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New results from the NA57 experiment

This article presents new results from the NA57 experiment, focusing on hyperon yields, blast wave analysis, and key findings. The aim of the experiment is to study the production of strange and multi-strange baryons in heavy ion interactions as a function of collision centrality and energy. The results show interesting patterns in hyperon yields and centrality dependence.

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New results from the NA57 experiment

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  1. NA57 New results from the NA57 experiment Giuseppe E. Bruno Università degli Studi di Bari and INFN, Bari, Italy For the NA57 Collaboration

  2. Outline • Aim of the experiment and experimental layout • Results on the hyperon yields • “Blast wave” analysis • Conclusions New results from the NA57 Collaboration G.E. Bruno

  3. Aim of the experiment K0  - - S K0 and hyperons are identified by reconstructing their weak decay modes into final states containing charged particles only s • NA57 studies the production of strange • and multi-strange baryons in heavy ion • interactions as a function of • collision centrality • collision energy ++- p +- +- p +- +K- p +- + c.c. A sketch of hyperon decay and detection in the NA57 set-up: New results from the NA57 Collaboration G.E. Bruno

  4. Layout of the NA57 experiment at CERN SPS Target: 1% Pb Scintillator Petals: centrality trigger MSD: Multiplicity Silicon Detector Tracking device: silicon pixel planes (5 x 5 cm2 ) Lever arm: double side mstrips Data samples collected by NA57 Sample size (trigger x 106) Beam momentum Data taking year System Pb-Pb 160 A GeV/c 230 + 230 1998 + 2000 Pb-Pb 40 A GeV/c 260 1999 p-Be 40 GeV/c 60 + 110 1999 + 2001 New results from the NA57 Collaboration G.E. Bruno

  5. Data corrected for acceptance and also for detector and reconstruction efficiency by Monte Carlo simulation In the acceptance window: Yield (i.e. particle per event) Transverse mass spectra (Tapp) Extrapolation to a common window: one unit of rapidity about ycm full range of pT Hyperon yield measurements New results from the NA57 Collaboration G.E. Bruno

  6. Centrality of the collision Centrality determination from charged particle multiplicity measurement Nwound from trigger cross section (Glauber model calculations) Pb-Pb 160 A GeV 0 I II III IV • 160 GeV: 56% most central events in 5 classes (bins 0 to IV) • <Nwound> = 624, 1214, 2093, 2902, 3491 • 40 GeV: determination of classes close to that for 160 GeV, • results presented here for bins I-IV New results from the NA57 Collaboration G.E. Bruno

  7. Hyperon yields at 40 and 160 A GeV/c Yield calculated for 42% most central Pb-Pb collisions (class I-IV) systematic errors • Particle yields are closer, a significant factor is observed for - • Anti-particle yields larger by a factor ~ 5 at 160 GeV/c Indication for larger baryon density at 40 A GeV/c New results from the NA57 Collaboration G.E. Bruno

  8. NA57 STAR Centrality: 5% forL ~10% for X,W (to be consistent with STAR) Comparison with higher energy (STAR at RHIC) NA57 STAR • anti-hyperon/hyperon ratios • increase with energy • energy dependence weaker for • species with higher strangeness STAR 130 GeV data from: C.Adler et al., Phys.Rev.Lett. 89 (2002), J.Castillo, nucl-ex/0210032, C.Suire, nucl-ex/0211017 and reference therein New results from the NA57 Collaboration G.E. Bruno

  9. 3 Enhancement = 2 Errors: Hierarchy of the enhancements (QGP prediction) <Yield> / <Npart> systematic 2 1 statistic (<Yield> / <Npart>)p-Be 1 No enhancement <Npart> • Significant centrality dependence of strangeness • enhancement for all particles except for  • Saturation for the two-three most central bins ? Particles made up of newly created quarks only Particles having quarks in common with the nucleon New: all available data on - and + 3x increase of statistics Centrality dependence of the yields at 160 A GeV/c Factor  20 for  New results from the NA57 Collaboration G.E. Bruno

  10. mT spectra in Pb-Pb at 160 A GeV/c Hydro-dynamical picture: the mT spectra are sensitive to the transverse flow Inverse slopes (MeV) K0   - + - + 2299 2897 2879 2975 31611 28016 32429 Systematic errors on the inverse slopes • V0,   10% •   15% s Preliminary In the relativistic domain (mT  m0) Blue shifted temperature: same slope for different m0 Non relativistic particles (pT  m0) Linear increase of the inverse slope with m0 Most central 55% of I New results from the NA57 Collaboration G.E. Bruno

  11. A “blast wave” analysis Ref: E. Schnedermann, J. Sollfrank and U. Heinz, Phys. Rev. C48 (1993) 2462. In the framework of model based on thermalization and hydro-dynamical transverse flow description Bessel functions Two parameters in this model: • It assume a constant profile for the transverse flow velocity • Other models can be implemented (e.g. (r)=S•[r/R]n) • A nice overview of different models can be found in: • G. Torrieri and J. Rafelski arXiv:nucl-th/0212091 Thermal “freeze-out” temperature Collective transverse flow velocity New results from the NA57 Collaboration G.E. Bruno

  12. “Particles” (K0,,-) and • “anti-particles” (,+,-,+) • show same thermal freeze-out parameters • Similar production mechanism • Few final stateinteractions • (short thermal freeze-out?) T (MeV)  2/ndf K0,,- 16010 0.390.02 12.7/15 ,+,-,+ 16716 0.390.03 12.2/22 Low pT part of  spectra needs additional study of systematics (in progress) Results of fit to NA57 data in full centraliy region Global fit with two parameters ! (i.e. thermal freeze-out temperature and transverse flow velocity) T=1617 MeV =0.3950.015 2/ndf=26.1/39 New results from the NA57 Collaboration G.E. Bruno

  13. +15 +0.07 120 0.46 WA97 IV HBT -0.10 -11 118/43 0.48±0.01 127  4 NA49 IV 0.48±0.01 46/41 122  2 HBT (WA97): F.Antinori et al., J. Phys. G 27 (2001) 2325-2344 NA49 @ QM02: M. Van Leeuwen et al., to be published in Nucl.Phys.A [arXiv:nucl-ex/0208014] Centrality dependence (i) Centrality T (MeV)  2/ndf • Suggested trend: the more central the collision the • higher  and the lower the freeze-out temperature New results from the NA57 Collaboration G.E. Bruno

  14. 2 contours in (T,) plane P(2 20) [%] Lines: results of HBT analysis ( /T=1.760.78 GeV-1) 2 T  Details for the 5% most central NA57 data sample (class IV) Centrality dependence (ii) Preliminary results consistent with NA49 using the same approach and with the WA97 HBT analysis of negative hadrons New results from the NA57 Collaboration G.E. Bruno

  15. Summary • Particle yields and ratios in Pb-Pb collisions: • energy dependence is weaker for  and stronger for  • indication of a larger baryon density at 40 GeV/c • Strangeness enhancement at 160 A GeV/c: • enhancement hierarchy with strangeness content (up to • a factor 20 for ) • significant centrality dependence of the enhancement for • all observed particles but for  (most peripheral class 0 • essential) • Very near future: enhancement pattern at 40 GeV/c • Transverse mass spectra at 160 A GeV/c: • all spectra well described by a common freeze-out • temperature and transverse flow velocity • hints for a centrality dependence of the thermal freeze-out New results from the NA57 Collaboration G.E. Bruno

  16. The NA57 Collaboration Physics Department, University of Athens, Greece ; Dipartimento IA di Fisica dell'Università e del Politecnico di Bari and INFN, Bari, Italy ; Fysisk Institutt , Universitetet i Bergen, Bergen, Norway ; Høgskolen i Bergen, Bergen, Norway ; University of Birmingham, Birmingham, UK ; Comenius University, Bratislava, Slovakia ; University of Catania and INFN, Catania, Italy ; CERN, European Laboratory for Particle Physics, Geneva, Switzerland; Institute of Experimental Physics Slovak Academy of Science, Kosice, Slovakia ; P.J. Safárik University, Kosice, Slovakia ; Fysisk institutt, Universitetet i Oslo, Oslo, Norway ; University of Padua and INFN, Padua, Italy ; Collège de France, Paris, France ; Institute of Physics, Prague, Czech Republic ; University ``La Sapienza'' and INFN, Rome, Italy ; Dipartimento di Scienze Fisiche ``E.R. Caianiello'' dell'Università and INFN, Salerno, Italy ; State University of St. Petersburg, St. Petersburg, Russia ; IReS/ULP, Strasbourg, France ; Utrecht University and NIKHEF, Utrecht, The Netherlands New results from the NA57 Collaboration G.E. Bruno

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