NA61/SHINE results on fluctuations and correlations at CERN SPS energies
NNuclear Physics A 00 (2020) 1–4
NuclearPhysics A / locate / procedia XXVIIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions(Quark Matter 2019)
NA61 / SHINE results on fluctuations and correlations at CERNSPS energies
Maja Ma´ckowiak-Pawłowskafor the NA61 / SHINE Collaboration
Faculty of Physics, Warsaw University of Technology, Koszykowa 75, 00-662 Warsaw
Abstract
The aim of the NA61 / SHINE strong interaction programme is to explore the phase diagram of strongly interactingmatter. The main physics goals are the study of the onset of deconfinement and the search for the critical point of stronglyinteracting matter. These goals are pursued by performing a beam momentum (13 A – 150 / A GeV / c ) and system size(p + p, p + Pb, Be + Be, Ar + Sc, Xe + La, Pb + Pb) scan. This contribution presents new results from NA61 / SHINE onfluctuations and correlations which include in particular quantum correlations, as well as multiplicity and net-chargefluctuations, proton density fluctuations and anisotropic collective flow. Obtained results are compared with otherexperiments and with model predictions.
Keywords: critical point, onset of deconfinement, fluctuations, intermitency, femtoscopy, flow
1. Introduction
NA61 / SHINE [1, 2] at the CERN Super Proton Synchrotron (SPS) is a fixed-target experiment pursuinga rich physics program including measurements for strong interactions, neutrino, and cosmic ray physics.The strong interactions program focuses on search for the critical point (CP) and study of the onsetof deconfinement (OD) of strongly interacting matter. NA61 / SHINE is the first experiment to perform atwo-dimensional scan, in beam momentum (13 A – 150 / A GeV / c ) and system size (p + p, p + Pb, Be + Be,Ar + Sc, Xe + La, Pb + Pb) of colliding nuclei.
2. Search for the critical point
The expected signal of a critical point is a non-monotonic dependence of various fluctuation / correlationmeasures in such a scan. A specific property of the CP - the increase in the correlation length - makes fluctu-ations its basic signal. Special interest is devoted to fluctuations of conserved charges (electric, strangenessor baryon number) [3, 4]. a r X i v : . [ nu c l - e x ] F e b / Nuclear Physics A 00 (2020) 1–4 æ W Æ - k / k - -h + h central A+A at 150/158A GeV/c NA61/SHINE preliminaryp+p Be+Be
EPOS 1.99Skellam æ W Æ k / k - -h + h central A+A at 150/158A GeV/c NA61/SHINE preliminaryp+p Be+Be
EPOS 1.99Skellam
Fig. 1. Preliminary NA61 / SHINE results on the system size dependence of κ /κ [ h + − h − ] and κ /κ [ h + − h − ] at 150 / A GeV / c asa function of the mean number of wounded nucleons, (cid:104) W (cid:105) . æ W Æ k / k - h + h EPOS 1.99 - h EPOS 1.99 + hPoisson central A+A at 150/158A GeV/c NA61/SHINE preliminaryp+p Be+Be Ar+Sc æ W Æ k / k - h + h EPOS 1.99 - h EPOS 1.99 + hPoisson central A+A at 150/158A GeV/c NA61/SHINE preliminaryp+p Be+Be æ W Æ k / k - h + h EPOS 1.99 - h EPOS 1.99 + hPoisson central A+A at 150/158A GeV/c NA61/SHINE preliminaryp+p Be+Be
Fig. 2. Preliminary NA61 / SHINE results on the system size dependence of multiplicity fluctuations of h + and h − at 150 / A GeV / c as a function of the mean number of wounded nucleons, (cid:104) W (cid:105) . In order to compare fluctuations in systems of di ff erent sizes one should use intensive quantities, i.e.quantities insensitive to system volume. Such quantities are constructed by division of cumulants κ i of themeasured distribution (up to fourth order), where i is the order of the cumulant. For second, third and fourthorder cumulants intensive quantities are defined as: κ /κ , κ /κ and κ /κ .Figure 1 shows the system size dependence of third and fourth order cumulant ratio of net-electriccharge at 150 / A GeV / c . Measured data are in agreement with EPOS 1.99 model [5, 6] predictions. Moredetailed examination of system size dependence of the same quantities for negatively and positively chargedhadrons (Fig. 2) shows very di ff erent system size dependence. Moreover, none of the measured quantitiesof h + and h − are reproduced by the EPOS 1.99 model. This disagreement indicates that we do not fullyunderstand the underling physics how fluctuations are induced. Thus, more detailed studies are needed.In search of CP a possible tool is proton intermittency which should follow power-law fluctuations nearCP. It can be checked by studying the scaling behaviour of 2 nd factorial moments F ( M ) with the cell sizeor, equivalently, with the number of cells in ( p x , p y ) space of protons at mid-rapidity (see Refs. [7, 8, 9]).For experimental data a non-critical background must be subtracted with mixed events. After subtraction,the second factorial moments ∆ F ( M ) should scale according to power-law for M >> φ comparable to theoretical predictions [10]. Figure 3 shows ∆ F ( M ) in semi-centralAr + Sc interactions at 150 A GeV / c . The di ff erence between left and right side of the figure is the consideredstatistics. Left hand side shows results released in 2018 [11]. These results indicate positive values of ∆ F which may be connected with the CP. Right hand side shows the same results but with higher statistics (208k Nuclear Physics A 00 (2020) 1–4 () median68% C.I.95% C.I.99.7% C.I.NA61/SHINE Ar+Sc 150 NA61/SHINE Ar+Sc 150, cent.10 - 15%, pur > 90%NA61/SHINE preliminary () median68% C.I.95% C.I.99.7% C.I.NA61/SHINE Ar+Sc 150 NA61/SHINE Ar+Sc 150, cent.10 - 15%, pur > 90%NA61/SHINE preliminary
Fig. 3. Comparison of preliminary NA61 / SHINE results on proton intermitency signal with lower (left) and higher (right) statistics (GeV) T m0 0.1 0.2 0.3 0.4 0.5 0.6 a NA61/SHINE Preliminary 0-20% c Be+Be @ 150A GeV/ - p - p + p + p + p + p + - p - p c (GeV/ T p0.04 − − v NA49 & NA61/SHINE preliminaryPb+PbPSD (VCAL) centrality 15-35% < 1.2 y } x ,SP proj Ψ { v - π PSD (NA49: VCAL) c GeV/ A c GeV/ A
30 GeV (NA49) A Fig. 4. Left: Preliminary NA61 / SHINE results on α parameter in Be + Be collisions at 150 A GeV / c . Right: NA49 [22] andNA61 / SHINE [27] preliminary results on energy dependence of v in Pb + Pb collisions. events vs 143k events) where ∆ F signal is weaker.At the CP, the spatial correlation function becomes a power-law ∼ r − ( d − + η ) , where d represents thenumber of dimensions. One can predict a critical exponent η (related to spatial correlations) for the QCDuniversality class, which is the 3D-Ising model for QCD [12, 13]. The predicted value of η at the CP is0.03631 [14]. For the random field 3D Ising η = . ± .
05 [15]. In the HBT analysis the momentumcorrelation function C ( q ) of produced particles is directly related to the normalized source distribution S ( r )via C ( q ) = + ( | ˜ S | ) , where ˜ S is the Fourier transformation of S ( r ). The data analysis was done by using aLevy distributed source function [16]. Since, it leads to the same power-law tails, the Levy exponent α wasassumed to be identical to the spatial correlation exponent η [17]. In the vicinity of the critical point, verylow α values (around 0.5) may be expected and this can be measured by investigating the Bose-Einsteincorrelation function C ( q ) = + λ e − ( qR ) α . Figure 4 (left) shows measured values of α parameter for pion pairs( π − π − , π + π + and π − π − + π + π + ) in 20% most central Be + Be collisions at 150 A GeV / c .All measured combinations indicate 1 < α < α values lower then 2 can be caused by anomalous di ff usion, QCD fractal structured jet fragmentation, andalso to some extend by the averaging over broad event class (e.g. centrality) [18, 19, 20, 21].
3. Bulk matter properties
Spatial asymmetry of the initial energy density in the overlaping region of the colliding relativistic nucleiis converted, via interactions between produced particles, to the asymmetry of momentum distribution of / Nuclear Physics A 00 (2020) 1–4 particles in the final state. The resulting asymmetry encodes important information about the transport prop-erties of the QCD matter created during the collision. Asymmetry is usually quantified with the coe ffi cients v n in a Fourier decomposition of the azimuthal distribution of produced particles relative to the reactionplane. The NA61 / SHINE has an unique way to estimate the reaction plane with the Projectile SpectatorDetector (for details see Refs. [22, 23]).The energy dependency of flow coe ffi cients is of particular importance. At the SPS energies it is expectedthat the slope of proton directed flow at mid-rapidity, dv / dy , changes its sign [24, 25, 26]. Directed flow of π − for Pb + Pb collisions at di ff erent energies is presented in Fig. 4 (right). The NA49 results [22] for 40 A GeV / c were obtained using spectator plane estimated with Veto calorimeter (VCAL). Directed flow of π − ,in particular the p T dependence where v changes its sign, shows collision energy dependence.
4. Summary
Numerous experimental results show no indications of the CP in Be + Be collisions at 150 A GeV / c . En-larged statistics of Ar + Sc interactions has not improved the significance of the proton intermitency signal,making it inconclusive. In order to qualitatively measure the CP signal, the background phenomena shouldbe studied in details. Measured system size dependences of multiplicity fluctuations show clear disagree-ment with model predictions.The measured directed flow of pions shows energy dependence, with the slope of negatively chargedpions changing sign at di ff erent collision centralities in Pb + Pb collisions (for details see Ref. [23]).
Acknowledgements:
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