From Lead to Helium: Discovery Potential for Jet Quenching in the Smallest Collision Systems

Abstract

We present perturbative quantum chromodynamics (pQCD) predictions for the modification to the yield of high-momentum particles in very light ion collisions - ${}^{10}\mathrm{B} + {}^{10}\mathrm{B}$, ${}^{6}\mathrm{Li} + {}^{6}\mathrm{Li}$, ${}^{4}\mathrm{He} + {}^{4}\mathrm{He}$, and ${}^{3}\mathrm{He} + {}^{3}\mathrm{He}$ - both with and without medium-induced energy loss. We show that there is non-trivial suppression expected from our partonic energy loss model in symmetric systems from ${}^{208}\mathrm{Pb} + {}^{208}\mathrm{Pb}$ to ${}^{3}\mathrm{He} + {}^{3}\mathrm{He}$ and in asymmetric systems $A + B$, and that the energy loss scales approximately with $(\sqrt{A B})^{1 / 3}$. Further, we find that deep inelastic scattering measurements in ${}^{3}\mathrm{He}$ and ${}^{6}\mathrm{Li}$ tightly constrain the nPDF baseline, making these isotopes a particularly clean environment for observing final-state partonic energy loss induced by the formation of a quark-gluon plasma in these very small systems.

Figures (2)

• Figure 1: $R_{AB}$ versus $(\sqrt{AB})^{1/3}$ for collisions of nuclei with mass numbers $A$ and $B$. Experimental data for $\pi^0$ and $h^{\pm}$ hadrons produced in $\mathrm{Pb} + \mathrm{Pb}$CMS:2016xef, $\mathrm{Au} + \mathrm{Au}$PHENIX:2008saf, $\mathrm{Xe} + \mathrm{Xe}$CMS:2018yyx, $p + \mathrm{Pb}$ATLAS:2022kqu, $p / d / {}^3 \mathrm{He} + \mathrm{Au}$PHENIX:2021dod, and preliminary $\mathrm{O} + \mathrm{O}$CMS:2025bta and $\mathrm{Ne} + \mathrm{Ne}$2969907 are shown in black. Partonic energy loss predictions (red) and NLO pQCD nPDF-only (blue) are shown for $\pi^0$ hadrons. $\mathrm{O} + \mathrm{O}$, $\mathrm{Ne} + \mathrm{Ne}$, $\mathrm{Pb} + \mathrm{Pb}$, and $\mathrm{Xe} + \mathrm{Xe}$ data were rebinned in 2969907 to $9.6 ~\mathrm{GeV} \leq p_T \leq 12 ~\mathrm{GeV}$, and other data were matched to the closest $p_T$ bins. Systematic theoretical uncertainties are shaded, experimental systematic uncertainties bracketed, and statistical uncertainties are smaller than the markers. Experimental and nPDF uncertainties are shown at $68\%$ CL; energy loss uncertainties represent an envelope of model predictions. A dashed line is fit to symmetric-system data; the solid point at unity denotes $R_{pp} \equiv 1$.
• Figure 2: Plot of $R_{AA}$ as a function of $p_T$ for $\pi^0$ hadrons produced in ${}^{3}\mathrm{He} + {}^{3}\mathrm{He}$ (left), ${}^{4}\mathrm{He} + {}^{4}\mathrm{He}$ (central), and ${}^6\mathrm{Li} + {}^6\mathrm{Li}$ (right) collisions. Predictions from our partonic energy loss model are shown for $0\text{--}100\%$ centrality (red) as well as $0\text{--}10\%$ centrality (green), where the band represents the envelope of all energy loss results computed with different variations in the theoretical uncertainty. The nPDF-only predictions are also shown (blue), where the lighter, outer band represents the nPDF and PDF uncertainties and the darker, inner band represents the scale uncertainties.