[论文解读] Collective flow of photons in strongly coupled gauge theories

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

High-Energy Particle Collisions Research被引用 8

一句话总结

本文研究了强耦合夸克-胶子等离子体（QGP）中的光子剪切黏度，发现由于热光子谱发生蓝移，强耦合下其黏度被抑制。这种抑制增强了直接光子的各向异性流，为重离子碰撞中尽管光子-介质相互作用微弱却观测到较大椭圆流这一难题提供了合理解释。

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

研究动机与目标

• 解决理论预测与重离子碰撞中直接光子大椭圆流实验观测之间的差异。
• 研究QGP中强耦合如何通过光子-部分子散射影响光子剪切黏度。
• 检验强耦合QGP中增强的光子-介质相互作用是否可解释直接光子出射量出人意料地大的原因。
• 评估非微扰QCD效应在改变QGP中电磁相互作用方面的作用。
• 探讨剪切黏度抑制对相对论性重离子碰撞及宇宙学中电磁探针的影响。

提出的方法

• 利用一阶电磁散射过程分析强耦合夸克-胶子等离子体中光子的剪切黏度。
• 应用有效场论技术，对有限温度下QGP中的光子-部分子散射进行建模。
• 利用强耦合下热光子谱的蓝移现象解释黏度抑制。
• 引入黏性流体力学模拟等离子体膨胀及其对光子流的影响。
• 将结果与微扰QCD和弱耦合QGP模型进行比较，以分离非微扰效应。
• 评估强耦合对光子输运性质（尤其是剪切黏度）的影响。

实验结果

研究问题

• RQ1 QGP中耦合强度增强如何影响光子的剪切黏度？
• RQ2 热光子谱的蓝移在强耦合下对光子黏度的修正起什么作用？
• RQ3 为何重离子碰撞中直接光子的椭圆流大于弱耦合模型的预测？
• RQ4 在强耦合QGP中，光子-部分子散射过程在光子-介质相互作用中占主导地位的程度如何？
• RQ5 色规范场中的非微扰QCD效应是否可增强电磁相互作用，从而影响光子流？

主要发现

• 由于热光子谱的蓝移，强耦合QGP中光子的剪切黏度被抑制。
• 这种抑制源于一阶光子-部分子散射过程，其在强耦合下被增强。
• 剪切黏度在禁闭相变附近迅速增加，表明输运性质具有临界行为。
• 光子黏度的抑制增强了直接光子的各向异性流，为实验观测提供了一种解释机制。
• QCD规范场中的非微扰效应显著改变了光子-介质相互作用，挑战了电磁探针中弱耦合的假设。
• 研究结果表明，在强耦合等离子体中，电磁探针（如光子）可能携带比以往认为更多的流信息。