Summary
The STAR Collaboration measured about 20% lower yields of recoiling charged hadrons and jets in high-activity than low-activity oxygen–oxygen collisions at RHIC. The result provides evidence for jet quenching in this relatively small collision system.
The STAR Collaboration has measured jet quenching in oxygen–oxygen collisions at the Relativistic Heavy Ion Collider (RHIC), reporting that yields of recoiling charged hadrons and charged-particle jets were about 20% lower in high-event-activity collisions than in low-activity ones. The measurements, published in Physical Review Letters on 20 October 2026, provide evidence for jet quenching in the relatively small system created by colliding oxygen nuclei at a centre-of-mass energy of 200 GeV per nucleon pair.
What STAR measured
STAR studied correlations between high-transverse-momentum charged hadrons and particles or jets recoiling in the opposite direction. The trigger hadrons had transverse momentum, or momentum perpendicular to the beam, between 7 and 30 GeV/c. The associated particles were charged hadrons with 3–7 GeV/c or charged-particle jets with transverse momentum above 8 GeV/c.
The team compared events selected for high activity with those selected for low activity. It normalized the associated-hadron and jet yields to the number of trigger hadrons. Those normalized yields were suppressed by approximately 20% in the high-activity group. The paper’s abstract says the data exclude an absence of suppression with high significance.
For charged-particle jets reconstructed with radius parameter R=0.5, STAR quantified the change as a transverse-momentum shift of 0.70 ± 0.15 (stat.) ± 0.10 (syst.) GeV/c. The collaboration interprets this shift as a measure of energy redistribution caused by interactions after the collision.
Why jet quenching matters
Energetic quarks and gluons produced in a collision develop into sprays of particles called jets. As they pass through matter produced in the collision, interactions can redistribute their energy, changing the observed yields and momentum of particles associated with the jet. This effect is known as jet quenching and is used to study the properties of the hot, dense matter created in heavy-ion collisions.
The STAR result extends measurements of this effect to oxygen–oxygen collisions. The collaboration says the findings provide strong evidence for jet quenching in this system and offer new insight into quark-gluon plasma formation in small collision systems. The result is based on a comparison of event-activity classes within oxygen collisions, rather than a claim that every jet loses a fixed fraction of its energy.