Heavy quarkonium: progress, puzzles, and opportunities (1010.5827v3)

Abstract: A golden age for heavy quarkonium physics dawned a decade ago, initiated by the confluence of exciting advances in quantum chromodynamics (QCD) and an explosion of related experimental activity. The early years of this period were chronicled in the Quarkonium Working Group (QWG) CERN Yellow Report (YR) in 2004, which presented a comprehensive review of the status of the field at that time and provided specific recommendations for further progress. However, the broad spectrum of subsequent breakthroughs, surprises, and continuing puzzles could only be partially anticipated. Since the release of the YR, the BESII program concluded only to give birth to BESIII; the $B$-factories and CLEO-c flourished; quarkonium production and polarization measurements at HERA and the Tevatron matured; and heavy-ion collisions at RHIC have opened a window on the deconfinement regime. All these experiments leave legacies of quality, precision, and unsolved mysteries for quarkonium physics, and therefore beg for continuing investigations. The plethora of newly-found quarkonium-like states unleashed a flood of theoretical investigations into new forms of matter such as quark-gluon hybrids, mesonic molecules, and tetraquarks. Measurements of the spectroscopy, decays, production, and in-medium behavior of c\bar{c}, b\bar{b}, and b\bar{c} bound states have been shown to validate some theoretical approaches to QCD and highlight lack of quantitative success for others. The intriguing details of quarkonium suppression in heavy-ion collisions that have emerged from RHIC have elevated the importance of separating hot- and cold-nuclear-matter effects in quark-gluon plasma studies. This review systematically addresses all these matters and concludes by prioritizing directions for ongoing and future efforts.

• The paper presents a comprehensive review of heavy quarkonium, highlighting new experimental state discoveries that challenge traditional models.
• It employs advanced effective field theories and lattice QCD simulations to calculate quarkonium properties with enhanced precision.
• The study guides future research by outlining experimental and theoretical directions to further unravel the complexities of high-energy particle physics.

Heavy Quarkonium: Progress, Puzzles, and Opportunities

The paper of heavy quarkonia, particular bound states of a heavy quark and its antiquark, continues to be an expansive field that has undergone significant progress and encountered notable challenges. This paper provides an extensive review and synthesis of the current status, theoretical advancements, and experimental observations in heavy quarkonium research, as recorded until the early 2010s. The findings discussed highlight both the strides made in understanding quarkonium systems and the puzzles that continue to intrigue the field.

Overview of Key Topics

1. Newly Identified States: The paper details the discovery of several new quarkonium states, both conventional and unconventional, across the charmonium and bottomonium spectrum. The unconventional states, including the likes of $X(3872)$ and various $Y$ states, pose challenges to traditional quarkonium models as they lie near threshold energies and exhibit properties inconsistent with the quark model predictions.
2. Theoretical Developments: The exploration of heavy quarkonia has been propelled by advancements in Effective Field Theories (EFTs) such as Nonrelativistic Quantum Chromodynamics (NRQCD) and potential NRQCD (pNRQCD). These frameworks have enabled more accurate calculations of quarkonium properties by systematically integrating out irrelevant high-energy modes and focusing on the dynamics at the energy scales pertinent to quarkonium.
3. Lattice QCD Simulations: Quarkonium states are also extensively studied using lattice QCD, which offers a non-perturbative method to directly calculate quarkonium masses and other properties. The spectrum calculations from lattice QCD play a significant role in corroborating theoretical predictions from EFT analyses.
4. Phenomena Around Thresholds: States near the open-flavor thresholds, like $X(3872)$, have received particular attention due to their complex nature. These states often necessitate models that go beyond typical quarkonium potential models, suggesting possible molecular-like structures or tetraquark states.
5. State Suppression in Medium: The suppression of quarkonium states in heavy-ion collisions is explored as an indicator of quark-gluon plasma formation, providing insight into deconfinement phenomena in quantum chromodynamics (QCD).

Significant Results and Implications

• The paper reports that the identification of new, more peculiar quarkonium-like states, particularly in the charmonium sector, reflects the observations that sometimes deviate from the quark model.
• Lattice-QCD-derived calculations show consistency with experimentally observed quarkonium masses and thereby affirm the utility of non-perturbative techniques in QCD for studying heavy hadrons.
• Future experimental facilities like the LHC and planned particle physics projects are expected to further deepen our understanding of quarkonium through studies of even higher heavy-flavor thresholds and more diverse decay modes.

Future Directions

• Advanced theoretical models will be essential for attaining a more comprehensive understanding of the unconventional states and their properties.
• Improved lattice QCD techniques with higher precision and reduced systematic uncertainties will enhance predictions, especially for states near threshold.
• The development and verification of models for quarkonium states within dense media conditions prevalent in heavy-ion collisions will continue to challenge theoretical frameworks.

The work outlined in the paper underscores the transformative progress in heavy quarkonium research, recognizing both the achievements and the enigmas that continue to propel scientific inquiries in high-energy particle physics. The ongoing integration of experimental findings with theoretical advances holds promise for unraveling the complexities of QCD and matter interactions at the most fundamental level.