Measurement of the Neutrino Mixing Angle θ23 in NOvA

This Letter reports new results on muon neutrino disappearance from NOvA, using a 14 kton detector equivalent exposure of 6.05×1020 protons on target from the NuMI beam at the Fermi National Accelerator Laboratory. The measurement probes the muon-tau symmetry hypothesis that requires maximal θ23 mixing (θ23=π/4). Assuming the normal mass hierarchy, we find Δm232=(2.67±0.11)×10−3  eV2 and sin2θ23 at the two statistically degenerate values 0.404 +0.030−0.022 and 0.624 +0.022−0.030, both at the 68% confidence level. Our data disfavor the maximal mixing scenario with 2.6σ significance.