**61**: 553-567

https://doi.org/10.1140/epjc/s10052-009-0885-6

Regular Article - Theoretical Physics

## Stirring strongly coupled plasma

^{1}
Physics Department, Shahrood University of Technology, Shahrood, Iran

^{2}
Center for Theoretical Physics, MIT, Cambridge, MA, 02139, USA

^{3}
Department of Physics, CERN, Theory Division, 1211, Geneva 23, Switzerland

^{*} e-mail: Urs.Wiedemann@cern.ch

Received:
10
October
2008

Published online:
27
February
2009

We determine the energy it takes to move a test quark along a circle of radius *L* with angular frequency *ω* through the strongly coupled plasma of
supersymmetric Yang–Mills (SYM) theory. We find that for most values of *L* and *ω* the energy deposited by stirring the plasma in this way is governed either by the drag force acting on a test quark moving through the plasma in a straight line with speed *v*=*L*
*ω* or by the energy radiated by a quark in circular motion in the absence of any plasma, whichever is larger. There is a continuous crossover from the drag-dominated regime (*ω*≲*π*
*T*(1−*v*
^{2})^{3/4}, meaning *ω*≲*π*
*T* and *L* small enough) to the radiation-dominated regime (*ω*≳*π*
*T*(1−*v*
^{2})^{3/4}). In the crossover regime we find evidence for significant destructive interference between energy loss due to drag and due to radiation as if in vacuum. The rotating quark thus serves as a model system in which the relative strength of, and interplay between, two different mechanisms of parton energy loss is accessible via a controlled classical gravity calculation. We close by speculating on the implications of our results for a quark that is moving through the plasma in a straight line while decelerating, although in this case the classical calculation breaks down at the same value of the deceleration as the one at which the radiation-dominated regime sets in.

*© Springer-Verlag , 2009*