Unifying the Diquark and Molecular Models for Exotic Hadrons

The strong nuclear force, responsible for packing protons and neutrons together into the tiny space of the atomic nucleus, is also the driving force that combines fundamental “quark” particles into compounds called “hadrons”. For 50-years after the development of the quark model, only two types of hadrons were known: 3-quark “baryons” (including protons and neutrons) and 1-quark/1-antiquark “mesons”.

Since 2003, however, 50 new particles have been discovered that are apparent “tetraquarks” or “pentaquarks”, and more are being found every year. Nevertheless, their internal structure, which influences their external properties and even how many there are, remains unresolved. The two most prominent models for this structure are to treat the new particles either as molecules of known hadrons, or as bound states of “diquarks”.

Under previous NSF funding, the PI developed a variant of the latter, the “dynamical diquark model”, that has been particularly successful in explaining data, and included PhD and undergraduate students in the research. Under the new grant, the PI will show (again, with junior colleagues) how to generalize the model by including physical effects that molecular models incorporate especially well. This work will produce the best unified model in existence for these new types of particle.

Nearly all particles discovered in the 21st century are either conventional heavy-quark hadrons that fill gaps in the quark model, or exotic hadrons that are likely tetraquarks or pentaquarks. Of the 50 exotics observed to date, no consensus has emerged regarding their spectrum or structure. The best-known alternatives are di-hadron molecules and bound states of diquark quasiparticles.

The PI recently developed a complete model of exotics based solely upon diquarks interacting via the Born-Oppenheimer (adiabatic) approximation, and obtained multiple quantitative results supported by existing measurements to a surprising degree. However, the remarkable proximity of many exotics to di-hadron thresholds suggests a strong molecular component as well.

Including both diquark and molecular components amounts to handling level crossings in a diabatic formulation. The PI will apply the atomic-physics diabatic formalism of configuration mixing to the mixing of diquark-based exotic hadronic states with amplitude enhancements that occur near di-hadron thresholds (a primary feature of molecular models), and obtain detailed predictions for the spectrum and decay properties of exotics, thus creating the most physically realistic model for exotics to date.

The approach is straightforward to implement, and as demonstrated by the PI’s recent publications, is accessible to both PhD students and advanced undergraduates.

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.