Comparison of the Color Glass Condensate to di-hadron correlations in proton-proton and proton-nucleus collisions

Abstract: We perform a detailed comparison of long range rapidity correlations in the Color Glass Condensate framework to high multiplicity di-hadron data in proton-proton and proton-lead collisions from the CMS, ALICE and ATLAS experiments at the LHC. The overall good agreement thus far of the non-trivial systematics of theory with data is strongly suggestive of gluon saturation and the presence of subtle quantum interference effects between rapidity separated gluons. In particular, the yield of pairs collimated in their relative azimuthal angle $\Delta\phi\sim 0$, is sensitive to the shape of unintegrated gluon distributions in the hadrons that are renormalization group evolved in rapidity from the beam rapidities to those of the measured hadrons. We present estimates for the collimated di-hadron yield expected in central deuteron-gold collisions at RHIC.

• The paper presents a CGC framework that quantitatively reproduces ridge effects observed in LHC pp and pA collisions.
• It employs non-linear evolution equations and unintegrated gluon distributions to capture gluon saturation phenomena and quantum interference.
• Findings confirm enhanced ridge yields in central pA collisions, underpinning future research into high-energy QCD dynamics.

Understanding Di-Hadron Correlations in High-Energy Collisions within the Color Glass Condensate Framework

This paper explores the intriguing phenomenon of long-range rapidity correlations observed between hadronic particles in high-energy collisions, specifically addressing proton-proton (pp) and proton-nucleus (pA) interactions at the Large Hadron Collider (LHC). Employing the Color Glass Condensate (CGC) effective field theory, the authors, Kevin Dusling and Raju Venugopalan, aim to provide a robust theoretical foundation for these correlations, famously dubbed the "ridge" effect, a striking feature that manifests as a narrow peak in the relative azimuthal angle centered around $\Delta \phi \sim 0$.

The essence of this study lies in the ability of CGC to encapsulate gluon saturation phenomena, which are believed to play a critical role in high-energy QCD processes. These saturation effects are captured by non-linear evolution equations that quantify the growth of parton densities at small values of the Bjorken-x variable. Intriguingly, the analysis reveals that the nearside collimation in these long-range rapidity correlations is attributed to novel interference effects between gluons separated in rapidity, which are coherently described by CGC.

Detailed Insights and Systematic Comparisons

The paper provides a comprehensive quantitative comparison of theoretical predictions against data from CMS, ALICE, and ATLAS at various collision energies and settings. The CGC formalism captures the intricate interplay between "Glasma" graphs—characterizing particle production from highly occupied gluon states—and the traditional di-jet configurations typically dominated by Balitsky-Fadin-Kuraev-Lipatov (BFKL) dynamics. By employing a rigorous integration over unintegrated gluon distributions (UGDs), which evolve following the rcBK equation, the authors manage to replicate many of the experimental observations with surprising fidelity.

Key numerical results are presented extensively, revealing strong agreement with empirical data from both pp and pA collisions, while still acknowledging areas requiring further exploration, such as nuances in the peripheral collision domains. In particular, the authors highlight the significant enhancement of ridge yields in central pA collisions, which stand in stark contrast to the corresponding pp observations, demonstrating a clear sensitivity of these results to the nature of the colliding systems.

Theoretical and Practical Implications

The results manifestly support the hypothesis that large x and small x gluon interactions in high-density environments lead to the emergence of gluon saturation and coherent quantum effects, not just as peripheral attributes but as central to the core dynamics of particle production at high energies. Such findings crucially inform our theoretical understanding of QCD in these dense regimes, where perturbative approaches merge with rich, non-linear phenomena.

On a practical level, the ability of the CGC framework to model these correlations across varied experimental conditions underscores its utility as a predictive tool that can potentially guide future experimental designs and analyses. Should further empirical validation be forthcoming, it could significantly reinforce our understanding of gluon interactions in nuclear matter at TeV scales.

Prospects for Future Research

Looking forward, this work sets the stage for a broad spectrum of inquiries into high-energy nuclear physics. It paves the way for detailed investigations into other complexities of saturation physics, enriches studies of quantum interference on long-range scales, and motivates an extension of CGC applicability to other particle collision energies and conditions. The authors' predictions for results from the RHIC, exploiting lower energy collisions, furnish an additional empirical frontier for continued validation of CGC insights.

The synthesis between theoretical precision and experimental verification evident in this study promotes a vigorous trajectory for research in high-energy QCD, potentially unveiling further unknowns within the tapestry of hadronic matter under extreme conditions.