Fully strange tetra- and penta-quarks in a chiral quark model

TL;DR

This work investigates fully strange multiquark systems within a chiral quark model using the Gaussian Expansion Method and Complex Scaling Method to treat bound, resonant, and scattering states on the same footing. By exhaustively including all $S$-wave configurations and color structures for $ss\bar{s}\bar{s}$ tetraquarks and $ssss\bar{s}$ pentaquarks, the study identifies a compact $J^P=1^+$ tetraquark near $2.3\,\text{GeV}$ that naturally interprets the X(2300), along with additional narrow exotic states featuring dominant hidden-color and $K$-type components. In the pentaquark sector, bound and narrow resonances appear in the $J^P=\tfrac{1}{2}^-$, $\tfrac{3}{2}^-$, and $\tfrac{5}{2}^-$ channels within $2.6-3.2\,\text{GeV}$, with rich internal color structures and strong channel mixing. The analysis of radii, magnetic moments, and wave-function compositions underscores the crucial roles of channel coupling and exotic color configurations in stabilizing these states, and the work proposes promising two-body decay channels to guide future experiments at facilities such as BESIII.

Abstract

Motivated by the recently reported resonant structure $X(2300)$, a strong candidate for a fully strange tetraquark with positive parity, we perform a systematic study of fully strange tetra- and penta-quark systems within a chiral quark model. Low-lying $S$-wave configurations of the $ss\bar s\bar s$ and $ssss\bar s$ systems are investigated using the Gaussian Expansion Method (GEM) combined with the Complex Scaling Method (CSM), which allows for a unified treatment of bound, resonant, and scattering states. For tetraquarks, all possible configurations: meson-meson, diquark-antidiquark, and K-type structures, with complete color bases, are incorporated, while baryon-meson and diquark-diquark-antiquark configurations are considered for pentaquarks. Several weakly bound states and narrow resonances are identified in both sectors. In particular, a compact fully strange tetraquark with $J^P=1^+$ is found near $2.3\,\text{GeV}$, providing a natural interpretation of the $X(2300)$ resonance. Additional exotic states with dominant hidden-color and K-type components are predicted in the mass ranges $1.6-3.1$ GeV for tetraquarks and $2.6-3.2$ GeV for pentaquarks. The internal structure of these states is analyzed through their sizes, magnetic moments, and wave-function compositions, highlighting the essential role of channel coupling and exotic color configurations. Finally, promising two-body strong decay channels are proposed to facilitate future experimental searches.

Figures (7)

• Figure 1: The $S$-wave configurations considered in this investigation for the fully strange tetra- and penta-quarks. Particularly, panel $(a)$ is a meson-meson structure, panel $(b)$ is a diquark-antidiquark arrangement, and four K-type configurations are from panel $(c)$ to $(f)$. Panel $(g)$ and $(h)$ is a baryon-meson and diquark-diquark-antiquark structure of pentaquark, respectively.
• Figure 2: The fully coupled-channels calculation of $ss\bar{s}\bar{s}$ tetraquark system with $J^P=0^+$ quantum numbers.
• Figure 3: The fully coupled-channels calculation of $ss\bar{s}\bar{s}$ tetraquark system with $J^P=1^+$ quantum numbers.
• Figure 4: The fully coupled-channels calculation of $ss\bar{s}\bar{s}$ tetraquark system with $J^P=2^+$ quantum numbers.
• Figure 5: The fully coupled-channels calculation of $ssss\bar{s}$ pentaquark system with $J^P=\frac{1}{2}^-$ quantum numbers.