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, D-23=1/2-sin(2)theta(23), is one of the key issues for understanding the origin of the neutrino masses and mixing. In the 3nu context we study the dependence of various observables in the atmospheric neutrinos on D-23. We perform the global 3nu-analysis of the atmospheric and reactor neutrino data taking into account the effects of both the oscillations driven by the solar parameters (Deltam(21)(2) and theta(12)) 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 Deltasin(2)theta(23)approximate to0.04, so that D(23)similar to0.04+/-0.07 (1sigma). Though the value of the shift is not statistically significant, the tendency of the allowed region to move towards smaller values of sin(2)theta(23) is robust. The shift is induced by the excess of the e-like events in the sub-GeV sample. We show that future large scale water Cherenkov detectors can determine D-23 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 theta(23)).