Symmetries of Doubly Heavy Baryons
For very heavy quarks, the two heavy quarks in a doubly heavy baryon are expected to form a pointlike, heavy diquark in an antitriplet color configuration. In this limit the dynamics of the light degrees of freedom `factorize' from the dynamics of the heavy diquark system, and a superflavor symmetry emerges which relates the properties of doubly heavy baryons to heavy mesons. The charm quark may not be heavy enough for the $cc$ diquark in a $Ξ_cc$ to be regarded as pointlike. However, there are indications from the results of a nonrelativistic constituent quark model that many of the consequences of factorization emerge even though the model does not assume a quark-diquark structure and the mean separation of the two heavy quarks in the model is not small. We discuss the consequences of factorization for the spectroscopy of doubly heavy baryons and compare these consequences to results from a quark model. We also discuss the possibility of treating the strange quark as a heavy quark and applying these ideas to the $Ξ$.