2. - How can Net-Charge Fluctuations be used as a signal of a Quark- Gluon Plasma (QGP) phase transition? - Definition of a simple fluctuation measure, some expectations - Influences from detector inefficiencies, background, resonance decays etc. - Guide through the most popular fluctuation measures - RHIC, PHENIX experiment, results from 130 GeV and 200 GeV - Comparison to toy model of hadronization from QGP - Results from other experiments O UTLINE 2

3. Phase Diagram of Nuclear Matter Early Universe QGP Neutron Stars ??? Ultra-Relativistic Heavy-Ion Collisions ??? 3

4. Q UARK G LUON P LASMA D ECONFINEMENT PHASE TRANSITION Signals? 4

5. charge more evenly distributed in plasma, due to the fractional charges of quarks Hadron GasQGP 5

6. N ET- C HARGE F LUCTUATIONS Event-by-Event Fluctuations of Net Electric Charge in Local Regions of Phase Space Decrease of Fluctuations proposed as a signal for the QGP Predictions range up to an 80% reduction Asakawa, Heinz, Müller : PRL 85 (2000) 2072 Jeon, Koch : PRL 85 (2000) 2076 6

7. For each event: Let n + and n - denote the nr of positive and negative particles, respectively. Net charge Nr of charged particles A very simple measure of net-charge fluctuations is then since the variance of Q scales with n ch. 7

8. Hadron gas, no correlations: QGP, no correlations: Hadronized QGP, (from Jeon & Koch paper): 8

9. But charge is a globally conserved quantity Instead of v(Q)=1, the hadron gas scenario yields: where p a is the fraction of charged particles falling into the detector acceptance among all charged particles in the event. Also charge asymmetry, , has been taken into account. where p + and p - are the probabilities that a particle is positive and negative, respectively. A better measure of net-charge fluctuations is which yields in the hadron gas scenario. 9

10. Charge is globally conserved What can we expect to see? 10

11. Efficiency Dependence 11

12. Uncorrelated Background Contribution 12

13. Neutral resonance decays (e.g. ,  )  ++ --  If the detector acceptance is large enough to catch both the decay products, correlations between n + and n - are introduced. That is, the fluctuations are reduced. 13

14. Decays of Neutral Resonances  width = 30º f res is the fraction of particles originating from neutral resonances 14

15. Decays of Neutral Resonances f res = 0.3 15

16. The ”jungle” of different fluctuation measures (part 1/3) 16

17. The ”jungle” of different fluctuation measures (part 2/3) 17

18. The ”jungle” of different fluctuation measures (part 3/3) H. Tydesjö : PhD Thesis, Lund University (www.hep.lu.se/staff/tydesjo/theses/) Nystrand, Stenlund, Tydesjö : PRC 68 (2003) 034902 18

19. Relativistic Heavy Ion Collider (RHIC) Au+Au Collisions 200 AGeV 19

20. ~ 400 Members 57 Institutions 12 Countries ~ 400 Members 57 Institutions 12 Countries Collaboration 20

21. Central Magnet Beam-Beam Counters Muon Arm Spectrometers Central Arm Spectrometers 21

22. Event Display Central Au+Au Collision ~ 400 tracks in central arms 22

23. N ET C HARGE F LUCTUATIONS RHIC 1 st run period ~ 500 000 events |z vertex | < 17 cm p T > 200 MeV/c PHENIX Collaboration : PRL 89 (2002) 082301 23

24. Centrality classes defined by BBC and ZDC 24

25. Acceptance window defined around midpoint of detector arm 25

26. - Reduction not as large as predicted for QGP - Consistent with RQMD simulation Global Charge Conservation 10% most central events 26

27. Fluctuations independent of centrality 27

28. N ET C HARGE F LUCTUATIONS RHIC 2 nd run period ~ 850 000 events |z vertex | < 17 cm p T > 200 MeV/c p T < 2 GeV/c H. Tydesjö : PhD Thesis, Lund University (www.hep.lu.se/staff/tydesjo/theses/) 28

29. 3 different track definitions dc+pc1 : Tracks from Drift Chamber and PC1 (like in Run1 analysis) dc+pc1+pc3 : Tracks matched with PC3 hit dc+pc1+(pc3 || emc) : Tracks matched with PC3 or EMCal hit 29 - Higher rate of data taking - More statistics - More detectors in operation

30. (part of the difference is due to pure geometrical effects) 30

31. 31

32. dc+pc1 Clear centrality dependence 32

33. dc+pc1+(pc3 || emc) Qualitatively, same result 33

34. dc+pc1+pc3 Qualitatively, same result 34

35. dc+pc1  r = 75 35

36.  r = 50 Hijing simulations 36

37. Hijing simulations  r = 50 Usingefficiency and charge asymmetry differences between track definitions are removed ( 1 – p a ) 37

38. Toy model of hadronization from QGP ++ --    ++ -- 00 38

39. 25% of particles generated with toy model  from  Gaussian distribution, and using PHENIX geometry: 39

40. Percentages show the fraction of particles generated with the toy model. The rest is from pure global charge conservation. Gaus(0.2,0.1) Toy model of hadronization from QGP 40

41. Measurements elsewhere 41 STAR (G. Westfall) : Quark Matter 2004

42. 42 NA49 : nucl-ex/0406013 Measurements elsewhere

43. CERES/NA45 (H. Sako) : Quark Matter 2004 Measurements elsewhere 43

44. - A comparison with a toy model of hadronization from a QGP shows that the initial predictions on a very drastic decrease of net-charge fluctuations seem to have been too optimistic. - A very intriguing net-charge fluctuation centrality dependence is seen at 200 A GeV. The decrease is not consistent with Hijing simulations. - A large data sample at 200 A GeV is right now being prepared for analysis. Higher statistics and even more stable conditions during data taking will improve the measurements further. S UMMARY 44