Charged-current antineutrino interactions on a hydrocarbon scintillator in the MINERvA detector are used to study activity from their final-state neutrons. To ensure that most of the neutrons are from the primary interaction, rather than hadronic reinteractions in the detector, the sample is limited to momentum transfers below 0.8 GeV/c. From 16 129 interactions, 15 246 neutral particle candidates are observed. The reference simulation predicts 64% of these candidates are due to neutrons from the antineutrino interaction directly but also overpredicts the number of candidates by 15% overall. This discrepancy is beyond the standard uncertainty estimates for models of neutrino interactions and neutron propagation in the detector. We explore these two aspects of the models using the measured distributions for energy deposition, time of flight, position, and speed. We also use multiplicity distributions to evaluate the presence of a two-nucleon knockout process. These results provide critical new information toward a complete description of the hadronic final state of neutrino interactions, which is vital to neutrino oscillation experiments.
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We are grateful to Juan Nieves, Ignacio Ruiz Simo, and Manuel Vicente Vacas for making their RPA and 2p2h code available for study and incorporation into this analysis. Samples of other neutrino event generator neutron predictions were prepared by Jake Calcutt, Luke Pickering, and Kendall Mahn using the nuisance framework. This document was prepared by members of the MINERvA Collaboration using the resources of the Fermi National Accelerator Laboratory, a U.S. Department of Energy, Office of Science, HEP User Facility. Fermilab is managed by Fermi Research Alliance, LLC (FRA), acting under Contract No. DE-AC02-07CH11359. These resources included support for the MINERvA construction project, and support for construction also was granted by the United States National Science Foundation under Grant No. PHY-0619727 and by the University of Rochester. Support for scientists for this specific publication was granted by the United States National Science Foundation under Grants No. PHY-1306944 and No. PHY-1607381. We are grateful for the United States National Science Foundation’s decade of direct support to the Soudan Underground Lab outreach program, including Grant No. PHY-1212342; this analysis originated as the research component for two summer undergraduate outreach interns. Support for participating scientists was provided by NSF and DOE (USA) by CAPES and CNPq (Brazil), by CoNaCyT (Mexico), by Proyecto Basal FB 0821, CONICYT PIA ACT1413, Fondecyt 3170845 and 11130133 (Chile), by CONCYTEC (Consejo Nacional de Ciencia, Tecnología e Innovación Tecnológica), DGI-PUCP (Dirección de Gestión de la Investigación-Pontificia Universidad Católica del Peru), and VRI-UNI (Vice-Rectorate for Research of National University of Engineering) (Peru); by the Latin American Center for Physics (CLAF); and NCN Opus Grant No. 2016/21/B/ST2/01092 (Poland). We thank the MINOS Collaboration for use of its near detector data. Finally, we thank the staff of Fermilab for support of the beamline, the detector, and the computing infrastructure.