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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.
The main experimental result is a pT‑dependent production ratio RD0(NeNe/OO) that deviates from expectations based only on modification of nucleon structure inside nuclei. Because heavy quarks such as charm are produced early in a collision and then interact with any hot medium that forms, a change in the D0 spectrum with system size and pT can signal energy loss in a QGP. The authors therefore interpret their result as consistent with the onset of QGP effects when going from oxygen to the slightly larger neon system. This connects to a broader context in which QGP signatures have been clearly seen in large heavy‑ion collisions and some QGP‑like signatures have been observed in smaller systems.
Important caveats apply. The result is presented as evidence consistent with QGP onset, not as definitive proof. The measurement covers a specific forward region of the detector and a limited pT range, and it relies on simulation and fitting procedures to correct for detector effects and to separate prompt and nonprompt contributions. Systematic uncertainties are present, and the largest single uncertainty in many bins comes from the determination of the relative number of inelastic OO and NeNe collisions. Further data, complementary measurements at other angles and energies, and detailed theoretical comparisons will be needed to confirm and quantify the presence of parton energy loss and other medium effects in these light‑ion collisions.