hierarchy. This increases to 3σ for both hierarchies by combining with T2K and NOνA. This is due to a preference of different θ ₂₃ values at the minimum χ 2 by T2K/NOνA and atmospheric neutrino experiments. A Liquid Argon type detector for atmospheric neutrinos with the same exposure can give higher octant sensitivity, due to the interplay of muon and electron contributions and superior resolutions. We obtain a ~ 3σ sensitivity for θ ₂₃ = 39° for normal hierarchy. This increases to ≿ 4σ for all values of δ CP if combined with T2K/NOνA. For inverted hierarchy the combined sensitivity is around 3σ.