SS-6: Deuteron Observables Beyond Binding: Scope and Limits of the Base-to-Base Picture

Abstract: SS-6 explores whether SS-5's deuteron bipyramid predicts further deuteron observables — the quadrupole moment, D-state admixture, magnetic moment, matter radius, and low-energy scattering parameters — with comparable zero-parameter economy. The definitive negative finding is that the rigid bipyramid's intrinsic quadrupole is -0.22 fm² (oblate), which projects to a spectroscopic contribution an order of magnitude smaller than and opposite in sign to the observed prolate +0.286 fm². This is not a failure of the mechanism but reveals that the quadrupole, radius, D-state, and magnetic moment are dominated by the orbital wavefunction at r ~ 2 fm rather than the bipyramid at ~0.4 fm. Two consistency checks: the universal Bethe-Peierls relation gives the np scattering length to -20%, and inverting the effective-range expansion recovers the effective range to +0.8%. Two new open problems are registered: derivation of the short-range potential and of the deuteron orbital wavefunction.