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Strangeness production at the CERN SPS

Strangeness production at the CERN SPS. Ladislav Šándor Slovak Academy of Science Institute of Experimental Physics Košice, Slovakia. Why strangeness ? Flavour equilibration Strangeness enhancements Search for the onset of deconfinement Collective flow and thermal freeze-out

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Strangeness production at the CERN SPS

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  1. Strangeness production at the CERN SPS Ladislav Šándor Slovak Academy of Science Institute of Experimental Physics Košice, Slovakia Ladislav Šándor

  2. Why strangeness ? Flavour equilibration Strangeness enhancements Search for the onset of deconfinement Collective flow and thermal freeze-out Nuclear modification factors Plan of talk selected results on strangeness production from SPS ion experiments, particularly NA49 and WA97/NA57 Ladislav Šándor

  3. 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 (J.Rafelski, B.Mueller) allowing to consider strange and particularly multi-strange baryons as promising probes signalling the QGP creation Why strangeness ? Ladislav Šándor

  4. Flavour equilibration ? Ladislav Šándor

  5. P.Braun-Munzinger, I.Heppe, J.Stachel, PL B465 (1999) 15 Thermal model fits Question: is thermal and chemical equilibrium achieved for hyperons ? Applying a statistical model which assumes equilibrium and testing data against model predictions gives a way to answer this question A realistic model assuming grand canonical ensemble applied to SPS mid-rapidity data on particle ratios in Pb-Pb collisions at 158 GeV high degree of thermal equilibration even for rare multi-strange particles Ladislav Šándor

  6. Thermal model fits (2) Interesting work done in recent years in development of statistical hadronization models and their application to growing experimental data • F. Becattini et al., Phys. Rev. C69 (2004) 024905 • J. Letessier, J. Rafelski. nucl-th/0504028 • T. Renk. J.Phys.G: Nucl.Part.Phys., 30 (2004) 1495 prediction of particle ratios obtained from comprehensive analysis of the SPS data for several relevant QGP observables in the framework of a model of the space-time evolution of the collisions Remarkable success of statistical hadronization model in describing data on particle yields in a wide energy range Ladislav Šándor

  7. Thermal model fits (3) A comprehensive analysis of data over a broad energy range (√sNN = 2.7– 200 GeV); A.Andronic, P.Braun-Munzinger, J.Stachel. nucl-th/0511071 Statistical quality of fit at 158 A GeV not satisfactory : inhomogeneous freeze-out scenario ? (D. Zschiesche, nucl-th/0505054) Ladislav Šándor

  8. Energy dependence of thermal parameters SPS RHIC similarity of chemical freeze-out at SPS and RHIC Ladislav Šándor

  9. Strangeness enhancements Ladislav Šándor

  10. First SPS signals on enhancement NA35 streamer chamber experiment studying S-S and p-S collisions at 200 A GeV observed ~ twofold increase of strange particle (K0,Λ) production with respect to negatively charged hadrons when going from p-S to S-S interactions J. Bartke et al., Z. Phys. C 48 (1990) 191; R. Stock et al., Nucl. Phys. A 525 (1991) 221c WA85 experiment, OMEGA spectro- meter, S-W and p-W data at 200 A GeV , central rapidity, pT > 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 collisions S.Abatzis et al., Nucl. Phys.A 525 (1991) 445c Ladislav Šándor

  11. WA85 and WA94 results Ladislav Šándor

  12. Strangeness enhancement: NA57 Pb-Pb data at 158 A GeV F. Antinori et al., J.Phys.G 30 (2004) S129-S138 NA57 data confirm the global enhance-ment pattern first observed by the WA97 in narrower centrality region WA97 data: F. Antinori et al., Nucl.Phys. A661 (1999) 130c _ Significant centrality dependence of strangeness enhancements for all hyperons except for L Ladislav Šándor

  13. ... and at 40 A GeV/c F. Antinori et al., J.Phys.G 30 (2004) S717-S724 systematic errors: 10% for L, X 15% for W Most peripheral class accessible in NA57 : <Nwound> = 62  4 [ ] (95 % confidence level) • Enhancements still there at 40 A GeV/c with the same hierarchy as for higher energy data:E(L) < E(X) Ladislav Šándor

  14. … and recent NA49 results Important contribution to the knowledge of centrality dependence NA49 and NA57 results qualitatively consistent Ladislav Šándor

  15. 100 10 X- 40 GeV 160 GeV 1 Energy dependence of enhancements NA57 data • steeperincrease with centrality at lower energy • enhancements in the most central collisions (classes 3-4) are larger at 40 GeV than at 160 GeV • qualitative agreement with canonical suppression prediction (A.Tounsi & K.Redlich, hep-ph/0111159) Ladislav Šándor

  16. Energy dependence (cont.) Agreement of data with the model only qualitative… Need for more refined model development and predictions for 200 GeV Au-Au and LHC … STAR data: J.Phys.G 31 (2005) S1057 Ladislav Šándor

  17. The onset of deconfinement ? Ladislav Šándor

  18. 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 Ladislav Šándor

  19. Ratio of K, Λ yields to pions • sharp peak of K+/π+ • ratio • smooth rise of anti-Λ/π hadronic models do not (yet ?) reproduce the sharp peak • similar peak in Λ/π • ratio • structure in K–/π– Ladislav Šándor

  20. Inverse slope parameters • inverse slope parameter T of K+ and K- constant at SPS • approximately constant pressure and temperature in • mixed phase ? Ladislav Šándor

  21. 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 - Es drops to predicted constant level above the deconfinement threshold mixed partonic hadronic M.Gazdzicki, M.Gorenstein. Acta Phys.Pol.B 30 (1999) 2705 onset of deconfinement at SPS ? Ladislav Šándor

  22. New phenomenon or conventional physics ? If the “horn” signals the onset of deconfine- ment, one is expecting strong event-by- event fluctuations, search for is going on … There are indications for a change of behaviour at lower SPS energy, however the relation of observed effects to the onset of deconfinenent is still unclear A number of recent theoretical activities aiming at more conventional explanation : B. Tomášik, E. Kolomeitsev. nucl-th/0512088 , J. Letessier, J.Rafelski. nucl-th/0504028 , J. Cleymans et al., nucl-th/0510283, J. Cleymans et al., Phys. Lett. B 615 (2005) 50 … Interesting result, waiting fair confirmation … Ladislav Šándor

  23. Transverse spectra,thermal freeze-out scenario Ladislav Šándor

  24. Transverse mass spectra Shape of spectra close to exponential: NA57, 158A GeV 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 The same shape of hyperon and anti-hyperon spectra NA44 data – L.G.Bearden et al., PRL 78(1997) 2080 Ladislav Šándor

  25. T versus particle mass 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 <βT>) and thermal motion (freeze-out tempera-ture Tfo) one needs to invoke models Relativistic hydrodynamics at early deconfined phase plus UrQMD transport at hadronic stage S.A.Bass, A.Dumitru. PR C61:064909,2000 A hydrodynamically inspired blast-wave approach is widely used (Schnedermann, Sollfrank & Heinz, PR C48 (1993) 2462) Ladislav Šándor

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

  27. Simple blast-wave fits, NA49 data Reasonable description of spectra at all SPS energies 158A GeV 30A GeV 20A GeV Taken from : The NA49 collaboration, CERN-SPSC-2005-041, Nov. 2005 kinetic freeze out at Tfo 120 –130 MeV, <βT >  0.4 – 0.5 at SPS Ladislav Šándor

  28. 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 (Tfo= 170 MeV, <βT> = 0.2 , fit B2) Analysis by G.E.Bruno (NA57) indication for early freeze-out of the W at top SPS energy ? Ladislav Šándor

  29. High(er) transverse momenta Ladislav Šándor

  30. Nuclear modification factors at SPS It seemed that QGP signatures such as “jet quenching” (parton energy loss in the dense deconfined medium) is absent at SPS energies M.M. Aggarwal, et al.,WA98 Collaboration, Eur. Phys. J. C 23 (2002) 225; X.N. Wang, Phys. Rev. Lett. 81 (1998) 2655. Re-analysis of WA98 data using more realistic pp-reference changed the situation D. d’Enterria. Phys.Lett.B 596 (2004) 32 Suppression consistent with behaviour at RHIC What about strange particles ?? Ladislav Šándor

  31. NA57 results on RCP for K0, Λ F.Antinori et al., Phys,Lett.B 623 (2005) 17 Similar patterns as at RHIC, described by calculations involving parton energy loss Ladislav Šándor

  32. Recent NA49 data From talk of T. Schuster (this conference) Apparently a consistent picture at SPS is developing, needs further analysis Ladislav Šándor

  33. Comparison of NA57 and STAR data • Similar K0-L relative pattern • observed at SPS and RHIC • energy • RCP values higher by • ~0.5 at SPS, no • significant • suppression observed NA57 STAR STAR data from: Phys.Rev.Lett. 92 (2004) 052302 Ladislav Šándor

  34. We have learnt quite a lot till now, analysis continuing (see talks of M.Mitrovski, T.Schuster, H.Helstrup, J. Milosevic) 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, NA57 and NA49 in central Pb-Pb collisions with respect to p-Be (p-p) 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 Conclusions Ladislav Šándor

  35. 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 possible early freeze-out of multi-strange hyperons A similar tendency in strangeness production at SPS and RHIC observed (chemical freeze-out temperature, strangeness enhancements, transverse flow, high pT spectra, …) Conclusions (cont.) Ladislav Šándor

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