Centrality-dependent nuclear modification from hard-soft correlations in the glasma

Abstract

We present first predictions from the saturation-physics-based framework IP-Glasma for the nuclear modification factor $R_{AB}$ as a function of centrality in $p$ + O, $p$ + Pb, O + O, and Pb + Pb collisions due only to initial-state effects. The same framework is responsible for both soft ($p_T \lesssim 3 ~\mathrm{GeV}$) and semi-hard ($5 ~\mathrm{GeV} \lesssim p_T \lesssim 20 ~\mathrm{GeV}$) particle production, enabling the study of initial-state correlations between bulk and intermediate-$p_T$ particle production from a first-principles framework. We show that, tuned only to HERA data, IP-Glasma accurately predicts the self-normalized multiplicity distributions in $p$ + O, $p$ + Pb, O + O, and Pb + Pb collisions; the minimum-bias nuclear modification factor in $p$ + O and $p$ + Pb collisions; the centrality-cut nuclear modification factor in $p$ + Pb collisions; and the anomalous suppression of $R_{AA}$ observed in very peripheral Pb + Pb collisions. We find that IP-Glasma predicts significantly less suppression than is measured in both $\sqrt{s_{NN}} = 5.36 ~\mathrm{TeV}$ O + O and $\sqrt{s_{NN}} = 5.02 ~\mathrm{TeV}$ Pb + Pb collisions at the Large Hadron Collider, in qualitative agreement with the scenario in which $R_{AA} < 1$ is due to final-state energy loss. We show that the inelastic nucleon-nucleon cross section ($σ_{\text{inel}}^{NN}$) produced by IP-Glasma is extremely sensitive to the area of the subnucleonic hotspots; the same values of the hotspot area that reproduce the measured $σ_{\text{inel}}^{NN}$ also reproduce minimum-bias $R_{pA}$. Finally, we show that the hard-soft correlations in IP-Glasma arise from event-by-event fluctuations in the color fields, which simultaneously drive enhanced production of both soft and hard particles.