Semileptonic charm decay as a test for the spectator model in the B_c-meson

The $b$ and the $c$ quark compete with each other in decays of the $B_c$-meson. We emphasize the need of obtaining reliable signals of $c \to s$ in order to assess the merits of the spectator approximation in calculating the relative strengths of the two types of decays. This, we argue, can be done by considering the decay $B_c \to B_s (B_s^*) l \bar{\nu_l}$, followed by semileptonic decays of the $B_s$. We suggest looking for like-sign dileptons together with a $D_s$ at the BTeV or LHC-B experiments, and show that such signals can be made background-free by suitable event selection.