Indications for the Onset of Deconfinement in Pb+Pb collisions at the SPS

Transcription

1 Indications for the Onset of Deconfinement in Pb+Pb collisions at the SPS P.Seyboth, MPI für Physik, München for the NA49 Collaboration Introduction Search for structure in the energy dependence of Inclusive particle yields E-by-E fluctuations: K/π, Q BE correlations Anisotropic flow System size dependence of fluctuations of N ch, <p T > Summary 1

2 Introduction: Pb+Pb collisions at top SPS energy Initial energy density exceeds the critical value predicted by lattice QCD ( 1 GeV / fm 3 ) Strong collective behavior anisotropic and radial flow transverse expansion of the matter droplet by factor 2 Proposed signatures for deconfinement observed Strangeness enhancement Low-mass dilepton enhancement J/Ψ,Ψ yield suppression Signatures not specific for deconfinement Search for a threshold by varying the energy for the largest collision system (central Pb+Pb reactions) 2

3 The NA49 Detector Target: 20cm liquid H 2 or Pb foil VCAL detects projectile spectators p/p 2 = 7 (0.3) 10-4 (GeV/c) -1 (VTPC-1, VTPC+MTPC) de/dx resolution 3 6 % TOF resolution ~ 60 ps 2,5 < p < 10 GeV : TOF + de/dx at midrapidity p > 7 GeV de/dx forward rapidity 3

4 Pb+Pb collision at 158A GeV 4

5 NA49 energy scan program of central Pb+Pb collisions search for structure in the energy dependence of observables which may be linked to the onset of deconfinement Beam Energy Centrality Event Statistics Taken in 158A GeV 10 % 800 k A GeV 20 % 3,000 k A GeV 7 % 300 k A GeV 7 % 700 k A GeV 7 % 400 k A GeV 7 % 300 k 2002 PRC 66(2003) preliminary 5

6 Transverse mass spectra at mid-rapidity 158A GeV many spectra measured well described by fireball model assuming common temperature and flow blast wave : constant β T ; π, d excluded from fit (Schnedermann,Sollfrank, Heinz : PRC 48 (1993) 2462) 6

7 early freezeout of multistrange hyperons? 7

8 Transverse mass spectra at lower energy: blast wave model describes spectra at all SPS energies 40A GeV 30A GeV 20A GeV kinetic freeze out at T MeV, β T at SPS (assuming radially constant β T ) 8

9 Rapidity spectra also: Ξ, Ω Extrapolation 4π yields (open dots from reflection at y cm = 0) 9

10 Rapidity spectra: energy dependence of width width close to Landau prediction for pions better agreement with linear increase with y beam clear mass hierarchy at SPS (exclude p, Λ, Ξ containing leading beam quarks) 4π and mid-rapidity ratios differ systematically 10

11 Particle yields statistical model (fits by F.Becattini et al, PRC69(2004)024905) ( NA49 data ) hadron species populated according to phase space probabilities (max.entropy) (Fermi,Hagedorn) strangeness sector not fully saturated (Rafelski) statistical model successful, T, µ B, γ S, V incease of γ S = from p+p to A+A shows: relaxation of canonical suppression not enough! 11

12 Global view - Phase diagram QGP 1st order Hadrons statistical model describes yields from SIS to RHIC energy T of hadrochemical freeze out increases SIS RHIC µ B decreases (increase of pion production) freeze out at E 1 GeV per particle dashed line (Cleymans and Redlich, PRL 81(1998) 5284 ) (Lattice-QCD phase boundary: Fodor,Katz JHEP04(2004)050) hadrochemical freeze out parameters approach the phase boundary at SPS 12

13 Energy dependence - π yields increase of <π>/<n W > with energy gets steeper in the SPS range π deficit changes to enhancement compared to p+p + < π > = 1.5 ( < π > + < π - > ) pions are most abundant produced particle species measure of entropy in statistical models SMES: statistical model of the early stage 1/4 s NN (Gazdzicki,Gorenstein : Acta Phys.Pol. B30(1999)2705) Increase of initial d.o.f. g between AGS and SPS? 13

14 Energy dependence - ratio of K,Λ yields to pions sharp peak of K + /π + ratio smooth rise of anti-λ/π ( s quarks) hadronic models do not (yet?) reproduce the sharp peak similar peak in Λ/π ratio structure in K /π ( s quarks) ± < π > = 1 + ( < π 2 > + < π > ) 14

15 Energy dependence of total yield of s,s quarks energy dependence of yields changes at 30A GeV sharp peak, then undersaturation of strangeness content s quark carriers: s - quark carriers: s - quark carriers: - K, Λ Σ Ξ K K, K 0 +,, 0 1) + 0 1) Σ Λ+ Σ, Σ ± 2) 0 ± 2) Ξ, Ω Ξ, Ξ, Ω - 0-3) ) 1) ) 3) <K > <K >, <K > <K > due to isospin symm. ± empirical factor < Σ > 0.6 < Λ> from hadron gas model if not measured 15

16 Energy dependence ratio of strange hadrons to pions strangeness to pion ratio peaks sharply at the SPS note : hadronic hadronic < K > = 2 ( < K mixed mixed > + < K partonic partonic > ) = 4 < K S 1/4 s NN 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 < N + N > 0.74 g 0.21 > E S S < π > S = g u + g d s + g onset of deconfinement at SPS? 16 g

17 Energy dependence inverse slope parameters m T dn dm T dy e - m T /T inverse slope parameter T of K + and K - constant at SPS approximately constant pressure and temperature in mixed phase? (L.van Hove, PLB89(1982)253 M.Gorenstein et al.,plb567(2003)175) 17

18 Energy dependence - average transverse mass NA49 preliminary Increase of <m T > for abundant final state particles ( π, K, p) slows sharply at the lowest SPS energy 18

19 More realistic model with initial QGP phase Y.Hama et al., Braz.J.Phys,34,322 (2004) hydrodynamic evolution in QGP and hadronic phases (SPheRIO) NeXus type initial conditions at t 0 = 1 fm/c (T 0 = MeV, ε 0 = GeV/fm 3 at SPS) describes the rise of pion multiplicity and saturation of T * for Kaons 19

20 hypothetical trajectories in the phase diagram (Lattice-QCD phase boundary: Fodor,Katz JHEP04(2004)050) large (event-by-event) fluctuations are expected when the system hadronises close to the predicted critical point 20

21 The Event-by-Event K/π ratio max.likelihood fit for each event σ 2 dyn = σ 2 data σ 2 mixed Beam Energy NA49 preliminary Increased fluctuation signal at lower beam energies 21

22 The Event-by-Event (p + p)/π ratio Beam Energy NA49 preliminary The distribution of the E-by-E p/π ratio is narrower for data than mixed events. Effect of baryon resonance decay? 22

23 Smaller in a QGP than in a hadron gas (Jeon,Koch,Asakawa,Heinz,Müller) Effect of hadronisation? Electric charge fluctuations ( C.Alt et al., nucl-ex/ ) Φ q = 2 < Z > < N > z = q - q - z 2 Z = N i= 1 (q i - q ) Central Pb+Pb collisions 158A GeV -0.6< y <2.4 Φq, GCC = 1 < Nch > / < Nch > tot 1 Global charge conservation 23

24 Remove effect of global charge conservation : Φ = Φ Φ q q q, GCC No clear energy dependence of charge fluctuations at SPS QGP signature possibly masked by effect of resonance decays 24

25 BE correlations: energy dependence of radius parameters k ~ 0.15 GeV/c T R long increases slowly R side constant R out constant no evidence of anomalies or soft point of EOS 25

26 Anisotropic flow v 2 Initial anisotropy ε in non-central collisions transformed into momentum anisotropy v 2 by pressure at early reaction stage σ/σ T % ε = (<y 2 >+<x 2 >)/(<y 2 >-<x 2 >) SPS RHIC Smooth increase with collision energy towards hydrodynamic model prediction 26

27 event plane from pions flattening of acceptance via event plane shifting technique correction for event plane resolution de/dx identification of Λ decay daughters Λ signal/background 5-11 Λ elliptic flow (preliminary) 27

28 v 2 (p T ) at SPS and RHIC Λ increase of integrated v 2 mainly due to increase of yield at large p T 28

29 Multiplicity fluctuations at 158A GeV (preliminary) selected range: 4 < y < 5.5 small correction δ 10-2 for fluctuation in proj. participants: (NA49 preliminary) width increases with energy! maximum of fluctuations at N part 70 29

30 dependence on acceptance : extrapolation to 4π : NA49 preliminary fluctuations increase linearly with acceptance p = <N acc >/<N 4π > all charged >> negatives, positives no plausible explanation yet 30

31 Fluctuations of <p T > vs system size at 158A GeV selected range: 4.0 < y π < 5.5 corrected for limited 2-track resolution Similar centrality dependence as N ch fluctuations Correlation of <p T > with <N ch > due to energy conservation as in p+p? (Mrowczynski,Rybczynski,Wlodarczyk nucl-th/040712) 31

32 Conclusion from the energy scan at the SPS structure observed at low SPS energy in central Pb+Pb collisions the rate of increase of pion production steepens the strange particle to pion ratio peaks sharply constancy of the slope of π and K m T -spectra no convincing anomalies in e-by-e fluctuations (K/π, charge), BE correlations and flow reaction scenario with onset of deconfinement at the SPS provides best explanation of the observed structure plan to continue SPS program with lighter nuclei: role of the initial volume in the onset of deconfinement search for effects due to the predicted critical point 32

33 The NA49 collaboration 33