Observation of Long-Range Near-Side Angular Correlations in Proton-Proton Collisions at the LHC

Abstract: Results on two-particle angular correlations for charged particles emitted in proton-proton collisions at center-of-mass energies of 0.9, 2.36, and 7 TeV are presented, using data collected with the CMS detector over a broad range of pseudorapidity (eta) and azimuthal angle (phi). Short-range correlations in Delta(eta), which are studied in minimum bias events, are characterized using a simple "independent cluster" parametrization in order to quantify their strength (cluster size) and their extent in eta (cluster decay width). Long-range azimuthal correlations are studied differentially as a function of charged particle multiplicity and particle transverse momentum using a 980 inverse nb data set at 7 TeV. In high multiplicity events, a pronounced structure emerges in the two-dimensional correlation function for particle pairs with intermediate transverse momentum of 1-3 GeV/c, 2.0< |Delta(eta)| <4.8 and Delta(phi) near 0. This is the first observation of such a long-range, near-side feature in two-particle correlation functions in pp or p p-bar collisions.

• The paper reports the emergence of a near-side ridge in high-multiplicity pp collisions at 7 TeV, indicating unexpected collective effects.
• The study employs detailed angular correlation analysis with an independent cluster model to quantify cluster size and decay width across multiple energies.
• The findings highlight significant discrepancies with Monte Carlo models like PYTHIA, urging a reevaluation of QCD modeling in small collision systems.

Observations of Angular Correlations in Proton-Proton Collisions at the LHC

The paper presents a detailed study of two-particle angular correlations in proton-proton (pp) collisions, conducted at center-of-mass energies of 0.9, 2.36, and 7 TeV, leveraging data from the CMS detector at the Large Hadron Collider (LHC). The motivation for this analysis stems from the desire to understand Quantum Chromodynamics (QCD) behavior at these high energy scales, especially concerning hadronization processes and potential collective phenomena arising from dense particle production.

Short-Range vs. Long-Range Correlations

The study investigates both short-range and long-range correlations, focusing on pseudorapidity ($\Delta \eta$) and azimuthal angle ($\Delta \phi$) differences between pairs of charged particles. Short-range correlations, detailed primarily for $|\Delta\eta | < 2$, were assessed using an independent cluster model. This model interprets local peaks in correlation functions as arising from decay products of "independent clusters," potentially mapping the extent and multiplicity into two quantifiable parameters: cluster size (K) and decay width ($\delta$).

The analysis reveals that the cluster size scales with collision energy while the pseudorapidity extent remains fairly constant across different energies. Comparisons with Monte Carlo models, particularly PYTHIA with the D6T tune, show discrepancies: although PYTHIA reproduces the $\delta$ parameter accurately, it underestimates the K values, suggesting limitations in modeling soft QCD processes.

Emergence of Long-Range Correlations

The study of long-range azimuthal correlations, specifically for $2.0 < |\Delta \eta| < 4.8$, uncovers a significant and novel near-side correlation structure in high-multiplicity events at 7 TeV, reminiscent of ridge-like features observed in heavy ion collisions—a phenomenon not previously seen in pp systems. This ridge, characterized by long-range correlations at $\Delta \phi \approx 0$, becomes evident in events with high charged particle multiplicity ($N_{\mathrm{trk}} \geq 110$) and for particles in the intermediate transverse momentum range (1-3 GeV/c).

This feature may suggest emergent collective behavior (or other QCD-related processes) in high-energy pp collisions, traditionally not anticipated in such small systems. None of the event generators, including PYTHIA and HERWIG++, can account for this effect, underscoring possible deficiencies in current theoretical models.

Methodological Rigorousness and Systematic Uncertainties

The methodology includes meticulous event selection, triggering strategies, and efficiencies corrections, ensuring reliable extraction of correlation functions. Systematic uncertainties are thoroughly addressed, with variations in track quality cuts, event selection biases, and potential discrepancies in MC models all contributing to the reported uncertainties. The procedures adopted for correcting tracking inefficiencies and event selection biases (including pile-up effects) are particularly noteworthy, highlighting the robustness of the analysis even under complex experimental conditions.

Implications and Future Directions

The findings have profound implications for high-energy physics, hinting at additional QCD dynamics at play in pp collisions at LHC energies. This opens the field for revisiting theoretical models to incorporate potential collective effects that were once thought exclusive to heavy ion collisions. Future research must strive to understand the physical origin of this long-range correlation phenomenon in small systems and to extend the observation to other collision energies and particle systems.

In conclusion, this paper lays a substantial groundwork for exploring novel QCD phenomena in high-multiplicity pp collisions and sets the stage for refining theoretical models to capture the unexpected collective-like behavior observed at the LHC.