ALP-ttbar Toy MC v1.6 — Preregistration: Magnetic-Field and Material-Budget Scan of the Two-Hypothesis Vertex Fit for Displaced Light ALP → e⁺e⁻

Preregistration (frozen before any v1.6 result) of version 1.6 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). v1.5 (doi:10.5281/zenodo.23924862) found no gain from a two-hypothesis vertex fit at 17-50 MeV in a 3.8 T field: a free vertex absorbs a small opening angle, leaving a mismatch below the hit resolution. This mismatch scales as 1/B, while photon conversions and bremsstrahlung scale with the material budget. v1.6 repeats the v1.5 analysis in eight configurations: a field scan (3.8, 2.0, 1.0, 0.5, 0.2 T), a material scan (0.5 and 0.25 of the nominal material at 3.8 T) and a combined point (2.0 T, half material). The document fixes in advance the configurations, the parameters that follow the field and material, a sanity gate, validation criteria (Y1-Y3, using the boundary mixture 1/2 chi2_0 + 1/2 chi2_1 learned in v1.5) and seven quantitative predictions (P1-P7). A related-work section notes that field and material optimisation, reduced-field running (ALICE) and field-based conversion rejection (phi_V) are established, and limits the claim to this specific search. The limitations note that only the transverse opening is used; a three-dimensional vertex fit and a Gaussian-sum filter are later steps. The PDF reproduces the frozen README.md unchanged (SHA-256 a9e4b64b2d6edb77f1b8e23044e46d07789b341c79324a841e825a6edb8835c4) and adds two figures made only from analytic formulas and a published v1.5 number. No v1.6 simulation had been run when it was written. The zip contains the 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.