The interplay between compact and molecular structures in tetraquarks

TL;DR

This paper investigates the dual nature of tetraquarks, showing how they can exhibit both compact and molecular structures due to strong coupling effects, within an effective field theory framework.

Contribution

It introduces an effective field theory approach treating compact tetraquarks as elementary particles to study their interplay with molecular states.

Findings

Strong coupling causes compact tetraquarks to evolve into molecular configurations.

Large-Nc QCD properties favor the formation of shallow bound molecular states.

The interplay explains the existence of many shallow bound tetraquark states.

Abstract

Due to the cluster reducibility of multiquark operators, a strong interplay exists in tetraquarks between the compact structure, resulting from the direct confining forces acting on quarks and gluons, and the molecular structure, dominated by the mesonic clusters. This issue is studied within an effective field theory approach, where the compact tetraquark is treated as an elementary particle. The key ingredient of the analysis is provided by the primary coupling constant of the compact tetraquark to the two mesonic clusters. Under the influence of this coupling, an initially formed compact tetraquark bound state evolves towards a new structure, where a molecular configuration is also present. In the strong-coupling limit, the evolution may end with a shallow bound state of the molecular type. The strong-coupling regime is also favored by the large-$N_c$ properties of QCD. The interplay between compact and molecular structures may provide a natural explanation of the existence of many shallow bound states.