[Paper Review] Glueballs from gluon jets at the LHC

Wolfgang Ochs, Peter Minkowski|arXiv (Cornell University)|Aug 2, 2011

Particle physics theoretical and experimental studies1 references3 citations

TL;DR

This paper proposes identifying glueballs—hypothetical bound states of gluons—by studying leading neutral clusters in high-energy gluon jets at the LHC. Using comparisons of x-distributions (Feynman-x) between quark and gluon jets, it suggests that a glueball candidate would appear enhanced in gluon jets but suppressed in quark jets, offering a clear signature to distinguish it from conventional q̄q states.

ABSTRACT

The existence of glueballs within QCD is uncontroversial but their experimental verification is still in doubt. We discuss the new possibilities for a search of glueballs as the leading object in gluon jets at the LHC. We summarize previous results from LEP which demonstrate a significant excess rate of electrically neutral leading clusters in comparison with MC models.

Motivation & Objective

• To address the long-standing experimental challenge of identifying glueballs, predicted by QCD but not yet conclusively observed.
• To leverage the LHC's higher energy and improved statistics to test the hypothesis that gluon jets produce enhanced neutral leading clusters due to glueball formation.
• To develop a method for distinguishing glueball candidates from conventional q̄q resonances using differential x-distributions in quark vs. gluon jets.
• To extend LEP findings of neutral cluster excesses in gluon jets by applying more precise, high-energy measurements at the LHC.
• To provide a clear experimental signature based on fragmentation behavior: glueballs should be leading in gluon jets but suppressed in quark jets.

Proposed method

• Selecting gluon jets at the LHC using high-transverse-momentum di-jet events and 3-jet events with low-momentum jets, leveraging DGLAP evolution to estimate gluon jet fractions.
• Using rapidity gaps (Δy > 1.7, up to Δy ~ 4) to isolate leading clusters and enhance sensitivity to color-neutralization mechanisms, including color octet (P₈) and triplet (P₃) neutralization.
• Measuring the invariant mass and x-distribution (x = p/p_beam) of leading clusters beyond rapidity gaps to identify resonances such as f₀(980), f₀(1500), and f₀(600).
• Comparing the x-spectra of candidate glueball states to reference spectra from well-known q̄q resonances (e.g., ρ, f₂(1270), φ(1020)) in both quark and gluon jets.
• Applying the principle that a true glueball should be suppressed in quark jets but leading in gluon jets, while mixed states or q̄q resonances would show opposite behavior.
• Utilizing data from LEP (OPAL, DELPHI, ALEPH) as a benchmark, where a 10–40% excess of neutral clusters in gluon jets was observed beyond MC expectations.

Experimental results

Research questions

• RQ1 Can the excess of neutral leading clusters observed in LEP gluon jets be confirmed and extended at the higher energies of the LHC?
• RQ2 Does the x-distribution of a resonance in gluon jets differ significantly from its x-distribution in quark jets, providing a signature for glueball dominance?
• RQ3 Can f₀(980), f₀(1500), or f₀(600) be identified as glueball candidates through their fragmentation behavior in gluon jets?
• RQ4 To what extent do color octet and triplet neutralization mechanisms (P₈, P₃) govern the charge distribution of leading clusters in high-energy gluon jets?
• RQ5 Can the suppression of a resonance in quark jets and its enhancement in gluon jets serve as a definitive signature for glueball production?

Key findings

• A 10–40% excess of neutral leading clusters (Q_lead = 0) in gluon jets at LEP, beyond Monte Carlo expectations, suggests a non-quark-string fragmentation component, possibly glueballs.
• At the LHC, gluon jet energies are expected to be ~10× higher than at LEP (typically >25 GeV), enabling better separation of leading clusters and improved resolution of x-distributions.
• The fraction of gluon jets in di-jet events is estimated at ~60% at LHC (p_T ~ 200 GeV), increasing to ~80% at higher p_T, providing high-purity samples for study.
• For rapidity gaps Δy ~ 4, the charge distribution of leading clusters approaches asymptotic limits: Q_lead = 0 (P₈) and Q_lead = 0, ±1 (P₃), enabling quantitative tests of color neutralization.
• Resonance x-spectra in gluon jets (e.g., f₀(980) in ππ, f₀(1500) in 4π) show potential for signal enhancement, but angular decay distortions from gap cuts can suppress signals if not corrected.
• The key diagnostic is the comparison of x-spectra: a true glueball should be leading in gluon jets but suppressed in quark jets, while q̄q states show the opposite behavior.