- 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

2

Phase Diagram of Nuclear MatterPhase Diagram of Nuclear Matter

Early Universe

QGP

Neutron StarsNeutron Stars??????

Ultra-RelativisticHeavy-Ion Collisions

???

3

QUARK GLUON PLASMAQUARK GLUON PLASMA

DECONFINEMENTDECONFINEMENT

PHASETRANSITION

Signals?4

charge more evenly distributed in plasma,due to the fractional charges of quarks

Hadron Gas QGP

5

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

6

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 .

7

Hadron gas, no correlations:

QGP, no correlations:

Hadronized QGP, (from Jeon & Koch paper):

8

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.

9

Charge is globally conservedapQv 1)(

What can we expect to see?

10

Efficiency Dependenceea ppQv 1)(

11

Uncorrelated Background Contribution)1(1)( bkga fpQv

12

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

Decays of Neutral Resonances

width = 30º

fres is the fraction of particles originating from neutral resonances 14

Decays of Neutral Resonances

fres = 0.3

15

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

2

2

)1(

~

n

nap

DD

2 , )(

nn

nn

chnDRv

21 , )(

QQn

Qvch

Q

16

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

1

chchch

nn

QQ

n

1

11 ap

nnn

n

n

ndyn

11

2

17

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

dependence ugly ~

, , )( chnDDRv

1)(4

and )( , 0 if

Qvn

Qvch

dyn

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dependence for correct not does

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dynor recommend wouldI

18

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

19

~ 400 Members

57 Institutions

12 Countries

~ 400 Members

57 Institutions

12 Countries

Collaboration

20

Central Magnet

Beam-BeamCounters

Muon ArmSpectrometers Central Arm

Spectrometers

21

Event Display

CentralAu+Au

Collision

~ 400tracks in

central arms

22

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

Centrality classes defined by BBC and ZDC

24

Acceptance window defined around midpoint of detector arm 25

- Reduction not as large as predicted for QGP- Consistent with RQMD simulation

Global ChargeConservation

10% most central events

26

Fluctuations independent of centrality

27

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

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

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

30

31

dc+pc1

Clear centrality dependence 32

dc+pc1+(pc3 || emc)

Qualitatively, same result 33

dc+pc1+pc3

Qualitatively, same result 34

dc+pc1

r = 75

35

r = 50

Hijingsimulations

36

Hijingsimulations

r = 50

Using efficiency and charge asymmetry differences between track definitions are removed

( 1 – pa )

37

Toy model of hadronization from QGP

pions. 3or 2 intofragment

, , ,

nscombinatioquark The

(70%)(30%)

abundant)(equally

uddudduu

+

-uu

uu

+-

0

apQv

118

5)(get we

, 0 if construct, With this

38

25% of particlesgenerated withtoy model

fromGaussian distribution, and using PHENIX geometry:

39

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

40

Measurements elsewhere

41

STAR (G. Westfall) : Quark Matter 2004

42

NA49 : nucl-ex/0406013

Measurements elsewhere

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

Measurements elsewhere

43

- 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

44