An effective topology-transfer nuisance for Genie-based neutrino-argon semi-inclusive analyses: a methodological proposal with MicroBooNE-constrained diagnostic

Abstract: Liquid-argon TPC oscillation analyses increasingly split samples into proton-tagged (Np) and proton-less (0p) topologies at the ~35 MeV tracking threshold, but the standard GENIE Reweight library has no dial that transfers events across this boundary. We propose a single empirical nuisance parameter, $f_{\rm mig}$, that fills this gap as a per-event reweight on nominal GENIE output, implementable without event regeneration. It is presented as a methodological diagnostic: an effective topology-transfer surrogate absorbing an unspecified combination of near-threshold proton mismodelling effects (proton-side FSI, nucleon-momentum distribution, RES $Q<sup>2$ shape, MEC kinematic shape), not a measurement of any single microscopic mechanism. As a reference scale, the position-dependent mean-field energy loss absent from GENIE's cascade is estimated from the $<sup>{40}$Ar density profile to be a few percent, consistent with proton-transparency differences reported between cascade and transport generators on MicroBooNE data. The primary constraint is a direct scan on the MicroBooNE simultaneous CC 0p/Np differential cross-section release with no sector-weight regularisation: $\hat{f}<em>{\rm mig} = 0.093 \pm 0.024$, $3.8σ$ (Wilks) / $3.2σ$ (empirical). A supporting NC $Δ$ radiative sideband fit gives $$\hat{f}</em>{\rm mig} = 0.050<sup>{+0.029}_{-0.022}$$, conditional on sector-weight priors; the two datasets share flux/detector systematics and are corroborating rather than independent. Introducing $f_{\rm mig}$ also relieves a compensatory CCRES sector-weight suppression in migration-blind fits. The extracted parameter is best understood as a flux-integrated effective nuisance direction for the MicroBooNE BNB configuration, suitable as a candidate GENIE dial for future LArTPC analyses pending detector-level validation.