Neutrino-flux model for COHERENT

We present the neutrino-flux model for the COHERENT experiment, which relies on pion-decay-at-rest neutrinos from Oak Ridge National Laboratory's Spallation Neutron Source (SNS). The neutrino flux is a common systematic for all neutrino cross-section measurements with COHERENT detectors. Our model is based on a Geant4 simulation of the SNS proton beam and Hg target, allowing us to predict variations in the neutrino flux based on changes to the beam energy or beamline infrastructure. Based on comparison to available low-energy pion-production data, we estimate a flux uncertainty of 10%, motivating the construction of a D2O detector to benchmark the SNS neutrino flux empirically.