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

Abstract

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