Probing the Structure of Exotic and Conventional Hadrons

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Quantum Chromodynamics (QCD)is the microscopic theory of the strong interactions based on the theory of quarks and gluons that allows us to understand how hadrons (baryons, including the protons and neutrons that make up an atomic nucleus, and mesons and their excitations) are formed. For over a half century, the only known hadrons fit into the categories of either 3-quark compounds (baryons) or quark-antiquark compounds (mesons). Since 2003, almost 3 dozen new "exotic" hadrons, fitting into the categories of 4-quark tetraquarks and 5-quark pentaquarks, have been discovered. That said, fundamental questions like the reason for the multiplicity and spacing between masses of these new particles, as well as the question of how the quarks inside them arrange themselves, are still unresolved. The goal of this project is to develop a physical paradigm (pioneered by the PI and his collaborators) called the dynamical diquark picture into a fully predictive model that will be used to compute the physical properties of exotic hadrons. This model will be developed with the participation of a graduate student mentored by the PI. The primary project research is the development of a predictive model to describe the heavy-quark exotic hadrons that have been (and continue to be) discovered in the charmonium and bottomonium sectors. Even fundamental issues about exotics like their basic structure are not yet settled. Recently, the PI and his collaborators developed the "dynamical" diquark picture -- in which combinations of two quark (or antiquark) combinations called diquarks rapidly separate from each other, but are still bound by QCD color confinement -- as a possible structure for tetraquark states. The development of a full model implementing this picture forms the PI's principle research in this project. The model will be developed using an effective field theory based upon the Born-Oppenheimer (B-O) approximation, partly in collaboration with senior researchers at other academic institutions. Parameters for the potential energy functions needed in the B-O approximation can be gleaned from the results of lattice numerical simulations. Working out all the implications of this model will form the Ph.D. project for a student mentored by the PI. The masses, decay modes, and internal structure of exotics predicted by the model will then be compared directly to experiment. Numerous yet-unseen states are likely to be predicted.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.