Measurement of the neutrino mixing angle $\theta_{23}$ 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 Δm322=(2.67±0.11)×10-3  eV2 and sin2θ23 at the two statistically degenerate values 0.404-0.022+0.030 and 0.624-0.030+0.022, both at the 68% confidence level. Our data disfavor the maximal mixing scenario with 2.6σ significance.