Role of Final-State Interaction Modeling in Neutrino-Energy Reconstruction and Oscillation Measurements

We present a quantitative demonstration that, without additional theoretical and experimental efforts, realistic variations in final-state interaction (FSI) modeling may alter reconstructed neutrino-energy spectra at next-generation long-baseline experiments by amounts comparable to, or larger than, variations induced by oscillation-parameter shifts at their projected precision. Using the DUNE flux and baseline as a case study, we show that these FSI-driven distortions can mimic the effects of changes in the oscillation parameters Δm_{32}^{2} or δ_{CP}, producing a potential degeneracy. Our analysis thereby underscores the urgent need for an improved characterization of FSI to enable robust constraints from near detectors through the development of theory-driven uncertainty parameterizations benchmarked with dedicated new measurements.