11 September 2025
Physics with small ions: jet quenching in Oxygen collisions at the LHC
In July 2025, ALICE recorded for the first time Oxygen-Oxygen (OO) collisions at a center-of-mass energy of 5.36 TeV per nucleon pair, collecting about ten times more data than originally expected thanks to the excellent performance of the LHC. Oxygen, being significantly lighter than lead, provides a unique opportunity to explore whether a quark-gluon plasma (QGP) can form in small systems. The first results, especially on the search for jet quenching, have been eagerly awaited.
The ALICE Collaboration has now released the first results on neutral pion production in OO collisions at the LHC, only nine weeks after the data taking. The results were presented today at the Initial Stages 2025 conference, providing the first hints of jet quenching in OO collisions.
A powerful way to test whether energetic particles lose energy in the QGP medium is to study the production of neutral pions at large transverse momentum. These particles are produced in large numbers when quarks and gluons fragment, and they quickly decay into two photons, which ALICE measures with its electromagnetic calorimeter. If quarks and gluons lose energy in the plasma, fewer high-energy neutral pions are observed - a clear signature of jet quenching.
This effect has already been demonstrated by the ALICE collaboration in Pb-Pb collisions (and for charged hadrons in Pb-Pb and Xe-Xe collisions), where neutral pion production is strongly suppressed compared to proton-proton (pp) collisions - a clear signature of jet quenching.
Remarkably, the new ALICE results in OO collisions show a similar suppression pattern: not only is the suppression considerable in size, but its dependence on transverse momentum closely resembles that observed in larger systems. This behavior stands in clear contrast to neutral pion suppression factors measured in p–Pb collisions.
The observed suppression provides the first strong indications of jet quenching in Oxygen–Oxygen collisions — the smallest ion system where this phenomenon has been seen so far. They mark the beginning of a series of measurements by the ALICE collaboration to better understand jet quenching and collective phenomena in light-ion collisions.
Nuclear modification factor (RAA) of neutral pion production in OO collisions, compared to previous charged hadron measurements in Xe-Xe and Pb-Pb collisions. While no significant suppression (RAA close to unity) is observed at high-pT in p-Pb collisions, a significant suppression is observed for neutral pion production in OO collisions. Comparison to charged hadron results in Xe-Xe and Pb-Pb collisions indicates that the suppression increases with larger ion size.