ALP-ttbar Toy MC v1.5: Post-registration Results for a Two-Hypothesis Vertex Fit (Conversion vs. ALP Opening Angle) of Displaced Light ALP → e⁺e⁻

Post-registration results 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). The analysis follows the preregistration doi:10.5281/zenodo.23905511, which was frozen on GitHub and Zenodo before any v1.5 result was computed. v1.5 fits every pair under two hypotheses on the identical simulated hits of v1.3/v1.4: a photon conversion (tracks parallel at the vertex) and an ALP (free opening angle). Pairs with a large chi-square difference are exempted from the material veto. Limit degradation from conversions (ctau = 1 mm) at 17 / 50 / 100 / 200 MeV: 126x / 68x / 28x / 9.1x, against 126x / 68x / 32x / 14x for v1.4. There is no gain at 17-50 MeV: a free vertex absorbs a small opening angle, and bremsstrahlung makes about 10% of conversions look opening-angle-like, as many as 17 MeV ALP pairs. Without the mass window the modest gain at 100-200 MeV nearly disappears. Three of six preregistered predictions were confirmed and three refuted. The preregistered null validation X1 failed. A post-hoc diagnosis shows why Wilks' theorem does not apply here: once the vertex is free, the opening angle is identified only at second order (chi-square rise proportional to the fourth power of the opening, measured slope 4.3), so the test lies on a parameter boundary and its null distribution is the mixture 1/2 chi2_0 + 1/2 chi2_1 (Chernoff 1954; Self and Liang 1987), confirmed quantitatively. The main result is unaffected because the threshold was scanned. Future directions: a Gaussian-sum filter with ACTS; a weak-field and low-material test (the residual mismatch scales as 1/B); a dimuon control study repeating the dielectron steps; extension to mu+mu- above 200 MeV and to other long-lived-particle signatures, including ALPs from B-meson decays; and boundary-aware reference distributions in future preregistrations. The PDF is the report; the zip contains the full code, outputs and Figures 1-9 (same files as the GitHub repository). Independent personal toy study, not affiliated with the authors of the referenced papers, the CMS, LHCb or ALICE Collaborations, or the FCC study. Only qualitative trends and ratios are meaningful.