Highlights from the NA61/SHINE

. NA61 / SHINE is a multipurpose fixed-target facility at the CERN Super Proton Synchrotron. The main goals of the NA61 / SHINE strong-interactions program are to discover the critical point of strongly interacting matter as well as to determine the properties of produced particles relevant for the study of the onset of deconfinement - the transition between the state of hadronic matter and the quark-gluon plasma. An analysis of hadron production properties is performed in nucleus-nucleus, proton-proton, and proton-nucleus interactions as a function of collision energy and size of the colliding nuclei to achieve these goals. The selected NA61 / SHINE results from this program are presented. In particular, the latest results from di ff erent reactions p + p, Be + Be, Ar + Sc, and Pb + Pb on hadron spectra and fluctuations are discussed. The NA61 / SHINE results are compared with worldwide experiments and predictions from various theoretical models, like EPOS, PHSD, UrQMD, and others.


The NA61/SHINE facility
The NA61/SHINE detector [1] is a large acceptance hadron spectrometer with excellent capabilities in charged particle momentum measurements and identification by a set of eight Time Projection Chambers as well as Time-of-Flight detectors.The high resolution forward calorimeter, the Projectile Spectator Detector (PSD), measures energy flow around the beam direction, which in nucleus-nucleus reactions is primarily a measure of the number of projectile spectator (non-interacted) nucleons and is thus related to the violence (centrality) of the collision.A set of beam detectors identifies beam particles and measures precisely their trajectories.The NA61/SHINE detector system was recently upgraded: readout rate was increased to 1kHz, new Vertex Detector and data acquisition and trigger systems were installed, extension of PSD was performed.The schematic view of upgraded system is shown in Fig. 1.

Study of the onset of deconfinement
NA61/SHINE performed a two-dimensional scan in collision energy (13A-150A GeV/c and system size (p+p, Be+Be, Ar+Sc, Xe+La, Pb+Pb) to study the phase diagram of strongly interacting matter.The main goals of NA61/SHINE are the search for the critical point and a study of the onset of deconfinemnet.
A plateau ("step") in the energy dependence of the inverse slope parameter T was observed by the NA49 experiment in Pb+Pb collisions for m T spectra of K ± .It was expected for the onset of deconfinement due to the presence of a mixed phase of hadron gas (HRG) and quark-gluon plasma (QGP) [2].In p+p interactions at SPS energies the inverse slope parameter T of m T spectra shows qualitatively similar enery dependence as in central Pb+Pb collisions ("step") and such a behaviour seems to emerge also in Be+Be reactions, as visible in Fig. 2. The values of the T parameter in Be+Be collisions are slightly above those in p+p interactions.The T parameter in Ar+Sc reactions is found between those in p+p/Be+Be and Pb+Pb collisions.Finally, rapid changes of the ratios K + /π + at mid-rapidity and ⟨K + ⟩ / ⟨π + ⟩ as function of collision energy ("horn") were observed in Pb+Pb collisions by the NA49 experiment.These were predicted by the SMES model [3] as a signature of the onset of deconfinement.These two ratios together with new NA61/SHINE results from Be+Be and Ar+Sc collisions are shown in Fig. 3.A plateau like structure is visible in p+p interactions.The ratio K + /π + at mid-rapidity as well as the ratio of total yields from Be+Be collisions is close to the p+p measurements.For the five analysed energies of Ar+Sc collisions, the ratio K + /π + at midrapidity and ⟨K + ⟩ / ⟨π + ⟩ are higher than in p+p collisions but show a qualitatively similar energy dependence -no horn structure visible.

Search for critical point
The expected signal of a critical point (CP) is a non-monotonic dependence of various fluctuation or correlation measures in NA61/SHINE energy -system size scan.Fluctuations of conserved charges (electric, strangeness or baryon number) are of special interest [8][9][10].To compare fluctuations in systems of different sizes, one should use quantities insensitive to system volume, i.e. intensive quantities.They are constructed by division of cumulants κ i of the measured multiplicity distribution (up to fourth order), where i is the order of the cumulant.For second, third and fourth order cumulants intensive quantities are defined as: κ 2 /κ 1 , κ 3 /κ 2 and κ 4 /κ 2 .Their reference values for no fluctuations are 0 and for independent particle production are 1.In case of net-charge, cumulant ratios are redefined to κ 2 / κ 1 h + − κ 1 h − , κ 3 / κ 1 h + − h − and κ 4 / κ 2 h + − h − in order to keep the same references.Figure 5 shows the system size and energy dependence of second, third and fourth order cumulant ratio of net-electric charge in p+p as well as central Be+Be and Ar+Sc interactions.So far, there is no clear difference between systems for higher order moments.More detailed studies are needed.
Another possible tool for search of CP is a proton intermittency.In the proximity of CP a local power-law fluctuations of the baryon density should appear which can be searched for by studying second factorial moments with the cell size or, equivalently, with the number of cells in space of protons at mid-rapidity [11][12][13].NA61/SHINE measures F 2 (M) using statistically independent points and cumulative variables.Preliminary results on F 2 (M) of mid-rapidity protons measured in 0-20% most central Ar+Sc collisions at 150A GeV/c and 0-10% most central Pb+Pb collisions at 30A and 13A GeV/c are presented in Fig. 6.The intermittency index ϕ 2 for a system freezing out at the QCD critical endpoint is expected to be ϕ 2 = 5/6 assuming that the latter belongs to the 3-D Ising universality class [14].Measured F 2 (M) of protons for Ar+Sc at 150A GeV/c and Pb+Pb at 30A and 13A GeV/c show no indication for power-law increase with a bin size which could indicate the presence of CP.

Strangeness production in p+p
Finally, NA61/SHINE provides new and unique results on strangeness particle production in p+p interactions.Figure 7 presents first rapidity distribution of K * (892) 0 produced in inelastic p+p interactions at 40 and 80 GeV/c [15] and the only result on Ξ (1530) 0 and Ξ (1530) 0 production in inelastic p+p interactions at 158 GeV/c [16].
The new measurements by NA61/SHINE of Ξ (1530) 0 and Ξ (1530) 0 produced in inelastic p + p interactions at 158 GeV/c as well as previously obtained results for π + , π − , K + , K − , p, p, K * (892) 0 , Λ, ϕ(1020), Ξ − and Ξ + (see Refs. [17][18][19][20][21][22][23] were fitted by different variants of the Hadron Resonance Gas Model (HRG).The Canonical Ensemble with fixed γ s = 1 and Canonical Ensemble with fitted strangeness saturation parameter γ s configurations were used.Figure 8 compares the measured multiplicities of particles produced in inelastic p + p interactions at 158 GeV/c with multiplicities of the same particles obtained from the HRG model in the CE formulation under two different model assumptions: γ s = 1 and fitted γ s .The  software package THERMAL-FIST 1.3 [24] was used for this purpose.For the small p + p system, the appropriate statistical system is the Canonical Ensemble, which has as parameters the temperature T , the radius R of the system at chemical freezeout, and the strangeness suppression factor γ s .The results plotted in Fig. 8 show significant discrepancies for the fitted parameters T and R between the fits with γ s = 1 and free γ s .Moreover, the fit with fixed γ s shows unacceptably large χ 2 /NDF = 29.This demonstrates that the statistical model fails when fixing γ s to one.The fit with free γ s finds γ s = 0.434 ± 0.028 and reproduces the measurements significantly better than with γ s = 1.Thus the statistical model indicates a strong suppression of strange particle production in p + p collisions at CERN SPS energies.

Figure 2 .
Figure 2. Inverse slope parameter T of m T spectra of K ± as function of collision energy.Most results are shown with statistical uncertainties only.For the p+p data the shaded band indicates systematic uncertainties.

Figure 3 .
Figure 3.Yields ratio K + /π + at mid-rapidity and total yields ratio ⟨K + ⟩ / ⟨π + ⟩ produced in p+p, Be+Be and Pb+Pb collisions as function of collision energy.

Figure 4 (
right) shows the rapidity densities dn/dy of Ξ + at mid-rapidity per mean number of wounded nucleons divided by the corresponding values for inelastic p+ p collisions as a function of ⟨N W ⟩. Apart from a slightly flatter rise the overall picture remains unchanged.

Figure 4 .
Figure 4. (Color online) The strangeness enhancement E at mid-rapidity as a function of average number of wounded nucleons ⟨N W ⟩ calculated as a ratio of rapidity density for Ξ − production (left) and Ξ + production (right) in nucleus-nucleus interactions per ⟨N W ⟩ divided by the corresponding value for p + p interactions.The enhancement factor is defined as E = 2 ⟨N W ⟩