Through the blackness -- high p_T hadrons probing the central region of 200 AGeV Au-Au collisions

Abstract

The energy loss of high p_T partons propagating through a hot and dense medium is regarded as a valuable tool to probe the medium created in ultrarelativistic heavy-ion collisions. The angular correlation pattern of hadrons associated with a hard trigger in the region of p_T ~1-2 GeV which exhibits a dip in the expected position of the away side jet has given rise to the idea that energy is lost predominantly to propagating collective modes ('Mach cones'). Recent measurements by the STAR collaboration have shown that for a high p_T > 8 GeV trigger the angular pattern of associate hadrons for p_T > 4 GeV shows the emergence of the expected away side peak. These di-jet events suggest that the away side parton may emerge occasionally without substantial energy loss. Since in such a back-to-back configuration one of the partons may travel through the central region of the fireball, the average in-medium pathlength is substantial and the expected energy loss is not only sensitive to the initial geometry of matter but also to the change of geometry due to expansion. We show that radiative energy loss is able to explain the dijet events provided that the expansion of the medium is taken into account.