1. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 1 What have we learnt studying strangeness production in Pb—Pb at SPS ? Ladislav Šándor Slovak academy of science Institute of experimental physics Košice, Slovakia

2. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 2 Plan of talk Why strangeness ? Flavour equilibration Strangeness enhancements Search for the onset of deconfinement Collective flow and thermal freeze-out … a review of selected results on strangeness production from the SPS heavy-ion experiments …

3. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 3 Why strangeness ? Strange quarks/antiquarks not present in the initial state of heavy-ion reaction, all the strangeness of final state produced in the course of interaction Thus, the strange hadrons bear an important information on the collision dynamics Specific predictions (see below) allowing to consider strange and particularly multi-strange baryons as promising probes signalling the QGP creation

4. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 4 Two historic QGP predictions restoration of chiral symmetry  increased production of s mass of strange quark in QGP expected to go back to current value ~ 150 MeV ~ T c  copious production of ss pairs, mostly by gluon fusion J.Rafelski: Phys. Rep. 88 (1982) 331 J.Rafelski and B.Müller: PRL 48 (1982) 1066 deconfinement  stronger effect for multi-strange baryons can be built easily using uncorrelated s quarks produced in independent microscopic reactions  enhanced production increasing with strangeness content of particle P.Koch, B.Müller and J.Rafelski: Phys. Rep. 142 (1986) 167

5. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 5 Flavour equilibration ?

6. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 6 Thermal model fits P.Braun-Munzinger, I.Heppe and J.Stachel, PL B465 (1999) 15 high degree of thermal equilibration even for rare multi-strange particles

7. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 7 NA49 data – statistical model fit hadron species populated according to phase space probabilities (max.entropy) (Fermi,Hagedorn) strangeness sector not fully saturated (Rafelski) statistical model successful, T, μ B, γ S, V larger value of γ S in A+A due to relaxation of local strangeness conservation F.Becattini et al, PR C69 (2004) 024905

8. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 8 Strangeness enhancements

9. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 9 Enhanced Λ production … WA85 experiment, OMEGA spectrometer, data at 200 A GeV/c, central rapidity, p T > 0.9 GeV/c Λ and anti-Λ production enhanced by a factor 1.7 with respect to negatives when going from p-W to S-W interactions S.Abatzis et al., Nucl.Phys. A525 (1991) 445c

10. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 10 WA85 and WA94 results

11. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 11 First WA97 data See, e.g. L.Sandor et al., NPB (Proc. Suppl.) 71 (1999) 270 Uncorrected Ξ and Ω signals from p-Pb and Pb-Pb 1995 data at 158 A GeV/c A striking difference between Ω/Ξ ratios in the p-Pb and Pb-Pb data seen: Enhancement by a factor 2.5 going from p-Pb to central Pb-Pb collisions ! p-Pb Pb-Pb

12. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 12 Final WA97 data An important pillar of the claim that evidence for a new state of matter had been found at the CERN SPS F. Antinori et al., Nucl.Phys. A661 (1999) 130c  Strangeness enhancement:

13. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 13 NA57 – wider centrality range... NA57 data confirm the global enhancement pattern first observed by the WA97 Significant centrality dependence of strangeness enhancements for all hyperons except for  _ 158 A GeV/c data

14. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 14... and data at 40 A GeV/c systematic errors: 10% for ,  15% for  Most peripheral class accessible in NA57 : = 62  4 (95 % confidence level)  Enhancements still there at 40 A GeV/c with the same hierarchy as for higher energy data: E() < E() [ ]

15. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 15 Energy dependence of strangeness enhancements...  enhancements in the most central collisions (classes 3-4) are larger at 40 GeV than at 160 GeV  steeper increase with centrality at lower energy 40 GeV 160 GeV  1 10 100  qualitative agreement with canonical suppression prediction (A.Tounsi & K.Redlich, hep-ph/0111159)

16. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 16 … or canonical suppression ? Agreement of data with the model only qualitative… Need for more refined model development and predictions for 200 GeV Au-Au and LHC …

17. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 17 Where is the point of the onset of deconfinement ? Important NA49 activity : Search for a threshold by varying the energy of the largest collision system (central Pb+Pb reactions) SPS energy scan: 20,30,40,80,158 GeV/nucleon The energy dependence of several observables shows anomalies at lower SPS energies

18. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 18 Ratio of K, Λ yields to pions sharp peak of K + /π + ratio smooth rise of anti-Λ/π ( s quarks) similar peak in Λ/π ratio structure in K – /π – ( s quarks) hadronic models do not (yet ?) reproduce the sharp peak

19. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 19 Inverse slope parameters inverse slope parameter T of K + and K - constant at SPS approximately constant pressure and temperature in mixed phase ?

20. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 20 Ratio of strange hadrons to pions Strangeness to pion ratio peaks sharply at the SPS SMES explanation: - entropy, number of s,sbar quarks conserved from QGP to freezeout - ratio of strange/nonstrange d.o.f. rises rapidly with T in hadron gas - E s drops to predicted constant level above the deconfinement threshold onset of deconfinement at SPS ? hadronic mixed partonic

21. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 21 Thermal freeze-out scenario

22. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 22 Transverse mass spectra Shape of spectra close to exponential: T – apparent temperature (free parameter of fit) reflecting both the transverse collective flow and thermal motion. SPS data suggest approximately linear dependenence of T on particle mass NA44 data – L.G.Bearden et al., PRL 78 (1997) 2080 NA57, 158A GeV

23. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 23 T versus particle mass WA97 5% most central data Ω+antiΩ deviate from a common trend ! H.Van Hecke, H.Sorge & Nu Xu: PRL 81 (1998) 5764 : Early freeze-out of multi-strange hadrons ?

24. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 24 T versus m, recent data Presented at SQM2003, Atlantic Beach WA97 and NA57 data compatible, SPS and RHIC trends similar To disentangle contributions from a pressure driven collective flow (transverse velocity ) and thermal motion (freeze-out tempera- ture T fo ) one needs to invoke models A hydrodynamically inspired blast-wave approach is widely used (Schnedermann, Sollfrank & Heinz, PR C48 (1993) 2462) Relativistic hydrodynamics at early deconfined phase plus UrQMD transport at hadronic stage S.A.Bass, A.Dumitru. PR C61:064909,2000 ~

25. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 25 Blast-wave fits, NA57 data Pb-Pb 40 A GeV/c NA57, JPG30 (2004) 823 Pb-Pb 158 A GeV/c Model : thermalization plus hydro-dynamical transverse flow description F. Antinori et al, J. Phys. G: Nucl.Part.Phys, 30 (2004) 823; 31 (2005) S127 Fits with n=1

26. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 26 Blast-wave fits, NA49 data 40A GeV 30A GeV 20A GeV kinetic freeze out at T fo  120 –130 MeV,  0.4 – 0.5 at SPS Reasonable description of spectra at all SPS energies

27. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 27 Freeze-out of multistrange hyperons NA49 data on Ω production C.Alt et al., PRL 94 192301 (2005) The data favor a low transverse expansion velocity and high freeze-out temperature (T fo = 170 MeV, = 0.2, fit B2) indication of an early freeze-out of the  at top SPS energy Analysis by G.E.Bruno (NA57)

28. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 28 Multistrange hyperons at RHIC STAR data, nucl-ex/0412041 Freeze-out of Ξ,Ω at temperature close to T ch – earlier decoupling ! Presence of elliptic flow of Ξ, Ω – collectivity built up at the partonic level

29. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 29 Conclusions  We have learnt quite a lot till now, analysis continuing (talks of B.Lungwitz, R.Lietava)  Thermal model fits of data at the top SPS energy show high degree of chemical and thermal equilibration, even for rare multi-strange hyperons  Enhancement of strangeness observed by WA97 and NA57 in central Pb-Pb collisions with respect to p-Be reaction, increasing with strangeness content of particle (QGP prediction)  Important energy scan performed by the NA49 experi- ment. Several anomalies found in energy dependence of some observables (strangeness/ pions ratio, inverse slopes of kaons, …) indicating possible onset of deconfi- nement in central Pb-Pb collisions at about 30 A GeV

30. XXXVth ISMD, Kroměříž, August 2005 L. Šándor, Košice 30 Conclusions (cont.)  The analysis of transverse mass spectra of strange particles in Pb-Pb at the top SPS energy suggests that after a central collision the system expands explosively and then it freezes-out when the temperature is ~120 - 140 MeV, with an average transverse flow velocity of about one-half of the speed of light. An indication for early freeze-out of multi-strange hyperons found  A discovery potential of the SPS not exhausted yet. Possible continuation of experimenting with ion beams could answer a number of actual questions