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Net-Charge Fluctuations in Au+Au Collisions

Net-Charge Fluctuations in Au+Au Collisions. Henrik Tydesjö. O UTLINE. - 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

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Net-Charge Fluctuations in Au+Au Collisions

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  1. Net-Charge Fluctuations in Au+Au Collisions Henrik Tydesjö

  2. OUTLINE - 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 2

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

  4. QUARKGLUONPLASMA PHASE TRANSITION DECONFINEMENT Signals? 4

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

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

  7. For each event: Let n+andn-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 nch . 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 pa is the fraction of charged particles falling into the detector acceptance among all charged particles in the event. Also charge asymmetry, e, 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. What can we expect to see? Charge is globally conserved 10

  11. Efficiency Dependence 11

  12. Uncorrelated Background Contribution 12

  13. p+ r j p- 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 fres is the fraction of particles originating from neutral resonances 14

  15. Decays of Neutral Resonances fres = 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. Collaboration ~ 400 Members 57 Institutions 12 Countries 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. NET CHARGE FLUCTUATIONS RHIC 1st run period ~ 500 000 events |zvertex| < 17 cm pT > 200 MeV/c 23 PHENIX Collaboration : PRL 89 (2002) 082301

  24. Centrality classes defined by BBC and ZDC 24

  25. Acceptance window defined around midpoint of detector arm 25

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

  27. Fluctuations independent of centrality 27

  28. NET CHARGE FLUCTUATIONS RHIC 2nd run period ~ 850 000 events |zvertex| < 17 cm pT > 200 MeV/c pT < 2 GeV/c 28 H. Tydesjö : PhD Thesis, Lund University (www.hep.lu.se/staff/tydesjo/theses/)

  29. - Higher rate of data taking - More statistics - More detectors in operation 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

  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 Djr = 75 35

  36. Hijingsimulations Djr = 50 36

  37. Using efficiency and charge asymmetry differences between track definitions are removed Djr = 50 ( 1 – pa ) Hijingsimulations 37

  38. p- p0 p+ p+ j j j p- Toy model of hadronization from QGP 38

  39. jfromGaussian distribution, and using PHENIX geometry: 25% of particlesgenerated withtoy model 39

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

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

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

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

  44. SUMMARY • - 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. 44

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