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 K+/π+ 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.
Highlights
The onset of deconfinement is a well established concept in strong interaction physics
At lower temperatures and/or baryon chemical potential, strongly interacting matter is confined inside hadrons, while at higher temperatures and/or baryon chemical potential, strongly interacting matter is believed to be in a deconfined state in QGP
The heavy systems are close to statistical model predictions for large volumes
Summary
The onset of deconfinement is a well established concept in strong interaction physics. The search for it is one of the goals of the current physics programme of the NA61/SHINE experiment by performing a energy and system-size scan at the CERN SPS. The analysis of this scan has revealed phenomena that has properties of an onset, but can not be ascribed to the onset deconfinement. This hits at an additional onset effect related to the size of the colliding systems
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