Narrowing of Balance Function and Hadronization Ti

me at RHIC EnergyDu Jiaxin, and Liu Liansho

uInstitute Of Particle Physics,

Huazhong Normal University (CCNU)

dujx@iopp.ccnu.edu.cn 2/13

Outline

About Balance Function

A Brief Introduction to AMPT Model

The Time Evolution in AMPT

Our Result of Balance Function

Summary

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Why (changed) balance function?

Clocking HadronizationClocking Hadronization

QGP SignalQGP Signal

BF is expected to be narrower for a scenariowith delayed hadronization, due to the formation of a quark-gluon plasma.

Early Hadronization

Large y

Late Hadronization Small y

y

Bass, Danielewicz, and Pratt, Phys. Rev. Lett. 85, 2689 (2000).

Charge-anticharge pairs are correlated in rapidity. Those who created earlier can separate further in rapidity.

dujx@iopp.ccnu.edu.cn 4/13

})()()()(

{2

1)|(

n

ynyn

n

ynynYyB W

21 yyy Relative rapidity

All the particles are within the rapidity window WY

Charge Balance Function in Yw

The width of the BF is defined by:

( | )

( | )W

i W iiY

i Wi

B y Y yy

B y Y

In our calculation [ 3.0,3.0]WY

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Result given by STAR

The narrowing of balance function as the increase of multiplicity is clearly discovered by experiments.

STAR, QM04.

AuAu @ 200GeV

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A brief Introduction to AMPT Modelinitial state

pre-equilibrium

QGP and hydrody- namic expansion

hadronization

hadronic phaseand freeze-out

Characteristic:

Quark-Parton phase included

Complete time evolution after parton produced

Two versions are available, we use the default version(v1.11).

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Four main components :

Initial Conditions: HIJING model

Partonic Interactions: ZPC model

Hadronization: LUND string fragmentation mechanism (PYTHIA).

Hadronic Interactions: ART model

Zi-Wei Lin, Che Ming Ko, Bao-An Li and Bin Zhang, Phys. Rev. C72 064901 (2005).

dujx@iopp.ccnu.edu.cn 8/13

AMPT is based on non-equilibrium dynamics. No equilibrium phase transition from parton phase to hadron phase.

A parton comes to hadronization only when it cease to interact with other partons.

Hadronization time in AMPT Model

No unique hadronisation time for the whole system. Each parton has its own hadronisation time.

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We defined:

ifr fr0

1 partonN

iparton

t tN

as the characteristic hadronization time for an event. Where is the number of partons in the event, is the freeze out time of the parton.

thiparton

Nifrt

Fig.2 distribution for b>7 and b<7 correspondinglyfrt

10mb g

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Two preliminary questions :

Balance Function in AMPT

BF become narrowing

Multiplicity increase

BF become narrowing

Delayed hadronization?Is the narrowing of Balance Function only caused by the multiplicity increase or really due to delayed hadronization?

How does the hadronization time vary as the multiplicity increase?How does the BF width vary when hadronization time increase but the multiplicity keep constant?

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Fig.3. .vs. for b>7 and b<7 correspondinglyfrt chN

Two centrality samples:Each centrality sample is divided into sub-samples according to multiplicity intervals;The resulting sub-samples are further divided into sub-samples by different mean hadroniztion time intervals.

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FIG. 3: for different and Au-Au@ 200 GeV.

wYy

frt chn

Our result:The width of BF decreases with the increasing of multiplicity.

In the same multiplicity interval, the width of BF is consistent of being constant, independent of the hadroni-zation time.

Using the narrowing of BF as a measure of hadroniza-tion time and as a signal of QGP is doubtful.

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Summary We use the average of hadronization time as the unique hadr

onization time of the whole system.

We calculate the width of BF in different multiplicity interval and hadronization time interval.

The width of BF decreases with the increasing of multiplicity.

In AMPT model, the width of balance function is consistent with being independent of hadronization time in a fixed multiplicity interval.

Based on our calculation of AMPT model, We concludes that using the narrowing of balance function in RHIC as a measure of hadronization time and as a signal of QGP is doubtful.