Method to measure quarkonium wave function using $B_c$ decay

We propose a method that enables a direct experimental probe of the quarkonium wave function defined in potential nonrelativistic QCD (pNRQCD) using the three-body decay of the $B_c$ meson. We show that the momentum distribution of the spectator $c$-quark in the partonic decay is proportional to the absolute square of the momentum-space wave function of the $B_c$ state. It would provide, for the first time, an experimentally accessible probe of the quarkonium wave function.