Abstract
Neutron production in GeV-scale neutrino interactions is a poorly studied process. We have measured the neutron multiplicities in atmospheric neutrino interactions in the Sudbury Neutrino Observatory experiment and compared them to the prediction of a Monte Carlo simulation using GENIE and a minimally modified version of GEANT4. We analyzed 837 days of exposure corresponding to Phase I, using pure heavy water, and Phase II, using a mixture of Cl in heavy water. Neutrons produced in atmospheric neutrino interactions were identified with an efficiency of $15.3\%$ and $44.3\%$, for Phase I and II respectively. The neutron production is measured as a function of the visible energy of the neutrino interaction and, for charged current quasi-elastic interaction candidates, also as a function of the neutrino energy. This study is also performed classifying the complete sample into two pairs of event categories: charged current quasi-elastic and non charged current quasi-elastic, and $\nu_{\mu}$ and $\nu_e$. Results show good overall agreement between data and Monte Carlo for both phases, with some small tension with a statistical significance below $2\sigma$ for some intermediate energies.
Highlights
During the past few years, great advances in our understanding of neutrino interactions in the 100 MeV ∼ 10 GeV energy range have been achieved
Production of neutrons in neutrino interactions is a complicated process that depends on neutrino-nucleon cross sections; on the interactions of the produced particles within the nuclear media, known as the final state interactions (FSIs); and on the hadronic interactions of the final state particles that propagate in the detector media
We identify whether the particle is electronlike or muonlike by exploiting the fact that the angular distribution of the emitted photons is much broader for electrons than for muons, due to the more pronounced electron scattering and secondary gamma-ray emission
Summary
During the past few years, great advances in our understanding of neutrino interactions in the 100 MeV ∼ 10 GeV energy range have been achieved. Neutrons produced in atmospheric neutrino interactions are successfully identified with the Sudbury Neutrino Observatory (SNO), a heavy water Cherenkov detector. Neutron captures on heavy water provide a higher-energy signal than conventional water Cherenkov detectors This increases their observable energy above the typical radioactive backgrounds and allows a higher neutron detection efficiency. In Phase II (the salt phase), the heavy water volume was doped with chlorine in salt form (NaCl) at 0.2% by weight, which considerably boosted the neutron capture cross section and signal energy. In Phase III, 3He proportional counters were deployed in the detector, which provided a completely independent means of neutron detection This last phase is not used in the current analysis due to the added complexity to the geometry, which would require further study to determine the impact on our reconstruction of atmospheric neutrino interactions. The results reported in this analysis correspond to data collected during 337.25 Æ 0.02 days for Phase I and 499.45 Æ 0.02 days for Phase II
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