News from NA61/SHINE

TL;DR

NA61/SHINE's strong interaction program investigates the onset of deconfinement and the QCD critical point via a fixed-target beam-momentum scan covering several system sizes from p+p to Pb+Pb. The paper consolidates recent measurements of particle spectra, fluctuations, correlations, femtoscopy, and a first direct open charm measurement, highlighting system-size dependencies and model discrimination. While horn/step-like structures and fluctuation signals are seen in heavy systems, no definitive critical-point signature emerges, and surprising isospin-violation in kaon production challenges conventional hadron gas expectations. The results constrain deconfinement scenarios and guide future data-taking, including a substantial open charm program and broader system-size studies.

Abstract

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.

Figures (6)

• Figure 1: Left and middle: horn plots -- $K^{+}/\pi^{+}$ ratio versus energy at mid-rapidity (left) and in $4\pi$ (middle). Right: example of step plot -- inverse slope parameter of transverse mass/transverse momentum spectrum of $K^{+}$. See Ref. NA61SHINE:2023epu for 0--10% Ar+Sc results and references to NA61/ SHINE published p+p and 0--20% Be+Be as well as world data. NA61/ SHINE Xe+La (0--20% central for 16.8 $\hbox{Ge V}$ and 0--10% central for lower energies) and 0--7.2% Pb+Pb results are preliminary. For Xe+La, $T$ and $K^{+}/\pi^{+}$ at mid-rapidity ($y \approx 0$) was in fact obtained for $0.4 < y < 0.6$. For NA61/ SHINE Pb+Pb, $T$ was obtained for $0.8 < y < 1.0$. For NA61/ SHINE points, vertical bars denote statistical uncertainties, and color bands – systematic ones.
• Figure 2: Energy dependence of $\langle \Lambda \rangle / \langle \pi \rangle$ mean multiplicity ratio (left) and $E_\mathrm{S}$ factor (right). NA61/ SHINE p+p data are preliminary (8.8 $\hbox{Ge V}$) and taken from Ref. NA61SHINE:2015haq (17.3 $\hbox{Ge V}$). NA61/ SHINE 0--10% central Ar+Sc points are preliminary. See Ref. QM2025_YB for references to world data. Assumptions used in the plots: $\langle \pi \rangle = 1.5 \cdot (\langle \pi^+ \rangle + \langle \pi^- \rangle)$; $\langle K + \overline{K} \rangle_{A+A} = 2 \cdot (\langle K^+ \rangle + \langle K^- \rangle)$; $\langle K + \overline{K} \rangle_{p+p} = 4 \cdot \langle K^0_\mathrm{S}\xspace \rangle$. For NA61/ SHINE points, vertical bars denote statistical uncertainties, and color bands – systematic ones.
• Figure 3: Lévy stability parameter $\alpha$ (constant fit to all $m_{\textrm{T}}$ values) versus energy in 0--20% central Be+Be collisions at $\sqrt{s_{\mathrm{NN}}}$ = 16.8 $\hbox{Ge V}$ (Ref. NA61SHINE:2023qzr with later estimate of systematic uncertainty) as well as in 0--10% central Ar+Sc collisions at 5.1--16.8 $\hbox{Ge V}$ (NA61/ SHINE preliminary). Vertical bars denote statistical uncertainties, and color boxes – systematic ones.
• Figure 4: Energy dependence of intensive quantities of net-electric charge distribution in Ar+Sc collisions NA61SHINE:2025whi compared to published p+p results SHINE:2023ejm. Vertical bars denote statistical uncertainties, and color bands -- systematic ones.
• Figure 5: Ratio of charged to neutral $K$ meson yields in nucleus-nucleus collisions as a function of collision energy. The black line shows the Hadron Resonance Gas (HRG) model predictions for $Q/B=0.4$. The black dots indicate the HRG predictions for $Q/B$ values corresponding to the ones in the experiments. For different nuclei, $Q/B$ corresponds to the electric charge over the baryon number of the whole system. The gray squares show UrQMD predictions. Vertical bars denote total uncertainties. Figure taken from Ref. NA61SHINE:2023azp with new preliminary Ar+Sc 8.8 $\hbox{Ge V}$ point added QM2025_YB.