First measurement of electron neutrino appearance in NOvA

We report results from the first search for ν_μ → ν_e transitions by the NOvA experiment. In an exposure equivalent to 2.74 × 10^(20) protons on target in the upgraded NuMI beam at Fermilab, we observe 6 events in the Far Detector, compared to a background expectation of 0.99 ± 0.11 (syst) events based on the Near Detector measurement. A secondary analysis observes 11 events with a background of 1.07 ± 0.14 (syst). The 3.3σ excess of events observed in the primary analysis disfavors 0.1π < δ_(CP) < 0.5π in the inverted mass hierarchy at the 90% C.L.