Hyperon enhancement at 40 and 158 A GeV/c NA57 For the NA57 Collaboration Tiziano Virgili* *Università degli Studi di Salerno and INFN, Italy.
Contents ’Physics motivation ’The NA57 experimental apparatus ’Multiplicity and centrality measurement (new this year) ’Strangeness production: transverse mass spectra ’Strangeness production: ratios and enhancements (new this year) ’Conclusions
Physics motivation • ’NA57 physics goals: • study of the dependenceof hyperon enhancementson ’interaction volumedown to Nwound 50 ’collision energy (data at both 40 and 158 A GeV/c ’Data samples:
Experimental apparatus B=1.4 T Target:1% Pb-Pb, 8% p-Be Scintillator petals: centrality trigger (Pb-Pb) MSD 1 & 2: multiplicity detector (Pb-Pb) Tracking device: silicon pixel planes (5 x 5 cm2 cross section, 50 x 500 mm2 cell size, total of 1 M channels) Lever arm: pixels + double sided mstrips
Reconstructed decay topologies: W- L + K-( BR = 67.8 % ) X- L + p-( BR = 99.9 % ) L p- + p( BR = 63.9 % ) K0 p+ + p-( BR = 68.6 % ) • Charged decay tracks measured in the pixel telescope 5 cm 5 cm s 30 cm L p- X- Strange particle detection
Selected signals (Pb-Pb) 40 A GeV 160 A GeV
Multiplicity measurement dσ/Nch (barn) 158 A GeV/c Nch dσ/Nch (barn) ’Multiplicity measured in the range: 2<η<4 (158 A GeV/c) and 1.9<η<3.6 (40 A GeV/c) ’Measurement of dN/dη at the peak 40 A GeV/c Nch
Pb-Pb 158 A GeV/c 0 1 2 3 4 Nch Centrality of the collision dσ/Nch (barn) ’Data are divided in 5 centrality classes ’N. of participants (wounded nucleons) are determined within the Glauber model. Nch
dN/dη measurement ’General agreement at 158 A GeV/c; disagreement with NA50 at 40 A GeV/c ’Agreement for the participants between NA50 and NA57; disagreement with NA49 (larger values at the same centrality)
Ratio of dN/dη (158 A GeV/40 A GeV) ln(17.3)/ln(8.8) (Corrected for centrality) ’Ratio close to a scaling with log(s)
Midrapidity Yield (i.e. particles per event in the central unit of rapidity) • extrapolated to a common • y-pT region: • Enhancement: Strangeness production: Transverse mass spectra, Yields and Enhancements • Double differential cross sections • for each particle fitted to: • assuming flat rapidity distribution in the selected acceptance regions • Inverse slope of mT distribution ( T = Tapp ) free parameter of the fit.
Transverse mass spectra (158 A GeV/c)
Inverse slopes(158 A GeV/c) Pb-Pb ’ In central and semi-central Pb-Pb collisions (bin 1,2,3,4) we measure compatible slopes for particle and its anti-particle ’ This symmetry is less evident in p-Be
Blast wave analysis Hydro-dynamical picture: the mT spectra are sensitive to the transverse flow Blast wave description of the spectra: Transverse flow velocity Transverse source size Surface velocity Ref: E Schnedermann, J Sollfrank and U Heinz, Phys. Rev. C48 (1993) 2462
Blast wave fit to strange particles 53% most central events n=1 T = 144 ± 7(stat) ± 14(syst) MeV <b> = 0.381 ±0.013(stat) ± 0.012(syst) • T and <β> depend weakly on n=0, ½, 1 • n=2 casedisfavoured by data (bad χ2)
With increasing centrality: Transverse flow velocity increases Freeze-out temperature decreases Earlier decoupling for peripheral collisions ? Centrality dependence of the thermal freeze-out in Pb-Pb at 158 A GeV/c 1s contours n=1 J. Phys. G, nucl-ex/0403016
’ Y/ sY ratio increase with energy ’Energy dependence weaker for particle with higher strangeness baryon density decreases with energy increase Energy dependence of ratios Centrality regions: NA57:0 - 11% STAR*:0 - 11% *Ref: Physics Letters B 567 (2003), 167.
3 2 10 10 Hierarchy of the enhancements (QGP prediction) 2 1 1 1 1 No enhancement Enhancements at 158 A GeV/c Factor 20 for 10 10 1 1 • Evidence of significant centrality dependence of enhancements in Pb-Pb (measurements in bin 0 essential) • Saturation for the two-three most central bins ?
10 (95 % confidence level) 10 1 1 Enhancements at 40 A GeV/c • Enhancements are still there at 40 A GeV/c, • with the same hierarchy as at 158 A GeV/c:E(Λ) < E(Ξ)
Conclusions (I): • Multiplicity measurement: • Disagreement at 40 A GeV/c with NA50 • Agreement with other experiments at 158 A GeV/c • Ratio close to simple scaling with ln(s) • Transverse mass spectra in Pb-Pb at 158 A GeV/c: • Symmetry between hyperon and anti-hyperon in central • and semi-central Pb-Pb collisions (bins 1,2,3,4), not in p-Be • Description by common freeze-out adequate • Evidence for a centrality dependence of the thermal • freeze-out parameters
Conclusions (II): • Energy dependence of ratios: ’weaker dependence for particles with lower strangeness content (larger baryon density at lower energy) • Strangeness enhancement 40 vs 158 A GeV/c: • ’Enhancement of midrapidity yields at 40 A GeV/cof thesame order as at 158 A GeV/c, and with the same hierarchy :E(Λ)<E(Ξ)
The NA57 Collaboration Athens, Greece; Bari, Italy; Bergen, Norway ;Birmingham, UK; Bratislava, Slovakia; Catania, Italy; CERN, Switzerland; Kosice, Slovakia; Oslo, Norway; Padua, Italy; Paris, France; Prague, Czech Republic; Rome, Italy; Salerno, Italy;St. Petersburg, Russia; Strasbourg, France; Utrecht, The Netherlands.