Measuring the deviation of the 2–3 lepton mixing from maximal with atmospheric neutrinos

The measurement of the deviation of the 2–3 leptonic mixing from maximal, [Formula Presented], is one of the key issues for understanding the origin of the neutrino masses and mixing. In the [Formula Presented] context we study the dependence of various observables in the atmospheric neutrinos on [Formula Presented]. We perform the global [Formula Presented]-analysis of the atmospheric and reactor neutrino data taking into account the effects of both the oscillations driven by the solar parameters ([Formula Presented] and [Formula Presented]) and the 1–3 mixing. The departure from the one-dominant mass scale approximation results into the shift of the 2–3 mixing from maximal by [Formula Presented], so that [Formula Presented] [Formula Presented]. Though the value of the shift is not statistically significant, the tendency of the allowed region to move towards smaller values of [Formula Presented] is robust. The shift is induced by the excess of the [Formula Presented]-like events in the sub-GeV sample. We show that future large scale water Cherenkov detectors can determine [Formula Presented] with accuracy of a few percent, comparable with the sensitivity of future long-baseline experiments. Moreover, the atmospheric neutrinos will provide unique information on the sign of the deviation (octant of [Formula Presented]).