ALP-ttbar Toy MC v1.8: Post-registration Results for an All-Pixel Three-Dimensional Vertex Fit with Reduced Material for Displaced Light ALP → e⁺e⁻

Post-registration results of version 1.8 of a small, numpy-only toy Monte Carlo study motivated by the displaced-dielectron extension (Alimena et al., arXiv:2610.38932) of the top-associated long-lived ALP strategy of Rygaard et al. (JHEP 10 (2023) 138). The preregistration was frozen and public before any v1.8 result (doi:10.5281/zenodo.24028384). v1.8 combines the two handles found in v1.6 and v1.7: the 3D two-hypothesis vertex fit with z measured at every layer (V-allpixel) at material scale factors f = 1, 0.5 and 0.25 (3.8 T), plus a truth-level diagnostic r–z covariance with the true momentum at every material surface (V-oracle). The sanity gate (100.000% identity with v1.6 and v1.7) and all three validations passed; there is no deviation in the method. Limit degradation from conversions (cτ = 1 mm, 17/50/100/200 MeV), V-allpixel: f = 1: 117/38.5/13.1/3.9×; f = 0.5: 56/17.9/5.7/2.2×; f = 0.25: 26.7/8.8/3.3/1.1× (v1.6 baseline 127/68/28/8.2×). Material and z coverage reinforce each other: the 17 MeV gain from z at every layer grows from 7.8% to 23% as the material is reduced. V-oracle removes most bremsstrahlung-faked r–z openings of conversions (7.4% → 1.9%) but improves 17 MeV only from 117× to 113×, so the 17 MeV limit is set mainly by multiple scattering relative to the small opening, which revises the v1.7 interpretation. Six of seven predictions confirmed; P6 refuted in its degradation part. Low-material all-pixel trackers are pursued by next-generation concepts such as ALICE 3, ALICE ITS3 and the FCC-ee detectors; this toy only mimics such a tracker and makes no statement about their performance. Independent personal project, not affiliated with the authors of the referenced papers or with any collaboration. Only qualitative trends and ratios are meaningful. GitHub: https://github.com/wspaik5/ALP_ttbar_Toy_MC_v1.8_Postregistration_AllPixel_Reduced_Material