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 N = 4 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 = Lω 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 πT (1−v 2 ) 3/4 , meaning ω πT and L small enough) to the radiation-dominated regime (ω πT (1 − v 2 ) 3/4 ). In the crossover regime we find evidence for significant destructive interference between energy loss due to drag and that 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 at which the radiation-dominated regime sets in.

Using the gauge/gravity duality we study the imaginary part of the static potential associated to the thermal width in finite temperature strongly coupled anisotropic plasma. We firstly derive the potential for a generic anisotropic background. Then we apply our formulas to a theory where the anisotropy has been generated by a space dependent axion term. We find that using our method there exist a peculiar turning point in the imaginary part of the potential, similar to the one appearing in the real part. The presence of anisotropy leads to decrease of the imaginary potential, where larger decrease happens along the anisotropic direction when the temperature is kept fixed. When the entropy density is fixed, increase happens along the parallel direction while along the transverse plane we observe a decrease. To estimate the thermal width we use an approximate extrapolation beyond the turning point and we find a decrease in presence of the anisotropy, independently of the comparison scheme used.

The effects of charge and finite 't Hooft coupling correction on drag force and jet quenching parameter are investigated. To study charge effect and finite 't Hooft coupling correction , we consider Maxwell charge and Gauss-Bonnet terms, respectively. The background is Reissner-Nordström-AdS black brane solution in Gauss-Bonnet gravity. It is shown that these corrections affect drag force and jet quenching parameter. We find an analytic solution of drag force in this background which depends on Gauss-Bonnet coupling and charge. We set Gauss-Bonnet coupling to be zero and find drag force in the case of Reissner-Nordström-AdS background. Also we discuss the relaxation time of a moving heavy quark in this gravity background.

The effect of finite-coupling corrections to the drag force on a moving heavy quark in the Super Yang-Mills plasma is investigated. These corrections are related to curvature-squared corrections in the corresponding gravity dual. The results are compared with the dual gauge theory. It is shown that curvature-squared corrections affect the drag force. It is shown that corrections to the drag force depend on the velocity of the heavy quark. The diffusion coefficient of non-relativistic heavy quarks is calculated from the drag force. In addition, we also calculate the drag force on a moving heavy quark in the Gauss-Bonnet background.

We study the energy loss of a rotating infinitely massive quark moving, at constant velocity, through an anisotropic strongly-coupled N=4 plasma from holography. It is shown that, similar to the isotropic plasma, the energy loss of the rotating quark is due to either the drag force or radiation with a continuous crossover from drag-dominated regime to the radiation dominated regime. We find that the anisotropy has a significant effect on the energy loss of the heavy quark, specially in the crossover regime. We argue that the energy loss due to radiation in anisotropic media is less than the isotropic case. Interestingly this is similar to analogous calculations for the energy loss in weakly coupled anisotropic plasma.Comment: 26+1 pages, 10 figures, typos fixe