Observation of Associated Near-side and Away-side Long-range Correlations in sqrt(s_NN)=5.02 TeV Proton-lead Collisions with the ATLAS Detector (1212.5198v3)

Abstract: Two-particle correlations in relative azimuthal angle (Delta-phi) and pseudorapidity (Delta-eta) are measured in sqrt(s_NN) = 5.02 TeV p+Pb collisions using the ATLAS detector at the LHC. The measurements are performed using approximately 1 mub^-1 of data as a function of p_T and the transverse energy (SumE_T) summed over 3.1 < eta < 4.9 in the direction of the Pb beam. The correlation function, constructed from charged particles, exhibits a long-range (2<|Delta-eta|<5) near-side (Delta-phi ~ 0) correlation that grows rapidly with increasing SumE_T. A long-range away-side (Delta-phi ~ pi) correlation, obtained by subtracting the expected contributions from recoiling dijets and other sources estimated using events with small SumE_T, is found to match the near-side correlation in magnitude, shape (in Delta-eta and Delta-phi) and SumE_T dependence. The resultant Delta-phi correlation is approximately symmetric about pi/2, and is consistent with a cos(2Delta-phi) modulation for all SumE_T ranges and particle p_T. The amplitude of this modulation is comparable in magnitude and p_T dependence to similar modulations observed in heavy-ion collisions, suggestive of final-state collective effects in high multiplicity events.

• The paper demonstrates significant near- and away-side long-range correlations, revealing a distinct cos2Δφ modulation in p+Pb collisions.
• The paper employs systematic subtraction of dijet contributions to isolate genuine correlation structures, highlighting scaling with forward transverse energy.
• The paper’s findings imply that ridge phenomena are not confined to high-multiplicity A+A events, challenging traditional QCD flow interpretations.

Overview of the Observation of Long-range Correlations in Proton-lead Collisions

The paper presents a comprehensive exploration of two-particle correlations observed in proton-lead (p+Pb) collisions at a center-of-mass energy of 5.02 TeV, using the ATLAS detector at the LHC. Focusing on the correlation functions in relative azimuthal angle and pseudorapidity, the paper introduced methods that discern essential features between charged particles over specified transverse momentum ranges and transverse energy in the Pb beam's direction. These methods helped reveal the correlation's nuances over different intervals and reinforced the potential investigations into Quantum Chromodynamics (QCD) phenomena at high parton densities.

Key Findings

1. Correlation Structure:
   • Significant observations were made of long-range correlations characterized by "ridge" phenomena on both the near-side and away-side of the collision event spectra. These correlations were quantified over $2 < |\Delta\eta| < 5$.
   • The analysis demonstrated how these correlation phenomena are symmetric around $\pi/2$ with a notable $\cos 2\Delta \phi$ modulation, providing insight into potential collective behaviors in the p+Pb collision environment.
2. Extraction and Analysis:
   • Through systematic subtraction of recoil dijet contributions, the paper isolated the long-range correlations effectively, establishing a methodology for future explorations in the field of particle physics at high energies.
   • The per-trigger yield was dissected, revealing that while near-side correlations began forming predominantly at higher event activity, the away-side exhibited a persistent existence across all activities.
3. Scale and Symmetry:
   • The magnitude of the observed correlations grows with the transverse energy deposited in the forward calorimeters, suggesting the scaling behavior of the ridge phenomena.
   • The amplitude of the $\cos 2\Delta \phi$ components observed reached approximately 0.01 at $\pT \sim 3$ GeV, pointing to the presence of correlations not just in high-multiplicity nucleus-nucleus (A+A) collisions but also present in p+Pb, thereby challenging the unique association of such phenomena with hot, dense QCD matter.

Implications for Future Research

These observations suggest essential amendments to existing models depicting parton interactions and structure at high densities, paving the way for further investigation into the nature of initial-state effects versus final-state collective flow, traditionally characterized in A+A collisions:

• Parton Density and QCD Saturation: The results strongly support ongoing LHC investigations in discerning the saturation effects of parton densities at low Bjorken-x in heavy-ion collisions. This could bridge important theoretical gaps in understanding gluon saturation and potential color glass condensate manifestations.
• Event Shape and Correlation Dynamics: Exploring dynamic symmetry and transformation of these ridges with varying collision types (p+p, p+Pb, A+A) can provide deeper insights into the collective behavior and hadronization processes in the QCD medium at unprecedented energy levels.
• Methodological Advances: Improved methodological techniques demonstrated in this analysis offer a framework for future explorations in distinguishing between near-side jets and long-range flow effects, critical for precise determination of QCD matter properties under extreme conditions.

In summary, the paper provides an essential building block in understanding long-range correlations in proton-lead collisions, offering a nuanced view into QCD dynamics under high-density conditions and paving the way for future research into both fundamental physics and detector technologies. It inches the scientific community closer to disentangling the complexities of strong force interactions at the subatomic level.