Systematic theoretical uncertainties in jet quenching due to gluon kinematics

Abstract

We find that the current radiative energy loss kernels obtained from the opacity expansion dramatically violate the collinear approximation used in their derivation. By keeping only the lowest order in collinearity terms, models based on the opacity expansion have ∼50% systematic uncertainty in the calculation of π 0 R AA in the most central RHIC collisions, resulting in a systematic uncertainty of ∼200% in the extracted medium density. Surprisingly, the inclusion of a thermal gluon mass of the order of the Debye screening scale affects R AA only at about the 5% level due to nonintuitive coherence effects. For some observables such as R AA , the effect of these uncertainties decreases with increasing jet energy; for others, such as the average number of radiated gluons, the effect is energy independent. We note that it is likely that the differences reported in the extracted values of medium parameters such asq by various jet energy loss models will fall within this collinear approximation systematic uncertainty; it is imperative for the quantitative extraction of medium parameters or the possible falsification of the hypothesis of weak coupling between the hard probes and the soft modes of the quark-gluon plasma medium that future radiative energy loss research push beyond the lowest-order collinear approximation.

Comparison of jet quenching formalisms for a quark-gluon plasma “brick”

“…III B, for the opacity expansions, where the resulting uncertainty is found to be about a factor 2 in the gluon spectra. The resulting uncertainty in R AA decreases with increasing p T [54]. In Sec.…”

“…6). However, owing to nontrivial interference effects, its effect on observables such as R AA is actually not very large [54].…”

“…Note the very large difference in the results for the two collinearly equivalent definitions of x and that for the result with a reduced θ max . Of course, this enormous difference implies a large (factor 2-3) uncertainty in the extraction of the medium parameters from leading hadron suppression data [54].…”

Zeroing in on jet quenching: a PHENIX perspective

“…In contrast, non-perturbative approaches, for example those based on AdS/CFT gauge gravity duality [1], seem to work well. Even within the pQCD framework, there are many models that are based on different and often uncontrolled approximations [2], which, when tuned to the same data, predict very different medium properties.…”

Jet Quenching in Relativistic Heavy Ion Collisions at the LHC

“…For energy loss, this distribution is integrated over k t and the behavior at large angles is linked to how the kinematic constraints on the parton are enforced and the assumption of collinearity. It has been shown that the choice of maximum opening angle θ max can change the results appreciably, leading to at the least a large systematic error [150,178] Clearly the assumption of collinearity is badly violated: for values of x ⇠ µ/E, dN g /dxdk T reaches its maximum value at k T ⇠ xE. For these values of x the emission spectrum is highly sensitive to the choice of k max :…”