Unquenched Charmonium and Beyond

TL;DR

This paper reviews the importance of unquenched models in hadron spectroscopy, highlighting their success in explaining new charmonium-like states and related anomalies that quenched models cannot account for.

Contribution

It advocates for unquenched approaches incorporating coupled-channel effects as essential for a comprehensive understanding of hadronic states and anomalies.

Findings

Unquenched effects explain the $X(3872)$ low-mass puzzle.

Unquenched models predict exotic states like $Z_c(3900)$ and $Z_b(10610)$.

Unquenched effects are universal across charmonium, bottomonium, and light-flavor sectors.

Abstract

The year 2024 marked the 50th anniversary of the discovery of the $J/\psi$ particle, which unveiled the charm quark and the charmonium spectrum, instigating the "November Revolution" in particle physics. This discovery catalyzed the development of quenched potential models, most notably the Cornell model, which provided a foundational quantitative description of the hadronic spectrum. However, the landscape of hadron spectroscopy has been profoundly transformed since the turn of the 21st century with the observation of numerous charmonium-like states, such as $X(3872)$, which exhibit properties starkly at odds with quenched model predictions. These discrepancies, exemplified by the "$X(3872)$ low-mass puzzle" and the "$Y$ problem" associated with vector states like $Y(4260)$, underscore the critical limitations of the quenched approximation and signal the necessity for a new theoretical paradigm. This review synthesizes recent advances in hadronic spectroscopy, arguing that the unquenched picture, which incorporates coupled-channel effects such as hadronic loops, is essential for a unified description of these new states and associated anomalies. We demonstrate how unquenched effects provide compelling solutions to long-standing puzzles in charmonium decays (e.g., the "$\rho\pi$ puzzle" and anomalous dipion transitions), predict and explain the existence of exotic charged states like $Z_c(3900)$ and $Z_b(10610)$ via mechanisms such as Initial Single Pion Emission, and offer a framework for understanding interactions between charmonia and with nucleons. Furthermore, we emphasize the universality of unquenched effects, extending their application to bottomonium and light-flavor sectors. With improving precision, we advocate systematic development of unquenched hadronic spectroscopy.