- The paper observes a novel long-range near-side correlation (ridge) in high-multiplicity pPb collisions at 5.02 TeV using CMS data.
- It employs two-particle correlation functions and the ZYAM method to analyze event multiplicity and transverse momentum dependencies.
- The findings challenge existing models like HYDJET and AMPT and imply unique initial- or final-state effects in small collision systems.
Observation of Long-Range, Near-Side Angular Correlations in Proton-Lead Collisions at the LHC
This paper, authored by the CMS Collaboration, details an empirical investigation into two-particle angular correlations within proton-lead (pPb) collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV, conducted using the CMS detector at the Large Hadron Collider (LHC). By analyzing approximately two million collision events, the study reveals a novel long-range correlation pattern, analogous to the ridge-like structures observed in high-multiplicity proton-proton (pp) and nucleus-nucleus (AonA) collisions.
Background
Two-particle angular correlations have been a crucial aspect of studies aiming to comprehend the dynamics in high-energy particle collisions, particularly due to their role in probing the underpinnings of quantum chromodynamics (QCD). These correlations are typically investigated using two-dimensional $\deta$-$\dphi$ correlation functions, where $\dphi$ and $\deta$ denote differences in azimuthal angle and pseudorapidity, respectively. In high-energy AonA collisions, large scale correlations, indicating collective behavior, are generally attributed to hydrodynamic flow, emphasized by the second (elliptic) and third (triangular) Fourier components related to the initial spatial anisotropy and its fluctuations.
Experimental and Analytical Approach
The data set under scrutiny was accumulated during a brief operational period of the LHC, employing a beam setup with protons at 4 TeV and lead nuclei at 1.58 TeV. Track-based minimum bias selection criteria ensured the quality of the data, and standard CMS algorithms were employed for event and track reconstruction. The efficacy of the tracking was corroborated by Monte Carlo simulations, ensuring accuracy in assessing tracking efficiency and rejectable noise from multiple vertices or incorrect associations.
The core analysis focused on constructing two-particle correlation functions across varying event multiplicities and transverse momenta ranges, employing a factoring technique to isolate correlated components. The Zero-Yield-at-Minimum (ZYAM) method was applied to discern correlation yields, considering potential overlap with uncorrelated background events due to non-dynamical pair association effects.
Key Findings
The standout observation is the emergence of a long-range, near-side correlation structure in high-multiplicity pPb events, reflecting a “ridge”-like formation. Specifically, the correlation exhibited a pronounced maximum at transverse momentum ($\pt$) intervals of 1–1.5 GeV/c and scaled approximately linearly with event multiplicity. Comparatively, this ridge phenomenon demonstrated a stronger intensity in pPb collisions than those documented in studies of pp events, despite similar multiplicity conditions. Notably, prevalent event generators like HYDJET and AMPT failed to replicate this near-side structure, suggesting inadequacies in existing models regarding initial state interactions and subsequent medium dynamics.
Implications and Future Work
These findings introduce substantial implications for theoretical models addressing the onset of collective phenomena in small systems such as pPb collisions. They suggest either the persistence of final-state effects akin to those in larger systems or point towards unique initial-condition effects not entirely captured in current hydrodynamic or color-glass condensate frameworks. The results also necessitate a reevaluation of particle production mechanisms across different collision geometries, potentially influencing the interpretation of QCD at forward rapidities.
Looking ahead, an extended exploration encompassing various ion configurations and energy scales could unveil further insights into the nature of particle interactions in high-density environments. Enhancements in theoretical modeling are anticipated to reconcile the observed discrepancies and accurately simulate the dynamics involved in such high-energy physics realms. The synergy of experimental advancements and theoretical refinement will be pivotal in deepening the understanding of quantum behaviors manifesting in such collision events.