Effect of isospin-dependent cluster recognition on the observables in heavy ion collisions

skillfulbuyerUrban and Civil

Nov 16, 2013 (3 years and 8 months ago)

74 views

Effect of
isospin
-
dependent cluster
recognition on the observables in
heavy ion collisions

Yingxun

Zhang (
张英逊
)

2012


8

10
日,

兰州


合作者:


Zhuxia

Li, (CIAE)

Chengshuang

Zhou, (CIAE, GXNU)

M.B.Tsang

(MSU)

China Institute of Atomic Energy



Transport model is one of the powerful methods for
understanding the mechanism of SHE synthesis and the
properties of nuclear matter

General view of transport models

Three parts in the transport models

1, Initial condition

2, Motion of equation

Cluster recognition
methods

3, Observables

1), Nucleonic potential

2), In medium XS

1),
proj
.,
targ
.,

2), density profile

3), BE,
rms

4),
Ebeam
, b

5), stability

A
, BUU type:
f(
r,p,t
)

one body phase space density

Two
-
body collision: occurs between test part.

Mean field

Solved with test particle methods

EOS, symmetry energy

B
, QMD type: solve N
-
body equation of motion

nucleon

Two body collision: occurs between nucleons

Rearrange whole nucleon
-
> large
flucturation

EOS, symmetry energy

Motion of Equation

also play very important roles on the final observables
as well as on EOS and in
-
medium XS!

Cluster recognition methods in the QMD and BUU models:


1.
MST,
Aichelin
, et.al., PR202(1991)

2.
ECRA,

C. O.
Dorso

and J.
Randrup
, Phys.
Lett
. B
301, 328 (1993).

3.
SACA,
R. K.
Puri

and J.
Aichelin
, J.
Comput
. Phys.
162, 245 (2000).

4.
MSTB,
P. B.
Gossiaux
, R. K.
Puri
, C.
Hartnack
, and J.
Aichelin
,
Nucl
. Phys. A
619,
379 (1997).

5.

Cluster Correlation
,
Danielewicz et al., NPA533 (1991) 712
,
A.Ono
, 2012

6.

Coalescence model
,
LWChen
, et.al., NPA 2004

Cluster recognition methods

Problems in current QMD models
simualtions
:

1 Z=1 largely overestimated, Z=2, underestimated,

A. Ono

K.
Zbiri
, A Le Fever, J.
Aichelin
, et.al, PRC75, 2007

2, Enhancements of the productions of neutron
-
rich isotopes observed in
isoscaling
,

Y
2
/ Y
1


T
Z
N
p
n
e
/
)
(






Not well predicted by the transport models

TXLiu
, et.al.,
Phys.Rev.C

69, 014603(2004)

The predicted final isotope
distributions are narrower than the
experimental data, ……

3, Strong enhancement of heavy fragments in neutron
-
rich reaction system,

124Sn+64Ni

112Sn+58Ni

P.Rustto
, et.al., PRC81

The result shows that the
dynamical process is about
twice as probable in the
neutron
-
rich system as in the
neutron
-
poor one.

This unexpected and significant
difference

….

~ 2 times

4, Predict more transparency than that observed experimentally in central collisions
at intermediate energy

Insufficient production of fragments in the mid
-
rapidity region

R.Nebauer
,
J.Aichelin
, NPA658(1999)

All the problems are not fixed by only changing the
EOS or in
-
medium XS in previous studies!

It naturally require an improvements on the
cluster recognition methods in the transport
models !

R
nn
0=
R
np
0=
R
pp
0=
R
0



3.5 fm

In Regular MST, nucleons with relative distance of coordinate
and momentum of
|
r
i


r
j

|<R
0

and |p
i


p
j

|<P
0

belong to a
fragment.

roughly be in the range of nucleon
-
nucleon interaction
,
and is
determined by fitting the global experimental data,
such as the IMF multiplicities.

However, previous algorithms do not address
the lack of
isospin

dependence in cluster recognition
, which is the main focus of this work.

Failed in details, such as problem 2) and 3)

Isospin

dependent MST

Rnn
0=
Rnp
0~
6.0 fm

Rpp0~3.0fm

Physical point of view:

1.
properties of neutron
-
rich nuclei, such as neutron skin or
neutron halo effect

2.

long
-
range repulsive Coulomb force between protons in
the cluster

3.

hints from neutron
-
rich heavy ion collisions

11Be

Isospin

dependent cluster recognition methods (
iso
-
MST)

Effect of
iso
-
MST on observables

112,124Sn+112,124Sn, b=2fm,
E_beam
=50AMeV

Reaction systems:

Transport models:

ImQMD05

Improved Quantum Molecular Dynamics model (ImQMD05)



the mean fields acting on nucleon
wavepackets

are derived
from

Skyrme

potential energy density functional

potential energy density functional:


EOS

H=
T+U+U_coul

Surface symmetry energy term

Detail of code: Zhang, et alPR
C71

(05) 024604, PR
C74

(06) 014602, PRC75,034615(07)., PL
B664

(08) 145,
PRC85(2012)024602



Isospin dependent nucleon
-
nucleon cross sections


are adopted, the medium corrections are

free
np
med
np





)
/
1
(
0


free
pp
nn
med
pp
nn
,
0
,
)
/
1
(








d
d
free
pp
nn
np
/
,
)
(
,


Cugnon, et al., Nucl.Instr.Meth.Phys. B111, 215(1996)



depend on the beam energy

Well reproduce the data of charge distribution, direct flow, elliptical flow and stopping
power (30
-
400AMeV)

Effect of
iso
-
MST on observables

YXZhang
,
Zhuxia

Li,
Chengshuang

Zhou,
MBTsang
, PRC85,051602(2012)(R)

1, Charge distribution

1, obviously reduce the yield of Z=1
part.


2, enhance the yield of fragments with
Z>=2.


3, strongly enhance the yield of heavy
fragments. (Z>=12)

Sn+Sn
,
Ebeam
=50AMeV

Rapidity distribution for
n,p

2,
n, p, t, He3
production

1, reduce the yield of both neutron and protons,


2, enhancement of the n/p, t/He3 ratios appears at mid
-
rapidity and lower kinetic
energy

n/p, DR(n/p), t/He3, DR(t/He3)

3, enhancement of the DR(n/p), DR(t/He3) ratios appears at mid
-
rapidity and lower kinetic energy

Zhang, et.al., PLB2008

3,isotope distribution and
isoscaling

isoscaling

YXZhang
,
Zhuxia

Li,
Chengshuang

Zhou,
MBTsang
, PRC85,051602(2012)(R)

1, enhance the production of the neutron
-
rich isotope, especially for neutron
-
rich
reaction system


2, predict larger values of
isoscaling

parameter, alpha

4, Effect of
iso
-
MST on equilibrium

MST:
Vartl
=0.58

Iso
-
MST:
Vartl
=0.62

YXZhang
,
Zhuxia

Li,
Chengshuang

Zhou,
MBTsang
, PRC85,051602(2012)(R)



the equilibrium or stopping power of the system

also depends on the detailed
description of cluster formation implemented in the transport models
as well as on
the mean field and the in
-
medium NN cross section.

Conclusion

1.
we introduce a
phenomenological
isospin

dependence in the description of
cluster formation in transport models by adopting different
R0 values for pp,
nn
, and
np
, Rpp
0= 3 fm and
Rnn
0=
Rnp
0= 6 fm.


2.
The
isospin
-
dependent minimum spanning tree method show
suppression of
Z = 1 particles and enhancement of fragments,
especially for heavier
fragments with
Z >=12.



3.
Furthermore,
we find
enhanced production of neutron
-
rich isotopes at mid
-
rapidity.

Consequently,
isospin
-
sensitive observables, such as the double
ratios,
DR(t/3He), and
isoscaling

parameter α increase to larger values.


4.
The widths of the longitudinal
and transverse rapidity distributions of
Z = 1

6
particles also
change,
the degree of equilibrium become higher.


5.
The
isospin

dependence of the cluster recognition can be
easily

implemented
and should be included in nuclear transport models.

Thanks