High-precision Penning trap spectroscopy of the ground state spin structure of HD +

We present high-precision spectroscopy of the ground-state hyperfine structure of HD⁺ at 4~T. We determine the bound-electron g factor, ge,bound = -2.002\,278\,540\,96(40), to a relative uncertainty of 2×10⁻¹⁰, the most precise determination of a bound-electron g factor of a molecular ion to date. The experimental value agrees with recently developed ab initio theory that now includes quantum-electrodynamical effects up to order α⁵ and has reduced the theoretical uncertainty by three orders of magnitude [O. Kullie et al., Phys. Rev. A 112 052813 (2025)]. In addition, we extract the scalar spin-spin interaction coefficients E₄~=~925\,395.758(41)$\,$kHz (electron-proton) and E₅~=~142\,287.821(22)$\,$kHz (electron-deuteron), which show a moderate tension with another state-of-the-art theoretical prediction [M. Haidar et al., Phys. Rev. A 106 042815 (2022)].