An LHCb study reports that the production of D0 mesons — particles that contain a charm quark — differs between collisions of neon nuclei and collisions of oxygen nuclei at a center‑of‑mass energy per nucleon pair of 5.36 TeV. The experiment measures a ratio of yields that changes with the particle transverse momentum (momentum perpendicular to the beam). This pattern cannot be explained by simple changes to the structure of protons and neutrons inside nuclei alone, and is consistent with the idea that a hot, dense state of deconfined quarks and gluons, called a quark–gluon plasma (QGP), begins to form as the colliding nuclei become larger.

The researchers used samples collected in 2025: 5.5 nb−1 of oxygen–oxygen (OO) collisions and 0.51 nb−1 of neon–neon (NeNe) collisions. They reconstructed D0 mesons through their decay to a kaon and a pion (D0 → K−π+). Yields were measured as a function of transverse momentum, pT, in the range 0.5–20 GeV and in the forward detector region corresponding to rapidity 2.0–4.5. For a clean comparison, the measured yields in each system were divided by the number of recorded inelastic collisions in that data sample.

To separate D0 mesons produced directly in the collision (prompt) from those coming from decays of longer‑lived b hadrons (nonprompt), the team fit the kaon–pion mass distribution and the distribution of a quantity related to how far the D0 decay appears from the collision point (the ln χ2IP). Detector effects and efficiencies were corrected using a mix of simulated events and data‑driven calibrations. Particle identification efficiencies came from D*+ → D0 π+ decays, and tracking efficiencies used K0S → π+π− decays. The overall efficiency corrections between OO and NeNe were small, near 1.03–1.04 across most pT bins, with a slightly larger correction at the highest pT.