Hadrons with Two Heavy Quarks

We explore the symmetries and systematics of the mass spectrum of hadrons with two heavy quarks. We also apply these symmetries to the strong decays of baryons with two heavy quarks. We explore the ramifications of a proposed symmetry that relates heavy diquarks to doubly heavy mesons. We present a method for determining how the excitation energy of a system containing two heavy quarks will scale as one changes the strength of the interactions and the reduced mass of the system. We use this to derive consequences of the heavy diquark-doubly heavy meson symmetry. We compare these consequences to the results of a quark model as well as the experimental data for doubly heavy mesons. We discuss the extension of the superflavor symmetry of Doubly Heavy Baryons (DHBs) to states which contain an excited heavy diquark, and we examine some of the consequences of this symmetry for the spectroscopy of DHBs and heavy mesons. Subsequently, we derive the consequences of this Heavy Diquark Symmetry (HDS) for the decay widths of DHBs. We compare these symmetry constraints to results from the 3P0 model for strong decays. The quark model we implement was not constructed with these symmetries and contains interactions which explicitly break HDS. Nevertheless these symmetries emerge. We argue that the 3P0 model and any other model for strong transitions which employs a spectator assumption explicitly respects HDS. We also explore the possibility of treating the strange quark as a heavy quark and apply these ideas to Ξ, Ξc, and Ξb baryons.