Long-range angular correlations on the near and away side in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV (1212.2001v4)

Abstract: Angular correlations between charged trigger and associated particles are measured by the ALICE detector in p-Pb collisions at a nucleon-nucleon centre-of-mass energy of 5.02 TeV for transverse momentum ranges within 0.5 < $p_{\rm T}^{\rm assoc}$ < $p_{\rm T}^{\rm trig}$ < 4 GeV/$c$. The correlations are measured over two units of pseudorapidity and full azimuthal angle in different intervals of event multiplicity, and expressed as associated yield per trigger particle. Two long-range ridge-like structures, one on the near side and one on the away side, are observed when the per-trigger yield obtained in low-multiplicity events is subtracted from the one in high-multiplicity events. The excess on the near-side is qualitatively similar to that recently reported by the CMS collaboration, while the excess on the away-side is reported for the first time. The two-ridge structure projected onto azimuthal angle is quantified with the second and third Fourier coefficients as well as by near-side and away-side yields and widths. The yields on the near side and on the away side are equal within the uncertainties for all studied event multiplicity and $p_{\rm T}$ bins, and the widths show no significant evolution with event multiplicity or $p_{\rm T}$. These findings suggest that the near-side ridge is accompanied by an essentially identical away-side ridge.

• The paper presents a detailed study of long-range angular correlations using two-particle correlation techniques in p–Pb collisions.
• The experimental analysis with ALICE detectors reveals symmetric near-side and away-side ridge structures in high-multiplicity events.
• The findings challenge simple jet-quenching models and support theories incorporating hydrodynamic flow as well as gluon-saturation effects.

Overview of Long-range Angular Correlations in p–Pb Collisions at 5.02 TeV

The paper investigates the measurement of angular correlations between charged trigger and associated particles in proton-lead (p–Pb) collisions, with the data collected by the ALICE experiment at the LHC. The analysis is conducted at a nucleon–nucleon center-of-mass energy of 5.02 TeV and focuses on a comprehensive range of transverse momentum. The paper aims to uncover underlying phenomena in high-energy particle collisions by examining long-range dijet-like structures and detailing their characteristics.

Experimental Setup and Methodology

The collisions, which were conducted during a pilot run at the LHC, utilized beams with energies of 4 TeV for protons and 1.58 TeV per nucleon for lead. The primary detectors used for data collection were components of the ALICE setup, including the Inner Tracking System (ITS) and the Time Projection Chamber (TPC), alongside the VZERO detector for triggering and event classification. Event and track selection was rigorous to ensure high data fidelity, relying on stringent criteria for vertex positioning, track integrity, and momentum thresholds.

For the primary analysis, two-particle correlations are reported as a function of azimuthal and pseudorapidity differences. These are expressed as associated per-trigger yields across four multiplicity classes, revealing insights into particle dynamics linked to initial state geometries and subsequent particle interactions.

Key Results and Interpretations

The paper identifies two remarkable, nearly symmetric, long-range ridge structures: one on the near-side (Δφ≈0) and one on the away-side (Δφ≈π). These ridges are evident in high-multiplicity events while absent in low-multiplicity samples, suggesting an intricate interconnection between initial geometry, particle density, and interaction dynamics.

The yields and widths of these ridge structures display no significant evolution with event multiplicity, indicating that previously reported near-side ridges in similar setups by the CMS collaboration appear to have comparable away-side counterparts in ALICE’s results. Significantly, this paper reports the away-side ridge in p–Pb collisions at these energy levels for the first time, a feature not observed in prior p–Pb analyses. Furthermore, the presence of identical near-side and away-side ridge yields at a given pᵀ range and event class suggests possible shared origins, aligning with theoretical formulations such as Colour Glass Condensate models.

Theoretical and Practical Implications

The observation of these ridge structures holds implications for theoretical models accounting for coherence phenomena and collective behavior in particle collisions. Specifically, the results challenge simplistic jet-quenching explanations and lean towards models incorporating hydrodynamic flow or gluon-saturation effects. Due to the experimental conditions akin to the rigorous criteria of high-energy nuclear physics, these findings provide valuable benchmarks for hydrodynamic calculations and theories dealing with complex QCD effects in non-thermalized mediums.

Prospects for Future Research

Further scrutiny into the observed ridge-like structures could implicate iterative comparisons with models encompassing collective flow dynamics, specifically those predicated on hydrodynamical expansion. Establishing a definitive discrepancy or convergence between theoretical predictions and the experimentally observed coefficients (v₂, v₃) remains an essential next step. Additionally, comparisons across collision systems and energy scales, notably in relation to d–Au collisions at RHIC energies, might yield refined understandings of universal versus system-specific particle production mechanisms.

In conclusion, this paper expands the discourse on long-range correlations in high-energy p–Pb collisions by documenting significant ridge phenomena and provides a fertile ground for theoretical exploration and model validation in describing collective phenomena within QCD frameworks.