Hadron Spectra in Au+Au Collisions by STAR Experiment at RHIC
Net-Charge Fluctuations in Au+Au Collisions
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Transcript of Net-Charge Fluctuations in Au+Au Collisions
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- 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
OUTLINEOUTLINE
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Phase Diagram of Nuclear MatterPhase Diagram of Nuclear Matter
Early Universe
QGP
Neutron StarsNeutron Stars??????
Ultra-RelativisticHeavy-Ion Collisions
???
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QUARK GLUON PLASMAQUARK GLUON PLASMA
DECONFINEMENTDECONFINEMENT
PHASETRANSITION
Signals?4
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charge more evenly distributed in plasma,due to the fractional charges of quarks
Hadron Gas QGP
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NET-CHARGE FLUCTUATIONS
Event-by-Event Fluctuationsof Net Electric Charge
in Local Regions of Phase Space
Decrease of Fluctuationsproposed as a signal for the QGP
Predictions range up to an 80% reduction
Asakawa, Heinz, Müller : PRL 85 (2000) 2072Jeon, Koch : PRL 85 (2000) 2076
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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 nch .
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Hadron gas, no correlations:
QGP, no correlations:
Hadronized QGP, (from Jeon & Koch paper):
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But charge is a globally conserved quantityInstead 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, , has been taken into account.
where p+ and p- are the probabilities that aparticle is positive and negative, respectively.
A better measure of net-charge fluctuations is
which yields
in the hadron gas scenario.
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Charge is globally conservedapQv 1)(
What can we expect to see?
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Efficiency Dependenceea ppQv 1)(
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Uncorrelated Background Contribution)1(1)( bkga fpQv
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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
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Decays of Neutral Resonances
width = 30º
fres is the fraction of particles originating from neutral resonances 14
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Decays of Neutral Resonances
fres = 0.3
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The ”jungle” of different fluctuation measures (part 1/3)
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The ”jungle” of different fluctuation measures (part 2/3)2
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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
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Relativistic Heavy Ion Collider (RHIC)Relativistic Heavy Ion Collider (RHIC)Au+Au Collisions 200 AGeV
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~ 400 Members
57 Institutions
12 Countries
~ 400 Members
57 Institutions
12 Countries
Collaboration
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Central Magnet
Beam-BeamCounters
Muon ArmSpectrometers Central Arm
Spectrometers
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Event Display
CentralAu+Au
Collision
~ 400tracks in
central arms
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NET CHARGE FLUCTUATIONS
RHIC 1st run period
GeVsNN 130
~ 500 000 events
|zvertex| < 17 cm
pT > 200 MeV/c
PHENIX Collaboration : PRL 89 (2002) 082301 23
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Centrality classes defined by BBC and ZDC
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Acceptance window defined around midpoint of detector arm 25
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- Reduction not as large as predicted for QGP- Consistent with RQMD simulation
Global ChargeConservation
10% most central events
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Fluctuations independent of centrality
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NET CHARGE FLUCTUATIONS
RHIC 2nd run period
GeVsNN 200
~ 850 000 events
|zvertex| < 17 cm
pT > 200 MeV/c
pT < 2 GeV/c
H. Tydesjö : PhD Thesis, Lund University (www.hep.lu.se/staff/tydesjo/theses/) 28
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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
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(part of the difference is due to pure geometrical effects)
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dc+pc1
Clear centrality dependence 32
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dc+pc1+(pc3 || emc)
Qualitatively, same result 33
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dc+pc1+pc3
Qualitatively, same result 34
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dc+pc1
r = 75
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r = 50
Hijingsimulations
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Hijingsimulations
r = 50
Using efficiency and charge asymmetry differences between track definitions are removed
( 1 – pa )
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Toy model of hadronization from QGP
pions. 3or 2 intofragment
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25% of particlesgenerated withtoy model
fromGaussian distribution, and using PHENIX geometry:
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Percentages show the fraction of particles generated withthe toy model. The rest is from pure global charge conservation.
Gaus(0.2,0.1)
Toy model of hadronization from QGP
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Measurements elsewhere
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STAR (G. Westfall) : Quark Matter 2004
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NA49 : nucl-ex/0406013
Measurements elsewhere
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CERES/NA45 (H. Sako) : Quark Matter 2004
Measurements elsewhere
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- A comparison with a toy model of hadronization froma 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.
SUMMARYSUMMARY
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