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 ×10²⁰ 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 3.3 σ excess of events observed in the primary analysis disfavors 0.1 π <δ_{C P}<0.5 π in the inverted mass hierarchy at the 90% C.L.