Probing the deviation from maximal mixing of atmospheric neutrinos

Abstract

Pioneering atmospheric muon neutrino experiments have demonstrated the near-maximal magnitude of the flavor mixing angle θ₂₃. But the precise value of the deviation D≡½−sin²θ₂₃ from maximality (if nonzero) needs to be known, being of great interest-especially to builders of neutrino mass and mixing models. We quantitatively investigate in a three-generation framework the feasibility of determining D in a statistically significant manner from studies of the atmospheric ν_μ, ν̅_μ survival probability including both vacuum oscillations and matter effects. We show how this determination will be sharpened by considering the up-down ratios of observed ν_μ̅- and ν̅_μ-induced events and the differences of these ratios in specified energy and zenith angle bins. We consider 1 Megaton year of exposure to a magnetized iron calorimeter such as the proposed India-based Neutrino Observatory detector ICAL, taking into account both energy and zenith angle resolution functions. The sensitivity of such an exposure and the dependence of the determination of D on the concerned oscillation parameters are discussed in detail. The vital use of matter effects in fixing the octant of θ₂₃ is highlighted.