[论文解读] Chiral drag force

Krishna Rajagopal, Andrey V. Sadofyev|arXiv (Cornell University)|May 27, 2015

High-Energy Particle Collisions Research被引用 9

一句话总结

本文提出了一种在强耦合手征性等离子体中作用于重夸克的新颖手征性拖拽力，该力源于轴向异常，并与等离子体的手征性不平衡成正比。通过在N=4超对称Yang-Mills理论中使用全息方法，研究发现该力的方向与外磁场或流体涡度平行或反平行，且与重夸克的电荷无关，在特定静止参考系中该力消失，从而为无耗散的手征性输运提供了直接证据。

ABSTRACT

We provide a holographic evaluation of novel contributions to the drag force acting on a heavy quark moving through strongly interacting plasma. The new contributions are chiral in that they act in opposite directions in plasmas containing an excess of left- or right-handed quarks and in that they are proportional to the coefficient of the axial anomaly. These new contributions to the drag force act either parallel to or antiparallel to an external magnetic field or to the vorticity of the fluid plasma. In all these respects, these contributions to the drag force felt by a heavy quark are analogous to the chiral magnetic effect on light quarks. However, the new contribution to the drag force is independent of the electric charge of the heavy quark and is the same for heavy quarks and antiquarks. We show that although the chiral drag force can be non-vanishing for heavy quarks that are at rest in the local fluid rest frame, it does vanish for heavy quarks that are at rest in a suitably chosen frame. In this frame, the heavy quark at rest sees counterpropagating momentum and charge currents, both proportional to the axial anomaly coefficient, but feels no drag force. This provides strong concrete evidence for the absence of dissipation in chiral transport, something that has been predicted previously via consideration of symmetries. Along the way to our principal results, we provide a general calculation of the corrections to the drag force due to the presence of gradients in the flowing fluid in the presence of a nonzero chemical potential. We close with a consequence of our result that is at least in principle observable in heavy ion collisions, namely an anticorrelation between the direction of the CME current for light quarks in a given event and the direction of the kick given to the momentum of all the heavy quarks and antiquarks in that event.

研究动机与目标

• 通过全息方法研究在强耦合、手征性等离子体中重夸克拖拽力的新贡献。
• 确定手征性异常——通过轴向异常——是否会产生一种与重夸克电荷无关的新类型拖拽力。
• 通过分析手征性拖拽力是否在特定参考系中消失，研究手征性输运的耗散特性。
• 探讨手征性拖拽力与手征性磁效应或手征性涡旋效应之间的联系，明确其在重夸克动力学中各自的作用。

提出的方法

• 在大Nc极限下的N=4超对称杨-米尔斯理论中应用规范-重力对偶，以建模具有有限化学势的强耦合等离子体。
• 在对偶的引力背景中同时包含流体梯度和手征费米子（左旋或右旋夸克）的化学势。
• 在全息作用量中引入陈-西蒙斯项，以捕捉轴向异常及其流体动力学后果。
• 通过计算在弯曲时空中的弦世界面嵌入，利用Nambu-Goto作用量提取重夸克所受的拖拽力。
• 在μ/T和流体梯度上进行微扰展开，以分离出手征性修正对拖拽力的贡献。
• 在重夸克静止的参考系中分析该力，表明反向流动的动量流与电荷流相互抵消，从而导致拖拽力消失，暗示无耗散。

实验结果

研究问题

• RQ1 轴向异常是否能产生一种与重夸克电荷无关的新手征性拖拽力？
• RQ2 该手征性拖拽力是否在重夸克静止的参考系中消失，从而表明输运过程无耗散？
• RQ3 手征性拖拽力在大小和方向上与手征性磁效应（CME）有何异同？
• RQ4 流体梯度和化学势在何种程度上改变了拖拽力，使其超出标准流体动力学修正？
• RQ5 能否通过事件间相关性观测到手征性拖拽力在重离子碰撞中的表现？

主要发现

• 由于轴向异常，在手征性等离子体中产生了一种新的手征性拖拽力，其方向与外磁场或流体涡度平行或反平行。
• 该手征性拖拽力与重夸克的电荷无关，对夸克和反夸克的影响相同，从而有别于CME电流。
• 当重夸克处于静止的适当惯性参考系中时，该拖拽力消失，此时反向流动的动量流与电荷流相互抵消。
• 该力的消失为手征性输运中不存在耗散提供了直接证据，证实了基于对称性的理论预测。
• 估计该手征性拖拽力可使底夸克获得高达约3 MeV的动量冲量，粲夸克约1 MeV，但这些数值可能被高估。
• 提出了一种事件间相关性可观测量，其与CME电流方向呈反相关，且与单个事件中所有重夸克及反夸克的净动量冲量相关。