Neutron skins from the DEAM fixed-coefficient pair interaction: zero-parameter results and predictions for exotic calcium isotopes

We present a phenomenological model of neutron skins built on the fixed-coefficient two-term central pair interaction of the Dynamic Interactive Atom Model (DEAM), in which no quantity is adjusted to data: the coefficients are fixed at the light-quark charge magnitudes (2/3, 1/3) as a structural ansatz; the length scale d = 2.068 fm follows from the saturation density; the cutoff r_c = (3/2)d is the inflection point of the pair potential; and the isovector base length b = 0.790 fm is set by the charged-pion Compton wavelength. A surface coordination-counting treatment with a self-consistent composition factor reproduces the neutron skins of 40,48Ca, 116,124Sn and 208Pb — including the negative skin of 40Ca — with unweighted RMS deviation 0.026 fm (error-weighted chi-square per datum = 0.58) over the five non-excluded cases, with no continuously adjusted parameters. The proton channel predicts a sign reversal of its contribution at N-Z ≈ (Z-1)/2; for calcium the reversal falls between 48Ca and 50Ca and steepens the skin growth — a mechanism-level, falsifiable intermediate position between the dramatic swelling originally extracted from interaction cross sections and the modest growth favored by their recent microscopic reanalysis. We report frozen-machinery values Δr_np(50Ca) = 0.215, Δr_np(52Ca) = 0.266, Δr_np(54Ca) = 0.316 (assuming the unmeasured r_ch = 3.589 fm), Δr_np(132Sn) = 0.29 and Δr_np(208Pb) = 0.235, all ± 0.03 fm, with binding falsification criteria. The 208Pb value is distinct from both the density-functional band (0.13–0.19 fm) and the PREX-2 central value (0.283 fm); MREX will be a decisive arbiter. As consistency checks, the same interaction reproduces the magic sequence 2-8-20-28-50-82-126 and binds the deuteron with the same cutoff. Predictions frozen on 7 July 2026; SHA-256 digests of the ancillary verification scripts are printed in Appendix C. The AI-assistance disclosure follows current APS policy. A Turkish parallel version is included as an additional file.