Looking for the phase transition - recent NA61/SHINE results

The fixed-target NA61/SHINE experiment (SPS CERN) looks for the critical point of strongly interacting matter and the properties of the onset of deconfinement. It is a two dimensional scan of measurements of particle spectra and fluctuations in proton-proton, proton-nucleus and nucleus-nucleus interactions as a function of collision energy and system size, corresponding to a two dimensional phase diagram (temperature T - baryonic chemical potential $\mu_B$). New NA61/SHINE results are presented here, such as transverse momentum and multiplicity fluctuations in Ar+Sc collisions compared to NA61/SHINE p+p and Be+Be data, as well as to earlier NA49 A+A results. Recently, a preliminary signature for the new size dependent effect - rapid changes in system size dependence was observed in NA61-SHINE data, labeled as percolation threshold or onset of fireball. This would be closely related to the vicinity of the hadronic phase transition region.


I. INTRODUCTION
The NA61/SHINE, understood as the Super Proton Synchrotron (SPS) Heavy Ion and Neutrino Experiment, is the continuation and the extension of the NA49 experiment [1,2]. It uses a similar experimental fixed-target setup to NA49 ( Figure 1) but with an extended research programme. Beyond an enhanced strong interactions programme, measurements of hadron production for neutrino and cosmic ray experiments are realized. The collaboration involves about 150 physicists from 15 countries and 30 institutions. It is the second largest non-LHC (the Large Hadron Collider) experiment at the CERN. The strong interaction programme of the NA61/SHINE is dedicated to the study of the onset of deconfinement and the search for the critical point (CR) of hadronic matter, related to the phase transition between hadron gas (HG) and quarkgluon plasma (QGP). The NA49 experiment studied hadron production in Pb+Pb interactions, while the NA61/SHINE collects data varying beam energy within the range of 13A-158A GeV and varying sizes of the colliding systems. This is equivalent to the two dimensional scan of the hadronic phase diagram in the (T, µ B ) plane, as depicted in Figure 2. The ion collisions research programme was initiated in 2009 with the p+p collisions used later on as reference data for heavy ion collisions.
Hadron-production measurements for neutrino experiments are just reference measurements of p+C interactions for the T2K experiment, since they are necessary for computing initial neutrino fluxes at J-PARC. It has been extended to measure the production of charged pions and kaons produced in interactions out of thin carbon targets and replicas of the T2K targets what is necessary to test accelerator neutrino beams [3]. Data taking began in 2007.
Collected p+C data also allow for better understanding of nuclear cascades in the cosmic air showers -necessary in the Pierre Auger and KASCADE experiments [4,5]. These are reference measurements of p+C, p+p, π+C, and K+C interactions for cosmic ray physics. The cosmic ray collisions with the Earth's atmosphere produce secondary air shower radiation. Some of particles produced in such collisions subsequently decay into muons, which are able to reach the surface of the Earth. Cosmic ray-induced muon production can allow the reproduction of primary cosmic ray composition if related hadronic interactions are known [6]. As seen in Figure 3, the phase structure of hadronic matter is involved. Progress in the theoretical understanding of the subject and the collection of more experimental data will allow us to delve further into the subject. While the highest energies achieved at the LHC and RHIC colliders provide data related to the crossover HG/QGP regions, the SPS fixed-target NA61/SHINE experiment is particularly suited to exploring the phase transition line of HG/QGP with the CR included. Heavy ion collision (HIC) experiments reproduce the conditions of the first 10 µs after the Big Bang, when a phase transition from the QGP to a hadron gas would have taken place [7]. It appears, however, that the QGP can be present in the core of massive neutron stars -particularly those with masses exceeding two solar masses [8,9]. That would correspond to the far lower right part of the phase plot, beyond the (T, µ B ) range covered by Figure 3. The CR has been long predicted for thermal quantum chromodynamics (QCDs) at finite µ B /T [10][11][12] although this was not unanimously accepted previously [13]. However, lattice QCD calculations are becoming more and more accurate, leading to the present conclusions that the cross-over region occurs at T c (µ B = 0) = 154 ± 9 MeV [14] and the location of a CR is not expected for µ B /T 2 and T /T c (µ B = 0) > 0.9 [15]. A more detailed exploration of QCD phase diagram would need both new experimental data with extended detection capabilities and improved theoretical models [16].
Another intriguing and far reaching possibility is the Big Bang phase transition scenario, referred to by Edward Witten as the "cosmic separation of phases" [17]. In the standard approach, the Big Bang QGP is almost matterantimatter symmetric and evolves to lower temperatures through the crossover region almost vertically to the temperature axis [18]. Edward Witten, using almost "back of envelope" arguments, pointed out the possibility of using the path of universe starting in the QGP phase from the high baryonic chemical potential region reaching almost zeroth temperature, with supercooled QGP. Hadronization then becomes quite an explosive phenomena with a necessary subsequent reheating (see e.g. [19,20]). Corresponding plots, taken from [19], are shown in Figure 4. Beyond cosmological effects (little/tepid inflation) such a possibility would change also our understanding of the hadronization effect in HIC processes.

A. Irregularities -the Horn
It was expected [23] that ratio K + /π + produced at HIC energies of about √ s N N ≈ 10 GeV should reach a rapid maximum when QGP formation begins. In 1998 there was not enough experimental data to fully check this hypothesis. Present collected results fully confirm the appearance of the horn, although there are still discussions about its relevance to the HG/QGP phase transition. Recent data from NA61/SHINE [24] show also a strong dependence of the effect on the size of the colliding objects, as seen in Figure 5.

B.
Irregularities -the Step Plateau: A step-in the inverse slope parameter T of the transverse mass spectra m T at mid-rapidity (0 < y < 0.2) plotted against the collision energy per nucleon ( Figure 6) is expected for the onset of deconfinement [23]. The effect increases with the size of colliding objects. Qualitatively, a similar structure is visible in p+p collisions, with Be+Be slightly above, consistent with the step structure.

C. Fluctuations
Experimental facilities of NA61 allows to measure fluctuations of various physical quantities which are sensitive to the vicinity of the critical point. These fluctuations can create a signature for the critical point. An analysis of fluctuations of various observables, particularly in a range of energies around 8 GeV per colliding nucleon pair at the center of mass in interactions of light nuclei (it corresponds to the beam energy of 30 GeV in the frame of a stationary target), is the main goal of the NA61/SHINE experiment. This is just the kinematical region where NA49 data indicate that the onset of deconfinement in central Pb+Pb collisions. It is mainly based on the observation of structures in the energy dependence of hadron production in central Pb+Pb collisions which are not observed in elementary interactions [21,22] Preliminary data presented in Figure 7 do not show any signs of critical behaviour [25].

III. SYSTEM SIZE DEPENDENCE
In the recent months, some unexpected results were observed by the NA61/SHINE experiment [24,26,27], concerning qualitative differences in system size dependence behaviour. It appears that in particular Be+Be results are very close to p+p at different collision energies. An example of such behaviour is presented in Figure 8. It looks as if with the increasing size of colliding systems, light clusters are produced more and more copiously, and at some density they start to overlap to reach percolation threshold. This effect would not depend on energy only, but also on the size of the system.
Intensive work now is ongoing to achieve more conclusive results, analyzing recently collected data from Xe+La and Pb+Pb.

IV. CONCLUSIONS
The Holy Grail of HIC -Quark Gluon Plasma -still remains an elusive object. Although there are no at present discussions concerning the very existence of this state of matter, there are still open problems connected with experimental signatures of many theoretical ideas and predictions in this field. The NA61/SHINE experiment acts in the energy region particularly suited for the appearance of phase transition effects. Beyond this, the fixed-target technology makes possible to perform 4π geometry measurementswhich are not accessible in collider-type experiments.
To date, collected and analyzed NA61/SHINE data related to theoretical predictions of fluctuations in the presence of CR have not shown any anomaly that could be attributed to this. These data relate to N − p T fluctuations in p+p, Be+Be, and Ar+Sc central events.
There is clear system size dependence of m T spectra that differs significantly between p+p and A+A events. This is the effect associated to the transverse collective flow.
The appearance of horn ( Figure 5) and step ( Figure 6) is in accordance with theoretical predictions for the onset of deconfinement in HIC due to mixed phase of HG and QGP [23].
The recent preliminary results of the NA61/SHINE concerning system size dependence may be also a signature for the new physical phenomena. There is a clearly visible jump between light and heavy systems. Be+Be results are very close to the p+p results, independently on the collision energy. In addition, multiplicity fluctuations, close to p+p value in Be+Be collisions, are strongly suppressed in Ar+Sc collisions.
For the CERN long shutdown 2019-2020, an NA61/SHINE detector upgrade system is foreseen. This would make the precise measurements of open charm and multi-strange hyperon production possible, which are also of great importance both for the neutrino physics programme as well as for the precision measurements of cosmic rays.