Heavy Baryon Mixing in Chiral Perturbation Theory

We discuss the SU(3) and heavy quark spin-symmetry breaking mixing between the Xi_c and Xi'_c 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 Xi_b -> Xi'_c l nu at zero recoil, and find that it is an order of magnitude smaller than naive power counting would suggest. It appears that Xi_b -> Xi'_c l nu is dominated by incalculable counterterms, and we discuss the implications for quark models based on the essential role of hyperfine interactions.