Geant4Reweight is an open-source C++ framework that allows users to 1) weight tracks produced by the GEANT4 particle transport Monte Carlo simulation according to hadron interaction cross section variations and 2) estimate uncertainties in GEANT4 interaction models by comparing the simulations hadron interaction cross section predictions to data. The ability to weight hadron transport as simulated by GEANT4 is crucial to the propagation of systematic uncertainties related to secondary hadronic interactions in current and upcoming neutrino oscillation experiments, including MicroBooNE, NOvA, and DUNE, as well as hadron test beam experiments such as ProtoDUNE. We provide motivation for weighting hadron tracks in GEANT4 in the context of systematic uncertainty propagation, a description of GEANT4s transport simulation technique, and a description of our weighting technique and fitting framework in the momentum range 0--10 GeV/c, which is typical for the hadrons produced by neutrino interactions in these experiments.
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