Eugenio Bruno, Giuseppe

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Latest results from NA57Giuseppe Eugenio Bruno University and INFN Bari, ItalyE-mail: giuseppe.bruno@ba.infn.itfor the NA57 collaboration: F Antinorik, P Bacone, A Badalà f , R Barbera f , A Belogiannia,I J Bloodworthe, M Bombarah, G E Brunob S A Bulle, R Caliandrob, M Campbellg, W Carenag,N Carrerg, R F Clarkee, A Dainesek, D Di Barib, S Di Liberton, R Diviàg, D Eliab, D Evanse,G A Feofilovp, R A Finib, P Ganotia, B Ghidinib, G Grellao, H Helstrupd, K F Hetlandd, A K Holme j,A Jacholkowski f , G T Jonese, P Jovanovice, A Juskoe, R Kamermansr, J B Kinsone, K Knudsong,V Kondratievp, I Králikh, A Kravcˇákovái, P Kuijerr, V Lentib, R Lietavae, G Løvhøiden j, V Manzarib,M A Mazzonin, F Meddin, A Michalonq, M Morandok, P I Normane, A Palmeri f , G S Pappalardo f ,B Pastircˇákh, R J Platte, E Quercighk, F Riggi f , D Röhrichc, G Romanoo, K Šafarˇíkg, L Šándorh,E Schillingsr, G Segatok, M Senél, R Senél, W Snoeysg, F Soramelk M Spyropoulou-Stassinakia,P Starobam, R Turrisik, T S Tveter j, J Urbáni, P van de Venr, P Vande Vyvreg, A Vascottog, T Vik j,O Villalobos Bailliee, L Vinogradovp, T Virgilio, M F Votrubae, J Vrlákovái and P Závadama Physics Department, University of Athens, Athens, Greeceb Dipartimento IA di Fisica dell’Università e del Politecnico di Bari and INFN, Bari, Italyc Fysisk Institutt, Universitetet i Bergen, Bergen, Norwayd Høgskolen i Bergen, Bergen, Norwaye University of Birmingham, Birmingham, UKf University of Catania and INFN, Catania, Italyg CERN, European Laboratory for Particle Physics, Geneva, Switzerlandh Institute of Experimental Physics, Slovak Academy of Science, Košice, Slovakiai P.J. Šafárik University, Košice, Slovakiaj Fysisk Institutt, Universitetet i Oslo, Oslo, Norwayk University of Padua and INFN, Padua, Italyl Collège de France, Paris, Francem Institute of Physics, Prague, Czech Republicn University “La Sapienza” and INFN, Rome, Italyo Dipartimento di Scienze Fisiche “E.R. Caianiello” dell’Università and INFN, Salerno, Italyp State University of St. Petersburg, St. Petersburg, Russiaq IReS/ULP, Strasbourg, Francer Utrecht University and NIKHEF, Utrecht, The NetherlandsNA57 at the CERN SPS has studied the production of strange particles in PbPb and pBe col-lisions. Hyperon enhancements at 40 A GeV/c are presented and compared to results at 160GeV/c beam momenta. The momentum spectra are analysed based on hydrodynamical modelsand freeze-out temperature, transverse and longitudinal ow velocities are extracted. Central-to-peripheral nuclear modication factors at 160 A GeV/c are calculated.International Europhysics Conference on High Energy PhysicsJuly 21st - 27th 2005Lisboa, PortugalSpeaker.PoS(HEP2005)126cCopyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(HEP2005)126Latest results from NA57 Giuseppe Eugenio Bruno1. Strangeness EnhancementAn enhanced production of strange particles in nucleus–nucleus collisions with respect toproton–induced reactions was suggested long ago as a possible signature of the phase transitionfrom colour-confined hadronic matter to a Quark-Gluon-Plasma (QGP) [1]. NA57 has measuredthe production of Λ, Ξ  , Ω  , their antiparticles and K0S in Pb–Pb and in reference collisions (p-Be and p–Pb) at 40 and 160 A GeV/c in the central unit of rapidity and at medium transversemomentum (p 0  5 GeV/c) [2, 3]. The Enhancement E is defined as the yield per participant(Y 	 Nwound  , Y ycm  0  5ycm  0  5 dy ∞0dN2dp  dy dp  ) relative to p-Be collisions. The hyperon enhance-ments at 40 and 160 A GeV/c are shown as a function of centrality in fig. 1. The arrow in the 2ndpanel of fig. 1 indicates the lower limit to the Ξ  enhancement in the four most central classes at95% confidence level. At 40 A GeV/c the enhancement pattern follows the same hierarchy with thestrangeness content predicted in a QGP scenario: E  Λ  E  Ξ  and E  Λ  E  Ξ   , as alreadyobserved at 160 A GeV/c where further E  Ξ  E  Ω  [3]. The rational behind these predictionsis that the s and s¯ quarks, abundantly produced in the deconfined phase would recombine to formstrange and multi-strange particles in a time much shorter than that required to produce them bysuccessive rescattering interactions in a hadronic gas. Comparing the measurements at the two1101 10 102 103pT > 0,   |y-ycm| < 0.5< Nwound >Particle / event / wound. nucl. relative to pBepBe PbPbΛΞ-1101 10 102 103pT > 0,   |y-ycm| < 0.5< Nwound >Particle / event / wound. nucl. relative to pBepBe PbPbΞ+Λ1101 10 102 103pT > 0,   |y-ycm| < 0.5< Nwound >Particle / event / wound. nucl. relative to pBeΛΞ-pBe pPb PbPb1101 10 102 103pT > 0,   |y-ycm| < 0.5< Nwound >Particle / event / wound. nucl. relative to pBeΛΞ+Ω-+Ω+pBe pPb PbPbFigure 1: Hyperon enhancements as a function of the number of participants (Nwound) at 40 (1st and 2ndpanels) and 160 (3rd and 4th panels) A GeV/c. The symbol  shows the systematic error.beam momenta: for the most central collisions the enhancements are slightly larger at 40 than at160 GeV/c, the increase with Nwound is steeper at 40 than at 160 GeV/c.2. Collective DynamicsThe expansion dynamics of the Pb–Pb collisions has been studied in the transverse and longi-tudinal directions by measuring, respectively, the m  ( ﬀ p2ﬁm2) spectra (details in ref. [5, 6])and the rapidity distributions (details in ref. [7]) of strange particles, based on a hydro-dynamicsinspired model [8]. A simultaneous description of all the measured particle spectra, both for rapid-ity and p  can be achieved with the following set of parameters (for the most central 53% of theinelastic Pb–Pb cross section at 160 A GeV/c ): freeze-out temperature T  144 ﬂ 7 MeV, averageflow velocities  β ﬃ 0  38 ﬂ 0  02 and  β  0  42 ﬂ 0  03, as shown in fig. 2. With increasingcentrality, the freeze-out temperature decreases and the transverse flow velocity increases. In thePoS(HEP2005)126126 / 2/2PoS(HEP2005)126Latest results from NA57 Giuseppe Eugenio Bruno0510152 2.5 3 3.5 4rapiditydN/dyK0   S0510152 2.5 3 3.5 4rapiditydN/dyΛ00.511.52 2.5 3 3.5 4rapiditydN/dyΛ00.511.52 2.5 3 3.5 4rapiditydN/dyΞ-00.10.20.30.42 2.5 3 3.5 4rapiditydN/dyΞ+00.10.20.30.42 2.5 3 3.5 4rapiditydN/dyΩ-+Ω+1101021031041051061071080 0.5 1 1.5 2 2.5ΛΛ–K0 SΩ-Ω– +Ξ-Ξ– +mT - m0   [GeV]1/mT dN/dmT  [a. u.] T=144±7 MeV<β^>=0.381±0.013χ2/ndf=37.2/48Figure 2: Rapidity distributions (left) and transverse mass spectra (right) of strange particles for the mostcentral 53% of the inelastic PbPb cross-section. Blast-wave ts are superimposed to the data point (fulllines). The dotted lines in the left panel show the distribution expected for a thermal source without anylongitudinal ow.longitudinal direction, instead, we do not observe a centrality dependence. The freeze-out tem-perature is lower at 40 (T  118 ﬂ 5 MeV) than at 160 A GeV/c, but the transverse velocity iscompatible at the two energies.3. Nuclear Modication FactorsAt the Relativistic Heavy Ion Collider (RHIC), the central-to-peripheral nuclear modificationfactorRCP  p  "!Ncoll # P!Ncoll # C $d2NCAA  dp  dyd2NPAA  dp  dymeasured for a large variety of particles has proven to be a powerful tool for the study of partonpropagation in the dense QCD medium expected to be formed in nucleus–nucleus collisions (see,e.g., [9]). At SPS energy, so far, only pi 0 RCP measurements were available [10]; the first results onthe particle-species dependence (unidentified negatively charged hadrons, K0S, Λ, and Λ) werereported recently by the NA57 Collaboration in [11]. Figure 3 (left panel) shows the results for0–5%/40–55% RCP nuclear modification factors. At low-p  RCP scales with the number ofparticipants for all particles except the Λ. With increasing p, K0S mesons reach values of RCP % 1:we do not observe the enhancement above unity that was measured in proton–nucleus relative topp collisions (Cronin effect [12]). An enhancement is, instead, observed for strange baryons, Λand Λ, that reach RCP & 1  5 at p  & 3 GeV  c. In fig. 3 (middle panel) we compare our K0S data topredictions (X.N. Wang) obtained from a perturbative-QCD-based calculation [13], including(thick line) or excluding (thin line) in-medium parton energy loss. The initial gluon rapiditydensity of the medium was scaled down, from that needed to describe RHIC data, according to thedecrease by about a factor 2 in the charged particle multiplicity. The data are better described bythe curve that does include energy loss. The prediction of a second model of parton energy loss(PQM) that describes several energy-loss-related observables at RHIC energies [14] is also inagreement with the value reached at high-pby the data. Figure 3 (right panel) shows the ratio ofPoS(HEP2005)126126 / 3/3PoS(HEP2005)126Latest results from NA57 Giuseppe Eugenio Bruno00.511.522.530 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5ΛΛh-K0SpT (GeV/c)RCP √sNN = 17.3 GeV    0-5% / 40-55%01230 1 2 3 4 5E loss (Wang)no E loss (Wang)pT (GeV/c)RCP 0-5% / 40-55%E loss (PQM)NA57 K0S00.511.522.50 2 4 6STAR, √sNN = 200 GeV, 0-5%/40-60%NA57, √sNN = 17.3 GeV, 0-5%/40-55%pT (GeV/c)RCP(Λ)/ RCP(K0 S)Figure 3: Left: RCP ratios for negatively charged particles (h ' ) and singly-strange particles in PbPb colli-sions at ( sNN ) 17 * 3 GeV [11]. The width of the shaded band centered at RCP ) 1 indicates the systematicerror due to the uncertainty on the values of + Ncoll , in each class; the band at low p - show the value ex-pected for scaling with the number of participants. Middle: K0S RCP . p -0/ compared to predictions [13, 14]with and without energy loss Right ratio of Λ RCP to K0S RCP, as a function of p - , at the SPS (NA57 at( sNN ) 17 * 3 GeV) and at RHIC (STAR at ( sNN ) 200 GeV [9, 15]).Λ RCP to K0S RCP, as measured from our data and by STAR at 1 sNN  200 GeV [9, 15] (note thatΛﬁΛ are considered by STAR and that the centrality range used for the peripheral class isslightly different in the two experiments). The similarity of the Λ–K pattern to that observed atRHIC may be taken as an indication for coalescence effects [16] at SPS energy.References[1] Rafelski J and Müller B 1982 Phys. Rev. Lett. 48 1066, ibidem 1986 56 2334[2] Antinori F et al. 2004 Phis. Lett. B 595 68-74[3] Bruno G E et al. 2004 J. Phys. G: Nucl. Part. Phys. 30 S717-S724[4] Tounsi A, Mischke A and Redlich K 2003 Nucl. Phys. A 715 565c[5] Antinori F et al. 2004 J. Phys. G: Nucl. Part. Phys. 30 823-840[6] Bruno G E et. al. 2005 J. Phys. G: Nucl. Part. Phys. 31 S127-S133[7] Antinori F et al. 2005 J. Phys. G: Nucl. Part. Phys. 31 1345-1357[8] Schnedermann E, Sollfrank J and Heinz U 1993 Phys. Rev. C 48 2462-2475[9] Adams J et al. 2004 Phys. Rev. Lett. 92 052302[10] Aggarwal M M et al. 2002 Eur. Phys. J. C 23 225; d’Enterria D 2004 Phys. Lett. B 596 32.[11] Antinori F et al. 2005 Phys. Lett. B 623 17[12] Cronin J et al. 1975 Phys. Rev. D 11 3105.[13] Wang X N 2000 Phys. Rev. C C61 064910; private communication.[14] A. Dainese et al., Eur. Phys. J. C38 (2005) 461; private communication.[15] M.A.C. Lamont, poster presentation at QM2005, Acta Phys. Slov. in print.[16] Hwa R C and Yang C B 2003 Phys. Rev. C 67 064902; Fries R J et al. 2003 Phys. Rev. C 68 044902;Greco V et al. Phys. Rev. C 68 034904.PoS(HEP2005)126126 / 4/4PoS(HEP2005)126

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