NA 57 Collaboration NA 57 Strange particle production

NA 57 Collaboration NA 57 Strange particle production in 160 and 40 A Ge. V/c Pb-Pb and p-Be collisions Domenico Elia INFN - Bari (Italy) on behalf of the D. Elia, INFN Bari (Italy) NA 57 Collaboration HQ 2004, Taos Valley - July 18 -24, 2004 1

NA 57 Collaboration Contents q Measuring strangeness with NA 57 q Particle yields and ratios: Ø energy and centrality dependence in Pb-Pb Ø enhancements with respect to p-Be q Conclusions Also @HQ 2004: Study of the m. T spectra of strange particles in Pb-Pb collisions at the SPS (talk by G. E. Bruno) D. Elia, INFN Bari (Italy) HQ 2004, Taos Valley - July 18 -24, 2004 2

NA 57 Collaboration Physics motivation § NA 57 physics goals: study of the dependence of hyperon enhancements on Ø interaction volume (centrality down to Nwound 50) Ø collision energy (data at both 40 and 160 A Ge. V) D. Elia, INFN Bari (Italy) HQ 2004, Taos Valley - July 18 -24, 2004 3

NA 57 Collaboration Physics motivation § NA 57 physics goals: study of the dependence of hyperon enhancements on Ø interaction volume (centrality down to Nwound 50) Ø collision energy (data at both 40 and 160 A Ge. V) § Data samples: System Beam energy Sample size Pb-Pb 160 A Ge. V (230+230) x 106 evts Pb-Pb 40 A Ge. V 240 x 106 evts p-Be 40 A Ge. V (60+110) x 106 evts WA 97 p-Be sample used as reference data at 160 A Ge. V D. Elia, INFN Bari (Italy) HQ 2004, Taos Valley - July 18 -24, 2004 4

NA 57 Collaboration Strange particle detection ls e nn § Reconstructed decay topologies: W- L + K - ( BR = 67. 8 % ) X- L + p - ( BR = 99. 9 % ) p- + p ( BR = 63. 9 % ) L K 0 s p+ + p- L measured in the pixel target M on c li es l te X- 5 cm Si ( BR = 68. 6 % ) § Charged decay tracks telescope 1 a ch e p co 30 cm p B - decay vertices Acceptance: Dy 1 around mid-rapidity D. Elia, INFN Bari (Italy) HQ 2004, Taos Valley - July 18 -24, 2004 5

NA 57 Collaboration Collision centrality § Centrality determination from measured charged particle multiplicity § Nwound from trigger cross section (Glauber + WNM calculations) Pb-Pb 160 A Ge. V 0 1 2 3 4 < Nwound > Class s / stot 160 Ge. V 0 42 – 53% 62 57 1 25 – 40% 121 119 2 11 – 23% 209 208 3 5 – 11% 290 292 4 0 – 5% 349 346 40 Ge. V § Same class definition at 40 and 160 Ge. V D. Elia, INFN Bari (Italy) HQ 2004, Taos Valley - July 18 -24, 2004 6

NA 57 Collaboration Yields and enhancements § Double differential cross sections for each particle fitted to: assuming flat rapidity distribution in the selected acceptance regions Inverse slope of m. T distribution ( T = Tapp ) free parameter of the fit. § Yield (i. e. particles per event) extrapolated to a common y-p. T region: § Enhancement: D. Elia, INFN Bari (Italy) HQ 2004, Taos Valley - July 18 -24, 2004 7

NA 57 Collaboration Energy dependence of hyperon ratios from SPS to RHIC Centrality region: 0 - 11% (both for NA 57 and STAR*) Ø Y / Y ratios increase with energy Ø Energy dependence weaker for particle with higher strangeness *Ref: Physics Letters B 567 (2003), 167. D. Elia, INFN Bari (Italy) HQ 2004, Taos Valley - July 18 -24, 2004 baryon density decreases with energy increase 8

NA 57 Collaboration Energy dependence of mid-rapidity yields from SPS to RHIC Centrality regions: NA 57: 0 -5% (L, K), 0 -11% (X, W) STAR*: 0 -5% (L), 0 -6% (K), 0 -10% (X, W) Ø stronger energy dependence for W- and all anti-hyperons *Refs: Physical Review Letters 89 (2002), 092301 Nucl. Phys. A 715 (2003) 470 c, nucl-ex/0307024 D. Elia, INFN Bari (Italy) HQ 2004, Taos Valley - July 18 -24, 2004 9

NA 57 Collaboration Centrality dependence of mid-rapidity yields D. Elia, INFN Bari (Italy) HQ 2004, Taos Valley - July 18 -24, 2004 10

NA 57 Collaboration Centrality dependence of mid-rapidity yields D. Elia, INFN Bari (Italy) HQ 2004, Taos Valley - July 18 -24, 2004 Yield <Nwound> 11

NA 57 Collaboration Centrality dependence of mid-rapidity yields Yields fitted to: Y <Nwound>a D. Elia, INFN Bari (Italy) HQ 2004, Taos Valley - July 18 -24, 2004 12

NA 57 Collaboration Centrality dependence of mid-rapidity yields Yields fitted to: Y <Nwound>a 160 A Ge. V 40 A Ge. V a c 2 / ndf 1. 20 ± 0. 04 3. 2 / 3 1. 49 ± 0. 04 1. 6 / 3 1. 21 ± 0. 05 0. 9 / 3 1. 52 ± 0. 04 6. 6 / 3 1. 00 ± 0. 07 8. 6 / 3 1. 18 ± 0. 08 4. 3 / 3 1. 40 ± 0. 04 6. 3 / 3 1. 85 ± 0. 13 6. 3 / 3 1. 30 ± 0. 11 5. 8 / 3 0. 84 ± 0. 60 1. 1 / 3 1. 45 ± 0. 28 0. 6 / 3 1. 45 ± 0. 86 0. 1 / 2 1. 49 ± 0. 36 0. 2 / 3 D. Elia, INFN Bari (Italy) Ø steeper centrality dependence of the 40 A Ge. V yields HQ 2004, Taos Valley - July 18 -24, 2004 13

NA 57 Collaboration Centrality dependence of hyperon enhancements at 160 A Ge. V/c [ ] systematic errors: 10% for L, X 15% for W Most peripheral class: <Nwound> = 62 4 Ø Significant centrality dependence of strangeness enhancements for _ all hyperons except for L (knowledge of yields in the most peripheral class essential) D. Elia, INFN Bari (Italy) HQ 2004, Taos Valley - July 18 -24, 2004 14

NA 57 Collaboration Centrality dependence of hyperon enhancements at 40 A Ge. V/c [ ] systematic errors: 10% for L, X 15% for W (95 % confidence level) Most peripheral class: <Nwound> = 57 4 Ø Enhancements still there at 40 A Ge. V with the same hierarchy as for higher energy data: E(L) < E(X) D. Elia, INFN Bari (Italy) HQ 2004, Taos Valley - July 18 -24, 2004 15

NA 57 Collaboration Energy dependence of hyperon enhancements Ø steeper increase with centrality at lower energy Ø enhancements in the most central collisions (classes 3 -4) are larger at 40 Ge. V than at 160 Ge. V (qualitative agreement with canonical suppression, Redlich et al, QM 2002) D. Elia, INFN Bari (Italy) HQ 2004, Taos Valley - July 18 -24, 2004 16

NA 57 Collaboration Conclusions q Energy dependence of yields and ratios: Ø stronger dependence for anti-particles than for particles (larger baryon density at 40 A Ge. V) Ø L and X- yields stay roughly constant with energy increase q Centrality dependence of the yields: Ø steeper centrality dependence of the 40 A Ge. V yields q Enhancements at 40 and 160 A Ge. V: Ø enhancements at 40 Ge. V similar to those at higher energy Ø same hierarchy at both energies: E(L) < E(X) Ø steeper centrality dependence at lower energy D. Elia, INFN Bari (Italy) HQ 2004, Taos Valley - July 18 -24, 2004 17