The MINERvA experiment observes an excess of events containing electromagnetic showers relative to the expectation from Monte Carlo simulations in neutral-current neutrino interactions with mean beam energy of 4.5 GeV on a hydrocarbon target. The excess is characterized and found to be consistent with neutral-current ${\pi }^0$ production with a broad energy distribution peaking at 7 GeV and a total cross section of $0.26\pm 0.02(\text{stat}\mathrm{.})\pm 0.08(\text{sys.})\times {10}^{-39}{\mathrm{cm}}^2$. The angular distribution, electromagnetic shower energy, and spatial distribution of the energy depositions of the excess are consistent with expectations from neutrino neutral-current diffractive ${\pi }^0$ production from hydrogen in the hydrocarbon target. These data comprise the first direct experimental observation and constraint for a reaction that poses an important background process in neutrino-oscillation experiments searching for ${\nu }_{\mu }$ to ${\nu }_e$ oscillations.

• Received 7 April 2016

DOI:https://doi.org/10.1103/PhysRevLett.117.111801