[Paper Review] Energy Loss and Medium Response via Two Particle Correlations in Heavy Ion Collisions

M. McCumber|arXiv (Cornell University)|May 4, 2009

High-Energy Particle Collisions Research1 references3 citations

TL;DR

This study uses two-particle correlations in mid-central Au+Au collisions at √sNN = 200 GeV to probe parton energy loss and medium response via reaction plane-dependent jet suppression. It finds a strong increase in away-side suppression with angle relative to the reaction plane, indicating that surviving back-to-back jets originate from partons crossing the nuclear overlap rather than from unmodified tangential emission, challenging theories predicting weak angular dependence.

ABSTRACT

In these proceedings, pair correlations between high pT final-state particles in central and mid-central Au+Au are presented as a function of trigger-angle with respect to the reaction plane. Jet tomography results like these add another dimension of experimental discrimination to energy loss calculations and initial state geometric descriptions. The observed increasing away-side suppression with respect to the nuclear overlap demonstrate that back-to-back production at high pT in mid-central collisions is not dominated by unmodified production produced tangentially to the nuclear overlap. Furthermore, these results present a challenge to theories that predict only a weak variation due to angle with respect to the reaction plane.

Motivation & Objective

• To investigate the origin of surviving back-to-back high-pT jets in mid-central heavy ion collisions by measuring their dependence on the reaction plane angle.
• To distinguish between two competing jet quenching mechanisms: partons crossing the nuclear overlap with partial energy loss versus only tangential partons surviving due to complete energy loss in a core region.
• To constrain theoretical models of energy loss and initial-state geometry by quantifying the angular dependence of away-side suppression.
• To test whether existing theoretical predictions for reaction plane-dependent suppression match experimental data from RHIC.
• To assess the role of initial-state soft production anisotropy in jet survival and medium response.

Proposed method

• Measures per-trigger yield (PTY) of high-pT pairs formed between neutral pions (4–7 GeV/c) and charged hadrons (3–4 and 4–5 GeV/c) in Au+Au collisions.
• Uses a two-source model to describe correlation functions, incorporating reaction plane trigger binning and resolution effects via modulation parameters α, β, and γ.
• Applies a reaction plane resolution correction to raw data, accounting for smearing effects using a fully anti-correlated systematic uncertainty band.
• Analyzes integrated away-side PTY as a function of reaction plane angle (ϕs) in centrality bins 0–20% and 20–60%.
• Performs composite χ² fits to test linear trends in suppression across ϕs, assuming identical percentage variation in PTY for both partner momentum windows.
• Compares results to theoretical predictions from Renk (nuclear overlap crossing) and Pantuev (black-core model) to assess compatibility.

Experimental results

Research questions

• RQ1 Does the suppression of away-side high-pT jet pairs depend significantly on the angle relative to the reaction plane in mid-central Au+Au collisions?
• RQ2 Which mechanism—partial energy loss during nuclear overlap crossing or tangential emission from a complete energy loss core—dominates the production of surviving back-to-back jets?
• RQ3 How do the measured angular dependencies of suppression compare to theoretical predictions assuming weak or strong reaction plane dependence?
• RQ4 To what extent do initial-state soft production anisotropies influence jet survival and the observed angular dependence?
• RQ5 Can the data constrain the geometric structure of the initial state in heavy ion collisions, particularly the distribution of soft matter?

Key findings

• Away-side suppression increases strongly with increasing angle relative to the reaction plane in mid-central Au+Au collisions, with a χ²/dof of 0.8 for the best-fit linear trend.
• The data rule out rising suppression trends (values above 1 in the ratio of in-plane to out-of-plane PTY) at more than 4σ significance.
• The results are incompatible with Renk’s prediction, which assumes weak reaction plane dependence and higher pT than measured.
• The Pantuev model shows a closer match but still predicts a weaker angular dependence than observed in the data.
• The steep angular dependence indicates that surviving back-to-back jets are primarily from partons that crossed the nuclear overlap with partial energy loss, not from unmodified tangential emission.
• The findings suggest that initial-state soft production anisotropy may play a significant role in jet survival, supporting models with non-uniform initial-state distributions.