Resolving parameter degeneracies in long-baseline experiments by atmospheric neutrino data

In this work we show that the physics reach of a long-baseline (LBL) neutrino oscillation experiment based on a superbeam and a megaton water Cherenkov detector can be significantly increased if the LBL data are combined with data from atmospheric neutrinos (ATM) provided by the same detector. ATM data are sensitive to the octant of θ23 and to the type of the neutrino mass hierarchy, mainly through three-flavor effects in e-like events. This allows to resolve the so-called θ23- and sign(Δm312)-parameter degeneracies in LBL data. As a consequence it becomes possible to distinguish the normal from the inverted neutrino mass ordering at 2σ C.L. from a combined LBL+ATM analysis if sin22θ13 > or approx. 0.02. The potential to identify the true values of sin22θ13 and the CP-phase δCP is significantly increased through the lifting of the degeneracies. These claims are supported by a detailed simulation of the T2K (phase II) LBL experiment combined with a full three-flavor analysis of ATM data in the HyperKamiokande detector