New molecular bonds existing in the strong interaction

Abstract: Similar to the covalent bond in chemical molecules induced by shared electrons, we proposed in [Commun. Theor. Phys. 74 (2022) 125201] the hadronic covalent bond induced by shared light quarks to explain the $T_{cc}(3875)$ and the deuteron. In this paper we improve and extend this mechanism to explain the $Z_c(3900)$, which is bound by the shared light quark-antiquark pair along with sea quark-antiquark pairs from the vacuum. Our analysis is based on the following forward and backward reasoning: a hadronic molecule exists, iff the attraction between its components is strong enough, iff the wave functions of its components significantly overlap with each other, iff the Pauli principle is well satisfied among all the shared quarks and antiquarks. Additionally, the $X(3872)$ is so unique that we need to further consider the annihilation of the shared light quark-antiquark pair, just in line with the reasoning that the production and annihilation of sea quark-antiquark pairs should be given equal consideration. Both the production and annihilation molecular bonds exist only in the strong interaction, not in the electromagnetic interaction, and they provide a quasi-static low-energy platform for studying the QCD confinement.

• The paper introduces hadronic covalent bonds, where shared light quarks create attractive forces analogous to chemical bonds.
• It proposes production and annihilation bonds to account for complex structures seen in states like Zc(3900) and X(3872).
• The work bridges quantum chromodynamics and molecular physics, offering a fresh framework for understanding exotic hadronic interactions.

Analysis of Novel Hadronic Bonding Mechanisms in Strong Interactions

The paper "New molecular bonds existing in the strong interaction" by Hua-Xing Chen explores pioneering concepts in the field of particle physics, expanding our understanding of the strong interaction through the lens of molecular analogs. This work does not merely bridge traditional atomic and molecular physics with quantum chromodynamics (QCD) but proposes novel frameworks for categorizing and explaining observed hadronic structures, specifically focusing on exotic states such as the $T_{cc}(3875)$, $Z_c(3900)$, and $X(3872)$.

Key Concepts and Bonding Mechanisms

Chen introduces the concept of a "hadronic covalent bond," likening it to the chemical covalent bond and suggesting it is induced by shared light quarks. The work is rooted in the hypothesis that when these quarks are antisymmetric—obeying the Pauli exclusion principle—the interaction becomes attractive enough to form a hadronic molecule. This is applied to the $T_{cc}(3875)$, interpreted as a $\bar D\bar D^*$ hadronic molecule with $(I)J^P=(0)1^+$, structured via shared u/d quarks between charm antiquarks.

However, this covalent bonding is limited in scope when addressing hadrons composed of both D and $\bar{D}$ mesons. To explicate structures such as the $Z_c(3900)$ and $X(3872)$, the paper proposes two additional bonding phenomena: the "production bond" and the "annihilation bond." Both arise from interactions with sea quark-antiquark pairs, facilitating molecular states that contribute to confining quarks within a quasi-static low-energy framework. The $Z_c(3900)$ is theorized to be such a structure, characterized by shared quark-antiquark pairs emerging from the vacuum.

Empirical Implications and Theoretical Expansion

The covalent and confined bonds hold practical implications for identifying new hadronic molecules, enabled by extending the theoretical framework of QCD to embrace complex color dynamics and spin-flavor symmetries. While the binding energies, estimated within a toy model, remain simplifications, they provide a roadmap for experimental validation.

Intriguingly, the paper discusses the potential for observing additional covalent molecules such as $\bar B^*\bar B$, $\bar D^{*}\Sigma_{c}$, and even analogs for hypernuclei like $^3_\Lambda H$. The strong, weak, and repulsive bonds quantified in these scenarios emphasize the intricate balance of forces governing molecular stability, analogous yet distinct from electromagnetic interactions.

Conclusion and Future Directions

Chen’s study offers nuanced insight into QCD confinement, presenting theoretical innovations that challenge dominant paradigms of hadronic interactions. By introducing mechanisms typically reserved for atomic physics into the context of particle physics, this research lays foundational groundwork for both experimental exploration and theoretical refinement. Future work should aim to validate the hypothesized states within high-energy experiments, probing the robustness of these concepts and their ability to predict and explain exotic hadronic phenomena.

Overall, this paper represents an intersection of established theoretical constructs and innovative interpretations, urging a recalibrated understanding of strong interactions that may soon redefine common approaches within hadron physics and QCD studies alike. As further developments emerge, these bonding models could crucially enhance both our knowledge of particle physics and the methodologies employed to investigate subatomic interactions.