News from NA61/SHINE

The main goal of the NA61/SHINE strong interaction program is to search for the critical point in the phase diagram of strongly interacting matter and to investigate phenomena related to the onset of deconfinement. In recent years, the program has expanded to include the study of open charm, aiming to understand the mechanisms of its production in heavy-ion collisions. This article presents a selection of recent results from the NA61/SHINE strong interaction program, including findings on particle spectra and yields, as well as fluctuations and correlations. Plans for the near future are also discussed.

💡 Research Summary

The NA61/SHINE experiment at the CERN Super Proton Synchrotron (SPS) continues its comprehensive strong‑interaction program, which aims to map the phase diagram of strongly interacting matter, locate the QCD critical point, and study the onset of deconfinement (OD). By performing a systematic beam‑momentum scan (13 A–150 A GeV/c, √s_NN = 5.1–17.3 GeV) with a variety of projectile–target combinations ranging from p+p to Pb+Pb, the collaboration has produced a wealth of new data.

A central focus of the paper is the investigation of the classic “horn” (K⁺/π⁺ ratio) and “step” (inverse slope parameter T of the transverse‑mass spectra) signatures of OD. New measurements for Be+Be, Ar+Sc, Xe+La and Pb+Pb confirm that the sharp horn observed by NA49 in central Pb+Pb persists at √s_NN = 7.6 GeV, while smaller systems display a much smoother energy dependence. The step‑like behavior of T is evident across all systems, with a plateau appearing already in Be+Be and becoming more pronounced for heavier nuclei. These results suggest that the structures associated with the onset of a mixed phase are not exclusive to the heaviest systems but evolve smoothly with system size.

The paper also presents recent Λ hyperon data. In central Ar+Sc collisions, both the mean multiplicity ⟨Λ⟩ and the mid‑rapidity yield show a flat energy dependence, mirroring the behavior seen in Pb+Pb. The ratio ⟨Λ⟩/⟨π⟩ and the strangeness‑to‑entropy proxy E_S do not exhibit a horn‑like maximum in Ar+Sc, although the trend follows that of heavy‑ion data. An upcoming low‑energy point (√s_NN = 5.1 GeV) will clarify whether a maximum appears for this intermediate system.

Searches for the critical point are pursued through several fluctuation observables. The collaboration has measured event‑by‑event transverse‑momentum and multiplicity fluctuations, intermittency of protons and negatively charged hadrons, and higher‑order cumulant ratios of net‑electric charge. No definitive critical‑point signatures (e.g., non‑monotonic peaks) have emerged. However, a Lévy‑stable source analysis of identical pion correlations reveals a modest minimum of the Lévy index α around intermediate SPS energies, hinting at a possible change in the emission source geometry. Higher‑order cumulant ratios κ₃/κ₁ and κ₄/κ₂ show a tentative non‑monotonic trend in 0–1 % central Ar+Sc collisions, though large statistical uncertainties preclude firm conclusions. These observations may be related to the OD or to proximity to a critical point, motivating further high‑statistics runs.

A major milestone reported is the first direct measurement of open charm production at SPS energies. Using a prototype vertex detector, NA61/SHINE extracted raw D‑meson yields in 0–20 % central Xe+La collisions at √s_NN = 16.8 GeV and corrected them with three event generators (AMPT, PHSD, Pythia/Angantyr). The resulting total multiplicities lie between the under‑prediction of microscopic transport models and the over‑prediction of statistical hadronization models, providing a stringent test of charm‑production mechanisms. Ongoing analysis of a large Pb+Pb data set (≈5 × 10⁸ minimum‑bias events) collected since 2022 will further refine these measurements.

Perhaps the most surprising result concerns isospin‑symmetry violation in kaon production. In central Ar+Sc collisions at √s_NN = 11.9 GeV, the ratio R_K = (⟨K⁺⟩ + ⟨K⁻⟩)/(2⟨K⁰_S⟩) exceeds unity by about 18 %, corresponding to a 4.7σ deviation from the expectation of exact isospin symmetry (R_K = 1). A similar, though slightly smaller, excess is seen at 8.8 GeV. These findings are not reproduced by the Hadron Resonance Gas model or UrQMD simulations and echo earlier observations in other heavy‑ion systems. The deviation appears to scale with the system’s charge‑to‑baryon ratio (Q/B), suggesting a novel source of isospin breaking beyond conventional electromagnetic effects.

Looking ahead, NA61/SHINE plans to accumulate 5 × 10⁸ minimum‑bias Pb+Pb events at 16.8 GeV by 2026, enabling high‑precision open‑charm studies. After the LHC Long Shutdown 3, the program will extend to intermediate‑mass systems (Mg+Mg, O+O, B+B) at several energies, with a pilot O+O run already recorded in 2025. These measurements aim to map the system‑size dependence of the K⁺/π⁺ ratio, the inverse‑slope T, and to test isospin symmetry in truly symmetric Z = N collisions (e.g., ¹⁶⁸O+¹⁶⁸O). Additionally, 24 M events of π⁺+C and π⁻+C at 158 GeV/c have been recorded to investigate whether the charged‑to‑neutral kaon ratio mirrors the isospin expectation in pion‑induced reactions.

In summary, the paper presents a comprehensive set of new results that refine our understanding of the onset of deconfinement, provide stringent constraints on charm‑production models, reveal unexpected isospin‑symmetry violation, and continue the systematic search for the QCD critical point. The forthcoming high‑statistics data and expanded system‑size program promise to deepen insights into the phase structure of strongly interacting matter.