Radiative and Collisional Jet Energy Loss in the Quark-Gluon Plasma at the BNL Relativistic Heavy Ion Collider

Abstract

We calculate and compare bremsstrahlung and collisional energy loss of hard partons traversing a quark-gluon plasma. Our treatment of both processes is complete at leading order in the coupling and accounts for the probabilistic nature of the jet energy loss. We find that the nuclear modification factor RAA for neutral π 0 production in heavy ion collisions is sensitive to the inclusion of collisional and radiative energy loss contributions while the averaged energy loss only slightly increases if collisional energy loss is included for parent parton energies E ≫ T . These results are important for the understanding of jet quenching in Au+Au collisions at 200 AGeV at RHIC. Comparison with data is performed applying the energy loss calculation to a relativistic ideal (3+1)-dimensional hydrodynamic description of the thermalized medium formed at RHIC.Introduction -Relativistic heavy ion collisions are designed to produce and study strongly interacting matter at high temperatures and densities. Experiments at the Relativistic Heavy Ion Collider (RHIC) have demonstrated that high p T hadrons in central A + A collisions are significantly suppressed in comparison with those in binary p + p collisions, scaled to nucleus-nucleus collisions [1,2,3]. This result has been referred to as jet quenching and has been attributed to the energy loss of hard p T partons due to induced gluon bremsstrahlung in a hot quark-gluon plasma. Bremsstrahlung energy loss has been calculated in several theoretical formalisms before [4,5,6,7,8,9]. Recently such bremsstrahlung calculations were implemented in models employing relativistic ideal (3+1)-dimensional hydrodynamics in order to calculate the nuclear modification factor R AA of neutral pions at RHIC [10,11,12]. Early estimates of the collisional energy loss which used asymptotic arguments indicated that the radiative energy loss is much larger than elastic energy loss [13]. Zakarhov compared radiative energy loss in the light-cone path integral approach and collisional energy loss employing the Bjorken method and concluded collisional energy loss is relatively small in comparison to the radiative one [7]. Renk derives phenomenological limits on radiative vs. collisional energy loss by considering quadratic vs. linear pathlength dependence and concludes that any elastic energy loss component has to be small [14]. In contrast, Mustafa and Thoma find that collisional energy loss has a significant influence on jet quenching [15,16]. Recent studies by Gyulassy and collaborators also point in this direction, see e.g. [17,18].The purpose of this study is to consistently incorporate collisional and radiative energy loss in the same formalism and to employ this formalism in a realistic description of energy loss of hard p T leading partons in the soft nuclear medium as described by (3+1)-dimensional hydrodynamics in 200 AGeV Au+Au collisions at RHIC.We will emphasize three points (the first two of which have been elucidated earlier [19] for bremsstrahlung energy loss). First, in many previous a...

Systematic theoretical uncertainties in jet quenching due to gluon kinematics

“…Most strikingly, we find that keeping only the lowestorder term in collinearity results in an ∼200% systematic uncertainty in the value of the extracted gluon rapidity density for central Au + Au collisions at top RHIC energies. While this paper does not quantify the uncertainties associated with the collinear approximation for other pQCD radiative energy loss models [23,27,34,35], the uncertainties in those models are almost certainly similar to the results quoted here. In particular, the sizable discrepancies between the extracted medium properties obtained by different energy loss groups [8,9,36] are probably within the current theoretical systematic uncertainty.…”

Collisional energy loss of heavy quarks

“…An additional contribution of suppression, potentially important for the range of smaller (to medium) momenta, is the collisional energy loss by elastic processes as discussed in [10,11,12] and with further modifications in [13,14,15,16,17,18]. It will turn out that in our approach a fairly large collisional energy loss is predicted, although some comparisons between collisional and radiative contributions [19,20] indicate another picture where the collisional loss should be relatively small (∼ 20%) compared to the radiative one. This conclusion was, however, based on a fixed coupling approach, and using a naive approximation for the required infrared regulator in the QCD cross section.…”

“…In the following work both aspects will be considered in a more precise way, aiming at providing Email address: meistren@uni-frankfurt.de (Alex Meistrenko) 1 M c ∼ 1.3 GeV, M b ∼ 4.6 GeV a computational framework to describe binary parton collision effects more reliably, which seems necessary as a baseline to understand quenching in heavy-ion experiments. Noted in this context is that certain observables might be more sensitive to the inclusion of binary collisions than the averaged energy loss, due to different probability distributions of radiative and collisional processes [13,19].…”

Effect of running coupling on photons from jet–plasma interaction in relativistic heavy-ion collisions

“…On the other hand, the authors of Ref. [16] claims that the collisional energy loss is sub-leading. However, in order to see the effects of energy loss on jet-photon one should also incorporate the radiative energy loss for completeness and this has to be done in the same formalism in a realistic scenario.…”