Evidence for parton energy loss in oxygen$-$oxygen collisions at $\mathbf{\sqrt{s_{\rm NN}}=5.36}$ TeV

Ultra-relativistic heavy-ion collisions create a hot and dense medium of deconfined quarks and gluons, the quark--gluon plasma (QGP), in which parton energy loss (``jet quenching'') is a key probe of hot medium properties. While parton energy loss has been firmly established in large systems such as Pb--Pb and Au--Au collisions, no unambiguous direct evidence exists in smaller systems such as high-multiplicity p--Pb and pp collisions. To probe the onset of parton energy loss at intermediate system size, measurements of neutral-pion production are presented in this Letter for oxygen--oxygen (OO) and proton--oxygen (pO) collisions recorded with the ALICE detector in July 2025, relative to a pp baseline. The nuclear modification factor $R_{\rm OO}$ is suppressed relative to unity with a transverse-momentum dependence similar to that observed in Pb--Pb collisions, consistent with a previous CMS measurement in OO collisions with charged particles. As $R_{\rm OO}$ contains contributions from both cold and hot nuclear matter effects, $R_{\rm pO}$ is also presented in order to constrain cold nuclear matter (CNM) contributions. $R_{\rm pO}$ is found to be compatible with unity, indicating that CNM effects alone cannot account for the suppression observed in $R_{\rm OO}$. Final-state effects are isolated using the measured double ratio $R_{\rm OO}/R_{\rm pO}^{2}$, which largely cancels CNM contributions and exhibits a significant suppression relative to expectations without energy loss at a 4.9$\sigma$ level. Theoretical models incorporating parton energy loss via different mechanisms predict a significant suppression of the $R_{\rm OO}/R_{\rm pO}^{2}$ relative to unity, consistent with the data. These findings establish parton energy loss in OO collisions, extending experimental evidence for jet quenching to the smallest nuclear system studied to date.