ALP-ttbar Toy MC v1.5 — Preregistration: Two-Hypothesis Vertex Fit (Conversion vs. ALP Opening Angle) for Displaced Light ALP → e⁺e⁻

Preregistration (frozen before any v1.5 result) of version 1.5 of an independent toy Monte Carlo series motivated by the displaced-dielectron study of long-lived axion-like particles in top-quark pair events (Alimena et al., arXiv:2610.38932), which extends the displaced-dimuon strategy of Rygaard et al. (JHEP 10 (2023) 138). v1.4 (doi:10.5281/zenodo.23880375) showed that a bremsstrahlung-aware fit gives well-calibrated conversion vertices, but the limit degradation from photon conversions stays large (126x / 68x / 32x / 14x at 17 / 50 / 100 / 200 MeV, ctau = 1 mm). One reason is on the signal side: under the conversion hypothesis, the ALP opening angle is absorbed by a shift of the fitted vertex. v1.5 fits each pair under both a conversion hypothesis and an ALP hypothesis (free opening angle) on identical simulated hits. Pairs with a large chi-square difference are exempted from the material veto. The document fixes in advance the fits, the selection, a sanity gate, validation criteria (X1, X2) and six quantitative predictions (P1-P6). A variant without the mass window (V-nomass) guards against double counting the opening-angle information. An outlook section (context only, not tested) notes that the residual mismatch scales as 1/B and that conversions and bremsstrahlung scale with the material budget, with references to ALICE reduced-field data, the FCC-ee detector concepts and ALICE 3. The PDF reproduces the frozen README.md unchanged (SHA-256 59e18a40f928f4eb3f08c26ab8c0c852d0a925510bf2828fac4f39f446a1524c) and adds two figures made only from an analytic formula and published v1.3/v1.4 numbers. No v1.5 simulation had been run when it was written. The zip contains the same files as the GitHub repository, including the script that draws the two figures. Independent personal toy study, not affiliated with the authors of the referenced papers, the CMS or ALICE Collaborations, or the FCC study. Only qualitative trends and ratios are meaningful.