Exotic States in Quarkonium Physics: Effective Theories of Heavy Mesonic Molecules and an AdS/QCD Model of Hybrid Quarkonium

Quantum chromodynamics (QCD), the theory of quarks and gluons, is known to be the correct description of strong nuclear interactions. At high energy and momenta, one can use QCD directly to compute quantities of physical interest related to the strong force. At low energies and momenta, one should use a different description in terms of the degrees of freedom relevant at that scale. Two approaches to achieve this end are effective field theories and gauge/gravity dualities. The former involves a field theory more or less like QCD itself, but with states which are composites of quarks and gluons. Then a perturbative expansion is made not in terms of the gauge coupling but instead in terms of the momentum of the fields. This approach dates back to the 1970s and is on firm theoretical footing. Gauge/gravity dualities are a newer and less understood technique, which relates the physics of the strong interactions to a different but likely equivalent theory in a higher dimensional space- time, where the quantity of interest can be computed more readily. We employ both effective field theories and gauge/gravity dualities to study the physics of exotic quarkonium states, that is bound states containing a heavy quark-antiquark pair which nevertheless cannot be be understood working only with the standard quark model of hadrons. Candidates for such states, long speculated to exist, have recently been observed at particle colliders, so that the theory of exotic quarkonium is now of great experimental importance.