[Paper Review] Collective flow of photons in strongly coupled gauge theories

Di-Lun Yang, Berndt Müller|arXiv (Cornell University)|Jul 15, 2015

High-Energy Particle Collisions Research8 citations

TL;DR

This paper investigates photon shear viscosity in a strongly coupled quark-gluon plasma (QGP), finding that it is suppressed at strong coupling due to blue-shifting of the thermal-photon spectrum. The suppression enhances anisotropic flow of direct photons, offering a resolution to the puzzle of large elliptic flow in heavy-ion collisions despite weak photon-medium interactions.

ABSTRACT

We investigate the shear viscosity of photons in a strongly coupled quark gluon plasma (QGP). We find that the shear viscosity due to the photon-parton scattering up to the leading order of electromagnetic coupling is suppressed when the coupling of the QGP is increased, which stems from the blue-shift of the thermal-photon spectrum at strong coupling. In addition, the shear viscosity rapidly increases near the deconfinement transition. We argue that the suppression of the photon shear viscosity in the QGP phase could lead to an enhancement of the anisotropic flow of direct photons in heavy ion collisions. The electromagnetic (EM) signatures such as leptons and photons produced in thermal plasmas play an important role in probing the properties of the plasmas, which have been widely investigated in relativistic heavy ion collisions and cosmology. It is generally believed that such EM probes could faithfully record the local information of the plasmas based on their weak interaction with the medium. However, recent observations of large elliptic flow of direct photons comparable to that of hadrons in relativistic heavy ion collisions results in a puzzle [1, 2]. Since the large portion of thermal photons is presumed to be generated in early times, only part of direct photons produced in late times could inherit the flow from their partonic or hadronic sources. It is shown in [3, 4] that the theoretical calculations, which incorporate the emission rate of prompt photons from perturbative quantum chromodynamics (QCD) and of thermal photons produced in the weakly coupled QGP and the rate from hadronic gas along with the plasma expansion dictated by viscous hydrodynamics, under-predict the elliptic flow compared with the experimental measurements. Different mechanisms relevant to the large flow of direct photons have been intensively studied in recent years [5–13]. Although it is found that the quantum electrodynamic (QED) plasma is rather viscous and the thermal leptons or photons barely interact with each other after they are emitted from the plasma [14–16], the scenario in the QGP is not so obvious since the interaction between the leptons/photons and the medium could be enhanced by the lepton-quark and photon-parton scatterings. Particularly, the photon-parton scattering dominates the photon-photon/lepton interaction in the order of EM coupling. Moreover, in the strongly coupled QGP (sQGP), the non-perturbative effect in the color sector may further modify the interaction with the EM sector. We thus consider the scenario when leptons/photons and

Motivation & Objective

• To resolve the discrepancy between theoretical predictions and experimental observations of large elliptic flow in direct photons from heavy-ion collisions.
• To investigate how strong coupling in the QGP affects photon shear viscosity via photon-parton scattering.
• To examine whether enhanced photon-medium interactions in strongly coupled QGP can explain the unexpectedly large flow of direct photons.
• To assess the role of non-perturbative QCD effects in modifying electromagnetic interactions in the QGP.
• To explore the implications of suppressed photon viscosity for electromagnetic probes in relativistic heavy-ion collisions and cosmology.

Proposed method

• Analyzes shear viscosity of photons in a strongly coupled quark-gluon plasma using leading-order electromagnetic scattering processes.
• Applies effective field theory techniques to model photon-parton scattering in the QGP at finite temperature.
• Uses the blue-shifting of the thermal-photon spectrum at strong coupling to explain viscosity suppression.
• Incorporates viscous hydrodynamics to simulate plasma expansion and its effect on photon flow.
• Compares results with perturbative QCD and weakly coupled QGP models to isolate non-perturbative effects.
• Evaluates the impact of strong coupling on photon transport properties, particularly shear viscosity.

Experimental results

Research questions

• RQ1 How does increasing the coupling strength in the QGP affect the shear viscosity of photons?
• RQ2 What role does the blue-shifting of the thermal-photon spectrum play in modifying photon viscosity at strong coupling?
• RQ3 Why is the elliptic flow of direct photons in heavy-ion collisions larger than predicted by weakly coupled models?
• RQ4 To what extent do photon-parton scattering processes dominate photon-medium interactions in the strongly coupled QGP?
• RQ5 Can non-perturbative QCD effects in the color sector enhance electromagnetic interactions and thus influence photon flow?

Key findings

• Shear viscosity of photons is suppressed in the strongly coupled QGP due to blue-shifting of the thermal-photon spectrum.
• The suppression arises from leading-order photon-parton scattering processes, which are enhanced at strong coupling.
• Shear viscosity increases rapidly near the deconfinement transition, indicating a critical behavior in transport properties.
• The suppression of photon viscosity enhances the anisotropic flow of direct photons, offering a mechanism to explain experimental observations.
• Non-perturbative effects in the QCD sector significantly modify photon-medium interactions, challenging the assumption of weak coupling in EM probes.
• The findings suggest that electromagnetic probes like photons may carry more flow information than previously thought in strongly coupled plasmas.