Doubly heavy baryons, heavy quark-diquark symmetry, and nonrelativistic QCD

In the heavy quark limit, properties of heavy mesons and doubly heavy baryons are related by heavy quark-diquark symmetry. This problem is reanalyzed in the framework of nonrelativistic QCD (NRQCD). We introduce a novel method for deriving potential NRQCD (pNRQCD) Lagrangians for composite fields from vNRQCD, which contains quarks and antiquarks as explicit degrees of freedom and maintains manifest power counting in the velocity via a label formalism. A Hubbard-Stratonovich transformation is used to eliminate four quark interactions in vNRQCD and then quarks and antiquarks are integrated out to get effective Lagrangians for composite fields. This method is used to rederive Lagrangians for the QQ̄ and QQ sectors of pNRQCD and give a correct derivation of the O(1/mQ) prediction for the hyperfine splitting of doubly heavy baryons.