The onsets of deconfinement and fireball of NA61/SHINE

TL;DR

The NA61/SHINE experiment investigates the onset of deconfinement and fireball formation in heavy-ion collisions, revealing unexpected behaviors in particle ratios that challenge current theoretical models and suggest new phenomena in relativistic nuclear matter.

Contribution

This paper presents new experimental results on particle production in various ion collisions, highlighting unexpected behaviors and proposing the possible onset of fireball formation.

Findings

Be+Be collisions behave like p+p, unlike other systems.

Ar+Sc is closer to Pb+Pb, indicating complex system-size effects.

Current models do not fully explain the observed behaviors.

Abstract

The NA61/SHINE experiment at the CERN SPS is pursuing a rich programme on strong interactions, covering the study of onset of deconfinement and aims to discover the critical point of strongly interacting matter by performing an energy and system-size scan at the full CERN SPS momentum range. These scans of p+p, p+Pb, Be+Be, Ar+Sc and Pb+Pb have been mostly completed with Xe+La last year (more Pb+Pb to to taken this year). Results from the different reactions are now emerging. As a surprise, some measurements did not scale smoothly. In particular, for the $\rm K^+/\piup^+$ ratio, Be+Be collisions behaved similarly to p+p (as superposition of nucleon collisions), while Ar+Sc was closer to Pb+Pb collisions. This step can not be explained by onset of deconfinement, and may indicate that there is also a onset of fireball in relativistic heavy ion collisions. A review of the results as well as possible interpretation will be presented. The theoretical models (SMES, PHSD) describe onset of deconfinement at the heaviest system relatively well. However, no model describes the behaviour of data at previously unmeasured collisions of light and intermediate size ions. The onset of fireball is not described by any model.