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Strangeness in Quark Matter 2006, Los Angeles, 26-31 MarchLadislav Šándor1 Strangeness production at the CERN SPS Ladislav Šándor Slovak Academy of Science.

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Presentation on theme: "Strangeness in Quark Matter 2006, Los Angeles, 26-31 MarchLadislav Šándor1 Strangeness production at the CERN SPS Ladislav Šándor Slovak Academy of Science."— Presentation transcript:

1 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor1 Strangeness production at the CERN SPS Ladislav Šándor Slovak Academy of Science Institute of Experimental Physics Košice, Slovakia

2 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor2 Plan of talk Why strangeness ? Flavour equilibration Strangeness enhancements Search for the onset of deconfinement Collective flow and thermal freeze-out Nuclear modification factors selected results on strangeness production from SPS ion experiments, particularly NA49 and WA97/NA57

3 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor3 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 (J.Rafelski, B.Mueller) allowing to consider strange and particularly multi-strange baryons as promising probes signalling the QGP creation

4 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor4 Flavour equilibration ?

5 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor5 Thermal model fits P.Braun-Munzinger, I.Heppe, J.Stachel, PL B465 (1999) 15 high degree of thermal equilibration even for rare multi-strange particles 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

6 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor6 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) J. Letessier, J. Rafelski. nucl-th/ T. Renk. J.Phys.G: Nucl.Part.Phys., 30 (2004) 1495 Remarkable success of statistical hadronization model in describing data on particle yields in a wide energy range 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

7 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor7 Thermal model fits (3) A comprehensive analysis of data over a broad energy range (√s NN = 2.7 – 200 GeV); A.Andronic, P.Braun-Munzinger, J.Stachel. nucl-th/ Statistical quality of fit at 158 A GeV not satisfactory : inhomogeneous freeze-out scenario ? (D. Zschiesche, nucl-th/ )

8 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor8 Energy dependence of thermal parameters similarity of chemical freeze-out at SPS and RHIC SPSRHIC

9 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor9 Strangeness enhancements

10 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor10 First SPS signals on enhancement WA85 experiment, OMEGA spectro- meter, S-W and p-W data at 200 A GeV, 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 collisions S.Abatzis et al., Nucl. Phys. A 525 (1991) 445c NA35 streamer chamber experiment studying S-S and p-S collisions at 200 A GeV observed ~ twofold increase of strange particle (K 0,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

11 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor11 WA85 and WA94 results

12 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor12 NA57 Pb-Pb data at 158 A GeV NA57 data confirm the global enhance- ment pattern first observed by the WA97 in narrower centrality region Significant centrality dependence of strangeness enhancements for all hyperons except for  _ Strangeness enhancement: WA97 data: F. Antinori et al., Nucl.Phys. A661 (1999) 130c F. Antinori et al., J.Phys.G 30 (2004) S129-S138

13 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor13... and 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() [ ] F. Antinori et al., J.Phys.G 30 (2004) S717-S724

14 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor14 … and recent NA49 results NA49 and NA57 results qualitatively consistent Important contribution to the knowledge of centrality dependence

15 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor15 Energy dependence of 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   qualitative agreement with canonical suppression prediction (A.Tounsi & K.Redlich, hep-ph/ ) NA57 data

16 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor16 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

17 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor17 The onset of deconfinement ?

18 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor18 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

19 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor19 Ratio of K, Λ yields to pions sharp peak of K + /π + ratio smooth rise of anti-Λ/π similar peak in Λ/π ratio structure in K – /π – hadronic models do not (yet ?) reproduce the sharp peak

20 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor20 Inverse slope parameters inverse slope parameter T of K + and K - constant at SPS approximately constant pressure and temperature in mixed phase ?

21 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor21 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 M.Gazdzicki, M.Gorenstein. Acta Phys.Pol.B 30 (1999) 2705

22 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor22 New phenomenon or conventional physics ? A number of recent theoretical activities aiming at more conventional explanation : 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 B. Tomášik, E. Kolomeitsev. nucl-th/ , J. Letessier, J.Rafelski. nucl-th/ , J. Cleymans et al., nucl-th/ , J. Cleymans et al., Phys. Lett. B 615 (2005) 50 … Interesting result, waiting fair confirmation …

23 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor23 Transverse spectra, thermal freeze-out scenario

24 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor24 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 The same shape of hyperon and anti-hyperon spectra

25 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor25 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 ) 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 ~

26 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor26 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

27 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor27 Simple blast-wave fits, NA49 data 158A 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 Taken from : The NA49 collaboration, CERN-SPSC , Nov. 2005

28 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor28 Freeze-out of multistrange hyperons NA49 data on Ω production C.Alt et al., PRL (2005) The data favor a low transverse expansion velocity and high freeze-out temperature (T fo = 170 MeV, = 0.2, fit B2) indication for early freeze-out of the  at top SPS energy ? Analysis by G.E.Bruno (NA57)

29 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor29 High(er) transverse momenta

30 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor30 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) 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 ??

31 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor31 NA57 results on R CP for K 0, Λ F.Antinori et al., Phys,Lett.B 623 (2005) 17 Similar patterns as at RHIC, described by calculations involving parton energy loss

32 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor32 Recent NA49 data From talk of T. Schuster (this conference) Apparently a consistent picture at SPS is developing, needs further analysis

33 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor33 Comparison of NA57 and STAR data NA57 Similar K 0 - relative pattern observed at SPS and RHIC energy RCP values higher by ~0.5 at SPS, no significant suppression observed STAR data from: Phys.Rev.Lett. 92 (2004)

34 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor34 Conclusions  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

35 Strangeness in Quark Matter 2006, Los Angeles, MarchLadislav Šándor35 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 ~ 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 p T spectra, …)


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