CMS measures suppression in oxygen and neon collisions

CMS has measured mesons in oxygen-oxygen and neon-neon collisions for the first time, using 2025 light-ion data at 5.36 TeV and comparing the yields with proton-proton collisions at the same energy. The collaboration presented the result as EXO-25-021 on June 24, 2026, during Hard Probes 2026 in Nashville, Tennessee.

The new measurement covers all three S-wave bottomonium states, (1S), (2S) and (3S), and shows a sequential suppression pattern. The excited (2S) and (3S) states were suppressed more strongly than the ground-state (1S), exactly the hierarchy physicists look for when a hot, dense medium is strong enough to break apart weakly bound quark-antiquark states. CMS quantified the effect with double ratios of ion-ion yields relative to proton-proton yields.

The oxygen and neon runs give CMS another calibration point for how suppression changes as the initial nuclear geometry shrinks, and how small a collision system can be while still showing partonic energy loss and quarkonium dissociation.

The new data also land on top of an earlier CMS light-ion result that already showed medium effects in the same 2025 campaign. At the Initial Stages 2025 conference, CMS found about 70% suppression of charged particles around transverse momentum of 6 GeV in oxygen-oxygen and neon-neon collisions, with no suppression at the highest pT near 100 GeV. CMS also found the suppression slightly stronger in neon-neon than in oxygen-oxygen, a comparison that fits the neon nucleus's bowling-pin shape.

CERN’s 2025 light-ion cycle included the first-ever oxygen-oxygen and neon-neon collisions at the LHC, along with proton-oxygen running over several days in late June and early July. The program had been in feasibility studies since 2019, and the 2025 LHC delivered a record amount of data.