Researchers with the MicroBooNE Collaboration report the first measurement on argon of muon‑neutrino charged‑current interactions that produce a neutral pion (π0) and at least one proton, using “transverse kinematic imbalance” variables. These variables probe small mismatches in momentum that reveal how the struck nucleon and produced particles are altered as they travel out of the nucleus. The study finds that current interaction models do not reproduce all of the measured features at once.

The team used data from the MicroBooNE detector corresponding to 1.10×10^21 protons‑on‑target from the Booster Neutrino Beam. They selected events with one muon, exactly one neutral pion, and at least one proton, and applied thresholds: muon kinetic energy above 40 MeV, leading proton kinetic energy above 60 MeV, and no charged pions or heavier mesons above 40 MeV. For that signal definition they report a flux‑integrated total cross section of 1.62×10^−38 cm^2 per argon nucleus. The new selection and reconstruction raise the analysis efficiency to 12.7% (from 8.6% in a previous MicroBooNE result) and the purity to 75.6% (from 69%).

Transverse kinematic imbalance (TKI) describes how the vector sum of outgoing momenta in the plane perpendicular to the incoming neutrino should vanish if the initial nucleon were at rest and nothing else happened. The authors reconstruct the three‑momentum of the muon, the leading proton, and the π0 (which decays to two photons) and form TKI observables such as the magnitude of the missing transverse momentum (δpT) and its orientation angle (δαT). Nonzero values of these observables point to nuclear effects like the initial motion of the struck nucleon (Fermi motion), interactions of hadrons inside the nucleus (final‑state interactions, or FSI), or to undetected particles.