Energy loss in a fluctuating hydrodynamical background

TL;DR

This paper investigates how event-by-event density fluctuations in a hydrodynamical background affect high transverse momentum energy loss and nuclear modification factors in heavy-ion collisions at RHIC, comparing radiative and elastic models.

Contribution

It provides the first detailed analysis of the impact of initial density fluctuations on high P_T energy loss and R_AA in heavy-ion collisions.

Findings

Density fluctuations significantly influence R_AA in central and noncentral collisions.

Both radiative and elastic energy-loss models show sensitivity to fluctuation size.

Angular modulation of R_AA with respect to the event plane is affected by fluctuations.

Abstract

Recently it has become apparent that event-by-event fluctuations in the initial state of hydrodynamical modelling of ultrarelativistic heavy-ion collisions are crucial in order to understand the full centrality dependence of the elliptic flow coefficient v_2. In particular, in central collisions the density fluctuations play a major role in generating the spatial eccentricity in the initial state. This raises the question to what degree high P_T physics, in particular leading-parton energy loss, which takes place in the background of an evolving medium, is sensitive to the presence of the event-by-event density fluctuations in the background. In this work, we report results for the effects of fluctuations on the nuclear modification factor R_AA in both central and noncentral sqrt(s_NN) = 200 GeV Au+Au collisions at RHIC. Two different types of energy-loss models, a radiative and an elastic, are considered. In particular, we study the dependence of the results on the assumed spatial size of the density fluctuations, and discuss the angular modulation of R_AA with respect to the event plane.