[Paper Review] Direct photon-charged hadron azimuthal correlations

A. Hamed|arXiv (Cornell University)|Jun 13, 2008

High-Energy Particle Collisions Research4 citations

TL;DR

This study presents the first measurement of direct photon-charged hadron azimuthal correlations in Au+Au collisions at √sNN = 200 GeV by the STAR experiment. Using high-pT direct photons as probes, it observes similar away-side suppression in γ-hadron correlations to that seen in di-hadron and single-particle RAA measurements, indicating consistent energy loss effects in the quark-gluon plasma across different observables.

ABSTRACT

Azimuthal correlations of direct photons at high transverse energy (8 $

Motivation & Objective

• To probe parton energy loss in the quark-gluon plasma using direct photons as unbiased probes of initial parton energy.
• To test whether γ-hadron azimuthal correlations can constrain in-medium energy loss mechanisms, especially those involving color factors and path-length dependence.
• To measure the suppression of away-side hadrons correlated with high-ET direct photons in Au+Au collisions and compare it to p+p and π⁰-hadron correlations.
• To assess the consistency of γ-hadron IAA with existing theoretical models tuned to single- and di-hadron suppression.
• To reduce background from π⁰ decays and fragmentation photons using BSMD shower shape cuts and a subtraction method based on R = Yn(γ-rich)/Yn(π⁰).

Proposed method

• Used level-2 high-pT tower trigger to select γ-jet events in 535 μb⁻¹ of 200 GeV Au+Au collisions and 11 pb⁻¹ of p+p data.
• Employed the Barrel Electromagnetic Calorimeter (BEMC) for full φ and |η| ≤ 1.0 coverage to detect high-ET photons.
• Applied shower shape cuts using the Barrel Shower Maximum Detector (BSMD) to separate direct photons from π⁰ → 2γ decays, achieving a γ-rich sample.
• Extracted near-side (Yn) and away-side (Ya) yields from dN/d(Δφ) distributions within |Δφ| ≤ 0.63 and |Δφ − π| ≤ 0.63, respectively.
• Used Equation (1): Yγ_direct+h = (Y^a_γ_rich+h − R Y^a_π⁰+h)/(1−R), with R = Y^n_γ_rich+h / Y^n_π⁰+h, to isolate direct photon-induced away-side yield.
• Corrected associated hadron yields for tracking efficiency as a function of multiplicity and applied systematic corrections assuming similar correlation patterns for π⁰, η, and fragmentation photons.

Experimental results

Research questions

• RQ1 How does the azimuthal correlation of direct photons with associated hadrons in Au+Au collisions reflect parton energy loss in the quark-gluon plasma?
• RQ2 To what extent is the away-side suppression of hadrons correlated with direct photons consistent with that observed in π⁰-hadron and di-hadron correlations?
• RQ3 Can γ-hadron correlations provide independent constraints on the energy loss mechanism, particularly regarding initial parton energy and color factor dependence?
• RQ4 How do systematic uncertainties in p+p reference measurements affect the extraction of IAA for γ-hadron correlations?
• RQ5 Is the suppression pattern in γ-hadron correlations consistent with theoretical models tuned to single-particle and di-hadron RAA data?

Key findings

• The IAA for direct γ-hadron correlations in the most central Au+Au collisions (0–10%) is found to be similar to that observed in di-hadron and single-particle RAA measurements.
• The away-side suppression in γ-hadron correlations decreases with increasing centrality, consistent with increasing energy loss in denser medium.
• The IAA for direct γ triggers shows a suppression pattern nearly identical to that of π⁰ triggers across all pT^assoc ranges, indicating similar energy loss effects despite different initial parton energies.
• The γ-rich sample, enriched via BSMD shower shape cuts, successfully suppresses π⁰ backgrounds while preserving signal from direct photons and other sources like η → 2γ and fragmentation photons.
• The measured IAA values are consistent with theoretical calculations that reproduce single- and di-hadron suppression, supporting the applicability of pQCD-based energy loss models.
• Systematic uncertainties in the p+p reference measurement lead to a relatively large scale uncertainty in IAA, which is expected to improve with larger future data samples.