Probing the Dependence of Partonic Energy Loss on the Initial Energy Density of the Quark Gluon Plasma
Considerable evidence now exists for partonic energy loss due to interaction with the hot, dense medium created in ultra-relativistic heavy-ion collisions. A primary signal of this energy loss is the suppression of high transverse momentum $p_{\mathrm{T}}$ hadron yields in A-A collisions relative to appropriately scaled $pp$ collisions at the same energy. Measuring the collision energy dependence of this energy loss is vital to understanding the medium, but it is difficult to disentangle the medium-driven energy loss from the natural kinematic variance of the steeply-falling $p_{\mathrm{T}}$ spectra across different $\sqrt{s_{\mathrm{NN}}}$. To decouple these effects, we utilize a phenomenologically motivated spectrum shift model to estimate the average transverse momentum loss $\Delta p_{\mathrm{T}}$ imparted on high $p_{\mathrm{T}}$ partons in A-A collisions, a proxy for the medium induced energy loss. We observe a striking correlation between $\Delta p_{\mathrm{T}}$ and Glauber-derived estimates of initial state energy density $\varepsilon_{\mathrm{Bj}}$, consistent across two orders of magnitude in collision energy for a variety of nuclear species. To access the path-length dependence of energy loss, we couple our model to geometric event shape estimates extracted from Glauber calculations to produce predictions for high-$p_{\mathrm{T}}$ hadron elliptic flow $v_2$ that agree reasonably with data.