The Physics of Glueballs (0810.4453v1)

Abstract: Glueballs are particles whose valence degrees of freedom are gluons and therefore in their description the gauge field plays a dominant role. We review recent results in the physics of glueballs with the aim set on phenomenology and discuss the possibility of finding them in conventional hadronic experiments and in the Quark Gluon Plasma. In order to describe their properties we resort to a variety of theoretical treatments which include, lattice QCD, constituent models, AdS/QCD methods, and QCD sum rules. The review is supposed to be an informed guide to the literature. Therefore, we do not discuss in detail technical developments but refer the reader to the appropriate references.

• The paper presents a comprehensive review on glueballs by integrating nonperturbative lattice QCD and constituent models to estimate their mass spectrum.
• It utilizes QCD sum rules and AdS/QCD correspondence to predict glueball trajectories, offering insights into theoretical challenges and model consistency.
• The study outlines experimental approaches, including radiative J/ψ decays and central production, to overcome the detection challenges of glueballs.

Overview of "The Physics of Glueballs"

The paper "The Physics of Glueballs" by Vincent Mathieu, Vicente Vento, and Nikolai Kochelev provides a comprehensive review of the theoretical treatments and experimental searches for glueballs, which are bound states of gluons predicted by Quantum Chromodynamics (QCD). The authors address various methods employed in the study of glueballs, such as lattice QCD, constituent models, AdS/QCD methods, and QCD sum rules, while also discussing experimental strategies for glueball detection.

Theoretical Approaches to Glueballs

1. Lattice QCD: This is a non-perturbative approach that discretizes space-time into a lattice, allowing for numerical simulations of QCD. The paper notes that lattice calculations have identified a spectrum of glueballs in pure gauge theory, with the scalar and tensor glueballs being the lightest states. The results show reasonably good agreement across various lattice studies, with the scalar glueball mass around 1.6-1.7 GeV.
2. Constituent Models: These models consider glueballs as composed of constituent gluons, often treated with relativistic or nonrelativistic kinetic terms. The authors discuss models like the MIT bag model, potential models, and gluon mass models. Such models offer insights into the glueball spectrum but require careful handling of the gluon's degrees of freedom to avoid unwanted states.
3. AdS/QCD Correspondence: Inspired by the AdS/CFT correspondence, this approach suggests a duality between a confining gauge theory and string theory in higher-dimensional spaces. The paper discusses predictions for glueball masses and trajectories that have similarities with lattice QCD findings.
4. QCD Sum Rules: This approach uses the operator product expansion and considers nonperturbative contributions through vacuum condensates and instanton effects. The predictions include mass estimates for glueballs and insight into glueball/quarkonium mixing, particularly highlighting the importance of instantons in resolving discrepancies between different sum rules.

Experimental Signatures and Challenges

Identifying glueballs experimentally is challenging due to expected mixing with conventional mesons and the lack of distinctive decay patterns. The authors stress that glueballs are more likely to be detected in "gluon-rich" processes such as central production in hadronic collisions, radiative J/ψ decays, and certain patterns in heavy-ion collisions.

• Radiative J/ψ Decays: This channel is favored because the J/ψ can decay into two gluons and a photon, providing a cleaner environment to search for glueballs.
• Central Production: In these processes, glueball production is possible through double pomeron exchange, which is a gluon-dominant interaction.
• Photoproduction: Although less likely, photoproduction might offer insights into the vector and tensor glueballs without quark-antiquark annihilation.

The authors analyze the potential impacts of glueballs in the quark-gluon plasma (QGP) phase formed in relativistic heavy-ion collisions. They suggest that specific decays, such as into KK or ππ, might be accessible in this environment, offering indirect evidence for glueball production.

Implications and Future Directions

This review suggests that confirming glueball existence would significantly advance understanding of nonperturbative QCD and the structure of the QCD vacuum. As experimental techniques and theoretical methods continue to evolve, refining constraints on glueball properties and improving the precision of lattice QCD and sum rule predictions remain crucial.

Future directions might include more detailed phase space analyses in collider experiments like BESIII, PANDA, and GlueX, as well as further lattice QCD studies incorporating dynamical quarks to understand the rich physics of glueballs in full QCD. Additionally, improvements in QCD-inspired models that capture the essence of flux tubes and gluonic interactions will be essential for a coherent picture of glueballs within the broader hadronic landscape.