Meson Decay Constants from Isospin Mass Splittings in the Quark Model

Decay constants of $D$ and $B$ mesons are estimated within the framework of a heavy-quark approach using measured isospin mass splittings in the $D$, $D^*$, and $B$ states to isolate the electromagnetic hyperfine interaction between quarks. The values $f_D = (262 \pm 29)$ MeV and $f_B = (160 \pm 17)$ MeV are obtained. Only experimental errors are given; possible theoretical ambiguities, and suggestions for reducing them, are noted.