News on charged and neutral hadron production from NA61/SHINE

Abstract

Strangeness production in high-energy hadronic and nuclear collisions remains a central topic in the study of strongly interacting matter under extreme conditions. The data collected by the NA61/SHINE experiment at the CERN SPS allow for a comprehensive scan of hadron production across various collision energies and system sizes. This article focuses on new results on charged and neutral hadron production in central collisions of medium-sized nuclei, such as Ar+Sc and Xe+La, in the SPS energy range. First, the energy and system-size dependence of the $K^+/\pi^+$ ratio is discussed, confirming the ``horn'' structure in central Pb+Pb collisions and its absence in smaller systems. Furthermore, the inverse slope parameter $T$ of kaon transverse mass (transverse momentum) spectra is analyzed, showing that intermediate-size systems reach values comparable to those observed in central Pb+Pb collisions. Second, normalized proton rapidity spectra $(\text{d}n/\text{d}y)/\langle W \rangle$ in central Be+Be, Ar+Sc, and Pb+Pb collisions are presented, demonstrating energy-dependent baryon transport as a function of system size. Building on this baseline, $\Lambda$ hyperon production in central Ar+Sc collisions is explored across SPS energies, where rapidity spectra, mean multiplicities ($\langle \Lambda \rangle$), and ratios to pions are compared with model predictions and existing data. Finally, an anomalous excess of charged over neutral kaons ($R_K > 1$) in central Ar+Sc collisions is reported, highlighting a potential unexpectedly large violation of isospin symmetry.