Transcript 幻灯片 1

Narrowing of Balance Function
and Hadronization Time
at RHIC Energy
Du Jiaxin, and Liu Lianshou
Institute Of Particle Physics,
Huazhong Normal University (CCNU)
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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2/13
Why (changed) balance function?
Bass, Danielewicz, and Pratt, Phys. Rev. Lett. 85, 2689 (2000).
 Clocking Hadronization
Charge-anticharge pairs
are correlated in rapidity.
Those who created earlier
can separate further in
rapidity.
 QGP Signal
Early Hadronization
 Large y
Late Hadronization
 Small y
BF is expected to be
y
narrower for a scenario
with delayed hadronization,
due to the formation of a quark-gluon plasma.
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3/13
Charge Balance Function in Yw
1  n  (y )    n  (y )   n (y )    n (y ) 
B(y | YW )  {

}
2
 n 
 n 
Relative rapidity y  y1  y2
All the particles are within the rapidity window
YW
In our calculation YW  [3.0,3.0]
The width of the BF is defined by:
  y YW
B( y | Y )   y


 B( y | Y )
i
i
i
W
i
i
W
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4/13
Result given by STAR
STAR, QM04.
AuAu @ 200GeV
The narrowing of balance function as the increase of
multiplicity is clearly discovered by experiments.
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5/13
A brief Introduction to AMPT Model
initial state
hadronic phase
and freeze-out
QGP and hydrodynamic expansion
pre-equilibrium
hadronization
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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Zi-Wei Lin, Che Ming Ko, Bao-An Li and Bin Zhang, Phys. Rev. C72 064901 (2005).
Four main components :
 Initial Conditions: HIJING
model
 Partonic Interactions: ZPC
model
 Hadronization: LUND string
fragmentation mechanism
(PYTHIA).
 Hadronic Interactions: ART
model
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Hadronization time in AMPT Model



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.
No unique hadronisation time for the whole system.
Each parton has its own hadronisation time.
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8/13
We defined:
tfr 
1
N parton
N parton
t
i 0
fri
as the characteristic hadronization time for an event. Where
N parton is the number of partons in the event, tfri is the freeze
out time of the ith parton.
 g  10mb
Fig.2 tfr distribution for b>7 and b<7 correspondingly
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9/13
Balance Function in AMPT
Multiplicity increase
BF become narrowing
?
Delayed hadronization
BF become narrowing
Is the narrowing of Balance Function only caused by the
multiplicity increase or really due to delayed hadronization?
Two preliminary questions :
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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10/13
Fig.3. tfr .vs. N ch for b>7 and b<7 correspondingly
Two centrality samples:
Each centrality sample is divided into sub-samples
according to multiplicity intervals;
The resulting sub-samples are further divided into subsamples by different mean hadroniztion time intervals.
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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 hadronization time.
FIG. 3:  y Y for different tfr
Au-Au@ 200 GeV.
w
and nch
Using the narrowing of BF
as a measure of hadronization time and as a signal of
QGP is doubtful.
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12/13
Summary

We use the average of hadronization time as the unique
hadronization 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.
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13/13