Octant sensitivity for large [theta]^sub 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 [theta] ^sub 23^. In this paper, we discuss the possibility of determining the octant of [theta] ^sub 23^ in the long-baseline experiments T2K and NO[nu]A in conjunction with future atmospheric neutrino detectors, in the light of non-zero value of [theta] ^sub 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/NO[nu]A and atmospheric neutrino experiments separately as well as the combined sensitivity. For the long-baseline experiments, a precise measurement of [theta] ^sub 13^, which can exclude degenerate solutions in the wrong octant, increases the sensitivity drastically. For [theta] ^sub 23^ = 39° and sin^sup 2^ 2[theta] ^sub 13^ = 0.1, at least ~ 2[sigma] sensitivity can be achieved by T2K + NO[nu]A for all values of δ ^sub CP^ for both normal and inverted hierarchy. For atmospheric neutrinos, the moderately large value of [theta] ^sub 13^ measured in the reactor experiments is conducive to octant sensitivity because of enhanced matter effects. A magnetized iron detector can give a 2[sigma] octant sensitivity for 500 kT yr exposure for [theta] ^sub 23^ = 39°, δ ^sub CP^ = 0 and normal hierarchy. This increases to 3[sigma] for both hierarchies by combining with T2K and NO[nu]A. This is due to a preference of different [theta] ^sub 23^ values at the minimum [chi] ^sup 2^ by T2K/NO[nu]A 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 ~ 3[sigma] sensitivity for [theta] ^sub 23^ = 39° for normal hierarchy. This increases to 4[sigma] for all values of δ ^sub CP^ if combined with T2K/NO[nu]A. For inverted hierarchy the combined sensitivity is around 3[sigma].