Jet Quenching in the Smallest Hadronic Collision Systems

We present perturbative quantum chromodynamics (pQCD) predictions for high-momentum particle yield modification in very light ion collisions - ${}^{10}\mathrm{B}+{}^{10}\mathrm{B}$, ${}^6\mathrm{L}\mathrm{i}+{}^6\mathrm{L}\mathrm{i}$, ${}^4\mathrm{H}\mathrm{e}+{}^4\mathrm{H}\mathrm{e}$, and ${}^3\mathrm{H}\mathrm{e}+{}^3\mathrm{H}\mathrm{e}$ - with and without medium-induced energy loss. We find non-trivial suppression in symmetric systems from ${}^{208}\mathrm{P}\mathrm{b}+{}^{208}\mathrm{P}\mathrm{b}$ to ${}^3\mathrm{H}\mathrm{e}+{}^3\mathrm{H}\mathrm{e}$ and in asymmetric $A+B$ systems, with the suppression scaling approximately as $R_{AB}\simeq (\sqrt{AB}{)}^{1\mathrm{/}3}$. Further, we find that ${}^3\mathrm{H}\mathrm{e}$ and ${}^6\mathrm{L}\mathrm{i}$ offer particularly clean environments for observing final-state partonic energy loss from quark-gluon plasma (QGP) formation in extremely small systems. Finally, we show that energy loss models generically predict $v_2\{\mathrm{S}\mathrm{P}\}\approx 0$ in small systems, indicating that the large measured $v_2>0$ in $p+{}^{208}\mathrm{P}\mathrm{b}$ is not due to energy loss.