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  • Open Access

Neutron measurements from antineutrino hydrocarbon reactions

M. Elkins et al. (MINERvA Collaboration)
Phys. Rev. D 100, 052002 – Published 5 September 2019
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  • INTRODUCTION
  • MINERvA EXPERIMENT
  • SIMULATION
  • EVENT SAMPLE AND NEUTRON SELECTION
  • MEASUREMENTS OF NEUTRON ACTIVITY
  • CONCLUSION
  • ACKNOWLEDGMENTS
    • References

    Abstract

    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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    • Received 15 January 2019

    DOI:https://doi.org/10.1103/PhysRevD.100.052002

    Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

    Published by the American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Nucleus-neutrino interactions
    Particles & Fields

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    Issue

    Vol. 100, Iss. 5 — 1 September 2019

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    • Figure 1
      Figure 1

      Neutron energy spectra (upper) and multiplicity (lower) from three popular neutrino event generators for 3 GeV antineutrinos interacting in CH. There exists a wide range of predictions, especially for the lowest-energy component of this spectrum (see the text for a description).

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    • Figure 2
      Figure 2

      Event display of a simulated event illustrating geometrical selections to avoid activity near the muon, event interaction point, and other charged hadron activity (a π− in this simulated event) with the remaining activity promoted to a neutron candidate. The aspect ratio for this figure exaggerates the transverse dimension by almost a factor of 2 in order to emphasize the detail. Activity from the π− in adjacent U and V planes near the interaction point is not shown.

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    • Figure 3
      Figure 3

      Distribution of neutrons produced by the genie model for the selected q3<0.8  GeV/c sample and the subset of neutrons that produced one or more neutron candidates. The ratio in the bottom panel is the efficiency to find at least one candidate from each neutron.

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    • Figure 4
      Figure 4

      Predicted candidate energy spectrum from genie neutrons in three energy ranges from the selected sample. The similarity of the spectra prevent a robust, direct calorimetric neutron energy measurement.

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    • Figure 5
      Figure 5

      Distribution of measured energy deposit Edep per neutron candidate, normalized by the total number of events. Data are shown with statistical uncertainties only; the simulation is shown with systematic uncertainties. The lowest bin only contains candidates down to the 1.5 MeV threshold. The lower panels contain ratios to the reference simulation for the data and for modifications to the genie and Geant4 simulations, which will serve as a benchmark and are described in the text and used hereafter. Bins with very large data statistical uncertainties are not shown.

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    • Figure 6
      Figure 6

      Time of the candidate relative to the time of the interaction. Data are shown with statistical uncertainties only; the simulation is shown with systematic uncertainties. Neutron candidates with energy deposits less than 10 MeV are shown for both ranges of q3 in the upper plots, and higher-energy candidates are the lower plots. Bins with very large data statistical uncertainties are not shown.

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    • Figure 7
      Figure 7

      Position of the candidate relative to the interaction point in the upstream/downstream direction. Data are shown with statistical uncertainties only; the simulation is shown with systematic uncertainties. Neutron candidates with energy deposits less that 10 MeV are shown for both ranges of q3 in the upper plots, and higher-energy candidates are the lower plots. Bins with very large data statistical uncertainties are not shown.

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    • Figure 8
      Figure 8

      Apparent 1/β of the particle causing the neutron candidates, expressed as a fraction of the speed of light. Data are shown with statistical uncertainties only; the simulation is shown with systematic uncertainties. Bins with very large data statistical uncertainties are not shown.

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    • Figure 9
      Figure 9

      Detector resolutions on the time and distance inputs to 1/β from the simulation, only for neutron candidates of which the origin was a neutron from the genie simulation. There is no timing bias, and the Gaussian fit to the timing resolution has σ=4.5  ns. At the speed of light, the mean and rms of the distance distribution correspond to 0.15 and 0.3 ns. Neutron multiple scattering effects are not included in the lower plot.

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    • Figure 10
      Figure 10

      Candidate multiplicity distribution for all six subsamples, 0<q3<0.4 (left) and 0.4<q3<0.8  GeV/c (right), with subpanels for the QE-rich, dip, and Δ-rich regions. The top plot shows the reference MnvGENIE-v1.1 simulation with a solid line and error band and two variations that turn off completely the 2p2h component and then also turn off the RPA component. The next row shows the difference from the reference simulation. The middle (lower) row of difference plots uses the modified Geant4 benchmark (modified genie benchmark) for all distributions.

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    • Figure 11
      Figure 11

      Candidate proton multiplicity distribution for all six subsamples, 0<q3<0.4 (left) and 0.4<q3<0.8  GeV/c (right), with subpanels for the QE-rich, dip, and Δ-rich regions. The top plot shows the reference MnvGENIE-v1.1 simulation with a solid line and error band and one variation that turns off completely the 2p2h component. The subpanels show the difference from the reference MnvGENIE-v1.1 for two additional 2p2h variations: the dotted line enhances only the pn initial states which give nn final states (FS), and the dashed line enhances only the pp initial states leading to pn final states.

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