Anisotropic flow of charged and strange particles in PbAu collisions at 158 AGeV measured in CERES experiment J. Milošević 1),2) 1)University of Belgrade and Vinča Institute of Nuclear Sciences, Belgrade, Serbia 2) Ruprecht Karls Universität, Heidelberg, Germany ICNFP, Kolimbary, Crete, Greece 1

ICNFP, Kolimbary, Crete, Greece2 Outline  CERES experiment  Elliptic flow as a form of collective movements  Fourier decomposition  Results  Comparison with hydrodynamic predictions and with results from other experiments  Conclusion

ICNFP, Kolimbary, Crete, Greece3 Shematic view of CERES/NA45 detector ≈30M PbAu collisions collected during 2000 data taking period =17.3 A GeV σ/σ geo = 0.21<η<0.86 in center-of-mass system

ICNFP, Kolimbary, Crete, Greece4 pxpx pypy x y z Non-central collision of two nuclei and appearance of the elliptic flow  Elliptic flow, v 2, is most famous mode of transversal anisotropic collective flow  Pressure gradient converts spatial anisotropy into momentum one  Experimentally it is observed as azimuthally anisotropic distribution of particles measured with respect to the event plane  Event plane is constructed in each event using momenta of particles themselves  Event plane is corrected to the detector inefficiency and event plane resolution  In difference of RHIC and LHC results, SPS data stay well below calculations of ideal hydrodynamics.

ICNFP, Kolimbary, Crete, Greece5 Particle identification and used method

ICNFP, Kolimbary, Crete, Greece6 Identification of charged particles (π ± и p)  Bethe-Bloch (charged pions and protons): 0.85dE/dx(π ±,p)| BB <dE/dx(π ±,p)| measured <1.15dE/dx(π ±,p)| BB  Compromise between high enough statistics and purity of the sample  Stil, there is admixture of p and K + (pbar и K − ) in π + (π − ) sample  For small (transversal) momenta (till 0.5 GeV/c) – identified protons Nucl.Phys.A 894 (2012)41-73

ICNFP, Kolimbary, Crete, Greece7 Reconstruction of particles with strange s quark (Λ и K 0 S )  Λ p+π − BR=63.9% cτ=7.89cm  K 0 S π + +π − BR=86,95% cτ=2.68cm  No match to primary vertex. p T -dependent opening angle cut on pair level.  Armenteros-Podolanski plot used to suppress background. Variables: 0<α=(q + L −q − L )/(q + L +q − L )<0.65 q T <0.125 GeV/c Nucl.Phys.A 894 (2012)41-73 }for Λ and q T >0.12 GeV/c for K 0 S

ICNFP, Kolimbary, Crete, Greece8 Corrections for the admixtures and proton elliptic flow extraction  Correction for admixture of negative kaons in the sample of negative pions v 2 (π − )=v 2 (“π − ”)+r K − [v 2 (“π − ”)−v 2 (K − )], where r K − =N K − /N π−  Positive pion case: v 2 (“π + ”)=[N π+ v 2 (π − )+N K + v 2 (K + )+N p v 2 (p)]/(N π+ +N K + +N p )  Extracted proton elliptic flow v 2 (p)=[(1+r K + +r p )v 2 (“π + ”) −v 2 (π − ) −r K + v 2 (K + )]/r p  Particle ratios, r, are obtained based on particle spectra measured by the NA49 experiment (PbPb collisions at the same energy) corrected for the acceptance and detection efficiency of the CERES/NA45 experiment

ICNFP, Kolimbary, Crete, Greece9 Results and comparison with other experiments

ICNFP, Kolimbary, Crete, Greece10 Differential elliptic flow of pions and protons  Negative pion v 2 corrected for the HBT and admixture of negative kaons  Positive pion v 2 corrected for the HBT effect but not for present admixtures  Beside admixture of positive kaons there is also significant admixture of protons in the sample of positive pions  Proton elliptic flow extracted from the positive pion sample (closed circles)  Elliptic flow of identified protons (stars)  It is negative in vicinity of 0.5GeV/c Nucl.Phys.A 894 (2012)41-73

ICNFP, Kolimbary, Crete, Greece11 Differential elliptic flow of Λ hyperons and K 0 S mesons  Λ and К 0 S are reconstructed in ϕ bins measured with respect to the event plane  NA49 and CERES results are in a good agreement  Magnitude of the elliptic flow measured by STAR experiment at RHIC is larger than the one measured at SPS due to the fact that energy of of the collision is bigger for 1 order of magnitude Nucl.Phys.A 894 (2012)41-73

ICNFP, Kolimbary, Crete, Greece12 NA49 and CERES elliptic flow of protons vs p T  (Pseudo)rapidity coverages overlap but not the same in two experiments: NA49: 0<y<2.1 CERES: 0.21<η<0.86  Similar centrality ranges: NA49: 0.0<σ/σ geo <0.125 CERES: =  Results from the two experiments are in a rather good agreement NA49 Collaboration Phys. Rev. Lett. 92 (2004) CERES/NA45 Collaboration Nucl.Phys.A 894 (2012)41-73

ICNFP, Kolimbary, Crete, Greece13 Comparison with hydrodynamic predictions

ICNFP, Kolimbary, Crete, Greece14 Pion flow for two classes of centralities compared to hydrodynamic predictions  For mid-central collisions (right) data suggests Т between 120 and 160 MeV  Similar behavior is noticed at the RHIC experiments too  Incomplete thermalization?  Growing viscosity?  Mixture of both of them?  Relativistic hydrodynamics for the 1-st order phase transition at T c =165 MeV  Calculations were performed for two freeze-out temperatures: T f =120 MeV and T f =160 MeV  For top-central collisions (left) there is a good agreement with ”standard” temperature Nucl.Phys.A 894 (2012)41-73 hydro by P. Huovinen

ICNFP, Kolimbary, Crete, Greece15 Elliptic flow of short kaons K 0 S  v 2 (K 0 S ) vs p T  Big statistical errors are due to strong cuts applied on secondary vertex selection  Systematic errors are significantly smaller with respect to the statistical ones: (+0.000,-0.003)  The same is true for v 2 (Λ) too  v 2 (K 0 S ) shows a tendency of falling below the curve T f =160 MeV for p T <1.1 GeV/c  A good agreement with the NA49 experiment Nucl.Phys.A 894 (2012)41-73 hydro by P. Huovinen

ICNFP, Kolimbary, Crete, Greece16 Elliptic flow of baryons (protons and Λ hyperons)  A characteristic baryonic flow: v 2 small at small p T and with an increase of p T it quickly grows  Both flows become negative in vicinity of p T ≈ 0.5 GeV/c  Deviation from ideal hydrodynamics  While v 2 (K 0 S ) shows a tendency of falling below curve T f =160 MeV, for barynic flow it is a fact  Deviation increases with mass increase  For η/s>0.42 and T f =120 MeV baryonic flow becomes negative. (C. Shen, U. Heinz, Phys. Rev. C 83 (2011) ) Nucl.Phys.A 894 (2012)41-73 hydro by P. Huovinen

ICNFP, Kolimbary, Crete, Greece17 Conclusion  We measured elliptic flow of charged and strange hadrons at center-of-mass energy = 17.3 GeV in PbAu collisions by CERES/NA45 experiment  Using particle ratios measured in PbPb collisions at the same energy by the NA49 experiment and elliptic flow measured at CERES experiment, the proton elliptic flow is extracted  Results obtained for different particle species were compared with those from NA49 and STAR experiments  Kinetic freeze-out temperature decreases going to more central collisions  Baryonic flow is not described by relativistic hydrodynamics  Baryonic flow becomes negative in vicinity of 0.5 GeV/c Incomplete thermalization? Increase of viscosity? Wrong initial conditions? Or mixture of all of tem?

ICNFP, Kolimbary, Crete, Greece18 Backup

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