On the width of the K⁻D atom ground state

Experiments at DAΦNE-Frascati and at J-PARC are scheduled to produce K⁻D atoms and observe their X-ray cascade down to the 1S ground state (g.s.), thereby measuring its strong-interaction width and shift away from a purely Coulomb state. A width Γ_1S≲1 keV will ensure good resolution of the X-ray transitions feeding the 1S g.s. Here we study the expected K⁻D 1S g.s. width from the perspective of global fits to level shifts and widths in heavier kaonic atoms across the periodic table, using K⁻ nuclear optical potentials constructed from K¯N chiral interaction models. Special attention is paid to the subthreshold energy at which the K¯N subsystem interacts in the K⁻D atom g.s. Within this approach we predict strong-interaction upward level shift of close to 700 eV and width of about 1.2 to 1.3 keV for the K⁻D atom 1S g.s., in fair agreement with genuinely three-body K⁻D atom calculations. Comparison is made with π⁻D atom phenomenology.