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

Abstract

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