Statistical analysis of pQCD energy loss across system size, flavor, $\sqrt{s_{NN}}$, and $p_T$

Abstract

We present suppression predictions from our pQCD-based energy loss model, which receives small system size corrections, for high-$p_T$ $\pi$, $D$ and $B$ meson $R_{AB}$ as a function of centrality, flavor, $\sqrt{s_{NN}}$, and $p_T$ from large to small collision systems at RHIC and LHC. A statistical analysis is used to constrain the effective strong coupling in our model to available high-$p_T$ suppression data from central heavy-ion collisions at RHIC and LHC, yielding good agreement with all available data. We estimate two important theoretical uncertainties in our model, stemming from: the transition between vacuum and hard thermal loop propagators in the collisional energy loss, and from the angular cutoff on the radiated gluon momentum. We find, consistently, that the extracted $\alpha_s$ remains relatively unchanged across heavy- and light-flavor final states and across central, semi-central, and peripheral collisions. We make predictions from our large-system-constrained model for small systems and find good agreement with photon-normalized $R^{\pi^0}_{d \text{Au}} \simeq 0.75 $ in $0-5\%$ centrality $d$ + Au collisions by PHENIX. However, we find strong disagreement with the measured $R^{h^{\pm}}_{p \text{Pb}} \gtrsim 1$ in $0-5\%$ centrality $p$ + Pb collisions by ALICE and ATLAS; we argue that this disagreement is due, in large part, to centrality bias. We make predictions for the ratio of suppression in ${}^3$He + Au and $p$ + Au collisions, which may in the future be used to disentangle final- from initial-state suppression in small systems. We then compare our results to various subsets of data, which allows us to estimate the preferred: low-$p_T$ scale at which non-perturbative processes become important, scales at which the strong coupling runs, and scale at which vacuum propagators transition to thermally modified propagators in collisional energy loss.