This paper reports the first measurement of charged-current coherent pion production by muon neutrinos on argon at sub-GeV energies. Using data from the MicroBooNE liquid argon time projection chamber (LArTPC) exposed to the Fermilab Booster Neutrino Beam, the collaboration measures a flux-averaged cross section of (9.1 ± 1.2_stat ± 1.2_syst) × 10^−40 cm^2 per argon nucleus. The neutrino beam in this analysis has a mean energy of 0.8 GeV and the result uses 1.26 × 10^21 protons on target (POT) collected from 2015 to 2020.

The experiment recorded interactions in an 85-ton active mass detector placed 470 m from the beam source. The detector’s active volume and fine-grained tracking allow separate reconstruction of the muon and pion that come out of the interaction. The team used detailed simulations of the neutrino flux and interactions, overlaid real ‘‘beam-off’’ data to reproduce cosmic-ray backgrounds, and ran a pattern-recognition reconstruction called Pandora to build particle tracks and to reject cosmic rays and other backgrounds.

Coherent charged-current pion production is a simple two-body process. A muon neutrino interacts with the whole nucleus and produces a muon and a charged pion while leaving the nucleus in its ground state. Because the nucleus does not break up, the muon and pion tend to go forward and carry most of the neutrino energy. That simple geometry makes the process useful as a possible in-situ way to check the neutrino flux used by long-baseline experiments, including the planned DUNE experiment.

The analysis relies on specific models in the simulation. The baseline interaction model implemented in the GENIE event generator uses the Berger–Sehgal treatment of coherent scattering, which builds on a theoretical idea called the partially conserved axial current (PCAC) hypothesis. The authors note that the connection between coherent production and pion–nucleus scattering is exact only at high energy, and that generator predictions differ at lower energies. In simulation the coherent signal is rare, predicted to be only about 0.15% of neutrino interactions, and the dominant experimental background comes from resonant pion production.