The semileptonic decays of the lowest-lying double-heavy baryons is treated in a quark model. For the $\Xi_{ bb}$, hyperfine mixing in the spin wave function leaves the total rate for decay into the lowest lying daughter baryons essentially unchanged, but changes the relative rates into the $\Xi_{ bc}$ and $\Xi_{ bc}^\prime$. The same pattern is obtained in the decays of the $\Omega_{ bb}$. For the $\Xi_{ bc}$, this mixing leads to factor of about 17 suppression in the decay rate to the $\Xi_{ cc}^*$, and a factor of two suppression in the total decay rate. For the $\Omega_{ bc}$, the decay to the $\Omega_{ cc}^*$ is suppressed by a factor of more than 30 from the unmixed case, and the total decay rate is decreased to about 40% of the decay rate obtained when mixing is ignored.
Categories: nucl-th hep-ex hep-ph