Heavy baryon mixing in chiral perturbation theory

We discuss the SU(3) and heavy quark spin-symmetry-breaking mixing between the Ξc and Ξc^' charmed baryons. Chromomagnetic hyperfine interactions are the leading source of spin-symmetry breaking and together with the SU(3)-breaking mass differences between the lightest pseudo Goldstone bosons gives the leading contribution to the mixing. Such contributions are computed in chiral perturbation theory and compared to quark model expectations. We also compute the leading contribution to the semileptonic decay Ξb→Ξc^'lνₗ at zero recoil, and find that it is an order of magnitude smaller than naive power counting would suggest. It appears that Ξb→Ξc^'lνₗ is dominated by incalculable counterterms, and we discuss the implications for quark models based on the essential role of hyperfine interactions.