[Paper Review] What shines brighter, Glasma or Quark-Gluon Plasma: a parametric estimate of photon production at early times in heavy-ion collisions

Jürgen Berges, Klaus Johannes Reygers+2 more|arXiv (Cornell University)|Jan 18, 2017

High-Energy Particle Collisions Research3 citations

TL;DR

This study performs a parametric analysis of photon production in ultrarelativistic heavy-ion collisions, comparing contributions from the Glasma (non-equilibrium) stage to the thermalized Quark-Gluon Plasma (QGP) using the bottom-up thermalization framework. It finds that Glasma contributions are significant—especially at high saturation scales—challenging the assumption that thermalized QGP dominates early photon emission.

ABSTRACT

Recent classical-statistical numerical simulations have established the thermalization scenario of Baier et al. as the correct weak coupling effective theory for thermalization in ultrarelativistic heavy-ion collisions. We perform a parametric study of photon production in the various stages of this bottom-up framework to ascertain the relative contribution of the off-equilibrium relative to that of a thermalized Quark-Gluon Plasma. Taking into account the constraints imposed by the measured charged hadron multiplicities at RHIC and the LHC, we find that Glasma contributions are important especially for large values of the saturation scale at both energies. These non-equilibrium effects should therefore be taken into account in studies where weak coupling methods are employed to compute photon yields.

Motivation & Objective

• To assess the relative importance of non-equilibrium Glasma photon emission versus thermalized Quark-Gluon Plasma emission in early-stage heavy-ion collisions.
• To evaluate whether weak coupling approaches that neglect non-equilibrium dynamics accurately describe early photon production.
• To incorporate experimental constraints from charged hadron multiplicities at RHIC and the LHC into the photon yield analysis.
• To determine under what conditions Glasma contributions dominate over thermalized QGP emission in the early thermalization phase.

Proposed method

• Utilizes the bottom-up thermalization scenario as the weak coupling effective theory for early-time dynamics in heavy-ion collisions.
• Performs a parametric study of photon production across different stages: Glasma (off-equilibrium) and thermalized QGP.
• Applies constraints from measured charged hadron multiplicities at RHIC and the LHC to fix model parameters such as the saturation scale.
• Compares the photon yield from the Glasma phase to that from the thermalized QGP phase under consistent physical conditions.
• Employs classical-statistical numerical simulations as a foundation for the thermalization scenario used in the analysis.
• Analyzes the dependence of photon production on the saturation scale, focusing on high values relevant to experimental conditions.

Experimental results

Research questions

• RQ1 How do Glasma and thermalized QGP contribute to early photon production in heavy-ion collisions?
• RQ2 Under what conditions does the Glasma phase dominate photon emission relative to the thermalized QGP?
• RQ3 To what extent do experimental constraints on charged hadron multiplicities affect the relative importance of Glasma contributions?
• RQ4 How do variations in the saturation scale influence the relative photon yields from non-equilibrium and thermalized stages?
• RQ5 Can weak coupling methods that neglect non-equilibrium effects accurately model early photon yields in heavy-ion collisions?

Key findings

• Glasm a contributions to photon production are significant, particularly at large saturation scales, challenging the assumption that thermalized QGP dominates early photon emission.
• The study finds that non-equilibrium effects from the Glasma phase must be included in weak coupling calculations of photon yields to achieve accurate results.
• At both RHIC and LHC energies, Glasma contributions are non-negligible and increase with higher saturation scales.
• The analysis shows that current weak coupling frameworks may underestimate early photon production if they omit Glasma dynamics.
• Experimental constraints from charged hadron multiplicities support the relevance of Glasma contributions in realistic collision scenarios.
• The results imply that future studies of early-time photon emission should explicitly account for off-equilibrium Glasma dynamics.