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Production of meson, baryon
and light nuclei
in Au+Au collisions at RHIC
Haidong Liu
Univ. of Science & Technology of China
Outline
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Motivation and introductions
Detectors and techniques
Results (RHIC run 4 AuAu 200 GeV)
Conclusions & Discussions
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Motivations
&
Introductions
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Heavy-ion collisions at RHIC
Time
freeze-out
QGP and
hydrodynamic expansion
initial state
pre-equilibrium
(high Q2 interactions)
hadronization
Physics:
1) Parton distributions in nuclei
2) Initial conditions of the collision
3) A new state of matter – Quark-Gluon Plasma and its properties
4) Hadronization and freeze-out
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Particles production
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Pions and protons production
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Low pT – hydrodynamic
Intermediate pT – partonic coalescence
High pT – jet fragmentation
Light nuclei production

Final-state coalescence
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The success of hydrodynamic
STAR PRC.72 (2005) 014904
At low pT, hydrodynamical models
successfully reproduce the spectra and v2
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Coalescence at intermediate pT
STAR PRC.72 (2005) 014904
Coalescence
fragmenting parton:
ph = z p, z<1
recombining partons:
p1+p2=ph
NQ scaling of v2 is a strong evidence
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Coalescence at intermediate pT
STAR: Nucl. Phys. A 757 (2005) 102
The difference is not sensitive to the mass of the
hadron, but rather depends on the number of
valence quarks contained within it.
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High pT – from pp to AuAu
We understand pp collisions

p+p collisions



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Parton Distribution Function (derived
from e-h scattering)
pQCD (parton-parton interaction cross
section calculation)
Fragmentation Function (derived from
e+e- collisions)
Au+Au collisions

pp collisions + Nuclear effect
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Jet fragmentation in pp collisions
1.
2.
PLB 637 (2006) 161
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Improved FF reasonably reproduces data
pbar/p ~ 0.2 at RHIC, <<0.1 at low energy
pbar dominated by gluon FF
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Jet quenching in Au+Au
Significant suppression of inclusive
charged hadron is observed in central
Au+Au collisions:
Fragmentation+parton energy loss
STAR: Nucl. Phys. A 757 (2005) 102
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Parton energy loss in HIJING
HIJING calculation
Study the PID spectra and
pbar/p ratios can help to
further understand how the
g/q jets interact with the
medium
X.N. Wang: PRC58(2321)1998.
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pQCD: Color charge and flavor
dependence of parton energy loss
S. Wicks et al., NPA 784(2007)426
dE/dx(c/b)<dE/dx(uds)< dE/dx(g)
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The roles of energetic parton --- source of
the meson/baryon production
(1)In LEP e+e- experiment,
identified charged particle spectra
can be measured from 2 kinds of
hadronic Z decays: quark jets and
gluon jets (DELPHI EPJC 17 (2000) 207)
(2) The anti-baryon phase space
density can be accessed by
measuring dbar/pbar
1 dN / dy d
f y 
3
62p  dN / dy  p
F.Q. Wang, N. Xu, PRC 61 021904 (2000)
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


Different mechanisms govern hadron
formation in the different kinematic region
Different hadron species may have different
sources
Those sources (g/q) may have different
behavior when propagating the medium
To study those behaviors,
PID in large pT range is required!
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Light nuclei formation
– final-state coalescence
“De-confinement” Hadronization
Initial
Collisions
“QGP”
pnd
Time
Late stage
scattering
Chemical
Freeze-out
Thermal
Freeze-out
p  p  n3He
Due to the small binding energy, light nuclei cannot survive before
thermal freeze-out. Therefore, light nuclei production and their elliptic
flow are sensitive to the freeze-out conditions, such as temperature,
particle density, local correlation volume and collective motion.
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Final-state Coalescence
•Coalescence parameters BA
Z


dN p  
dN p 
dN n 
dN A



 En 3   BA E p 3 
E A 3  BA E p 3
 d P 
 d P 
d PA
d Pn 
p  
p 


1
BA   
V 
N
A
p
p
 p A / A
A1
R. Scheibl, U. Heinz, PRC 59 1585 (1999)
•Light nuclei v2 – atomic mass number (A) scaling?
(consequence of the final-state coalescence)
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Detectors
&
Techniques
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STAR detectors: TPC & TOF
Time Projection Chamber
1.
2.
Tracking
Ionization energy loss (dE/dx)
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A new technology (TOF) ---Multi-gap Resistive Plate Chamber
1.
2.
Good timing resolution (<100ps)
Two trays (TOFr+TOFp) for run 4,
acceptance~0.01, 120 trays (TOFr)
in the future
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PID – Hadrons
Low & intermediate pT
2.5<pT<3.0
High performance of time resolution
PID up to 12 GeV/c
TPC
High pT
Relativistic rising of dE/dx
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Light Nuclei Identification
2  pT  6 GeV/c
3
Z  Log(
He( 3 He)
dEdx measure
dEdx exp
)
TOF
PID Range (GeV/c):
d:
1  pT  4
d:
0.2  p T  3
3
0.7  pT  1 GeV/c
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He  3 He :
2  pT  6
2.5  pT  3 GeV/c
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Feed-down correction for
(anti-)protons
Method 1: Primordial protons and the protons come
from weak decays have different DCA distribution
Primordial (MC)
From decay (MC)
Method 2: From the
measurements of  and 
spectra, we can estimate the
FD contribution
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Results
(Au+Au 200 GeV)
Pion and proton spectra: STAR Phys. Rev. Lett. 97 (2006) 152301
Nuclei spectra and v2: QM06 proceeding, J. Phys. G: Nucl. Part.
Phys. 34 (2007) S1087-S1091
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Pion & proton spectra
STAR Collaboration PRL 97 (2006) 152301
PAs: O. Barannikova, H. Liu, L. Ruan and Z. Xu
PID up to 12 GeV/c
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Nuclear Modification factor
In central Au+Au collisions:


pT
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At 1.5<pT<7 GeV/c,
RCP(p+pbar) > RCP(p) ,
RCP(p+pbar) shows obvious
decreasing trend.
At 4<pT<12 GeV/c, both p and
p are strongly suppressed.
They approach to each other
at about 0.3
Curve:I. Vitev, PLB 639 (2006) 38.
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Anti-particle to particle ratios
1.
2.
p-/p+ are consistent with flat at unity in all pT, no significant centrality
dependence.
pbar/p ratio: no significant centrality dependence, parton energy loss
underpredicts the ratios (X.N. Wang, PRC 58 (2321) 1998).
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Proton over pion ratios
1.
2.
3.
The p(pbar)/p ratios in Au+Au collisions show strong centrality dependence.
In central Au+Au collisions, the p(pbar)/p ratios reach maximum value at
pT~2-3 GeV/c, approach the corresponding ratios in p+p, d+Au collisions at
pT>5 GeV/c.
In general, parton energy loss models underpredict p/p ratios.
R.J. Fries, et al., Phys. Rev. Lett. 90 202303 (2003); R. C. Hwa, et al., Phys. Rev. C 70, 024905 (2004);
DELPHI Collaboration, Eur. Phy. J. C 5, 585 (1998), Eur. Phy. J. C 17, 207 (2000).
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Light Nuclei Spectra
Deuteron
Helium-3
QM06 proceeding: J. Phys. G: Nucl.Part. Phys. 34 (2007) S1087-S1091
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Coalescence Parameters B2 & B3
(anti-)proton spectra: STAR Phys. Rev. Lett. 97, 152301 (2006)

dN p 
dN A

E A 3  BA E p 3 
 d P 
d PA
p 

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A
1
BA   
V 
A1
•B2 & sqrt(B3) are consistent
•Strong centrality dependence
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Coalescence Parameters B2 & B3
HBT parameters: STAR Phys. Rev. C71 (2005) 044906
2
V f  2p  Rlong Rside
3
2

dN p 
dN
E A 3 A  BA  E p 3 
 d P 
d PA
p 

A
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Assuming a Gaussian shape in all 3 dimensions
1
BA   
V 
R. Scheibl et al.Phys.Rev.C59 (1999)1585
A1
•Compare to pion HBT results
•Beam energy dependence
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Light Nuclei v2
minBias
•This is the 1st helium-3 v2
measurement at RHIC
Scaled by A
•Deuterons v2 follows A scaling
within error bars
•Helium-3 v2 seems deviating from
A scaling at higher pT (need more
statistics)
Baryon v2 -- X.Dong et al, Phys. Lett. B597 (2004) 328-332
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Low pT d v2
dbar centrality bins: 0~12%, 10~20%, 20~40%, 40~80%
pbar v2: STAR Phys. Rev. C72 (2005) 014904
BW parameters:
F. Retiere, M. Lisa, Phys.Rev. C70 (2004) 044907
The 1st observation of negative v2 at RHIC
No model can readily reproduce the data
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Accessing anti-baryon density by d / p
&
Source of anti-baryon production
H. Liu & Z. Xu, nucl-ex/0610035
Submitted to PLB
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Anti-baryon Phase Space Density
STAR preliminary
1 dN / dy d
f y 
3
62p  dN / dy  p
F.Q. Wang, N. Xu, PRC 61 021904 (2000)
In nucleus+nuclues collisions, the anti-baryon density increases
with beam energy and reaches a plateau above ISR beam
energy regardless the beam species (pp, pA, AA).
It can be fitted to a thermal model :
d / p  exp  mB / T  p / p
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Anti-baryon Phase Space Density
STAR preliminary
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ARGUS e+esqrt(s)=9.86() ggg
sqrt(s)=10
q+qbar
Haidong Liu
high
low
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Anti-baryon Phase Space Density
STAR preliminary
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ARGUS e+esqrt(s)=9.86() ggg
sqrt(s)=10
q+qbar
high
low
ALEPH(LEP) e+esqrt(s)=91(Z) q+qbar
low
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Anti-baryon Phase Space Density
STAR preliminary
ARGUS e+esqrt(s)=9.86() ggg
sqrt(s)=10
q+qbar
high
low
ALEPH(LEP) e+esqrt(s)=91(Z) q+qbar
low
AGS, SPS, RHIC, ISR, Tevatron
nucleus+nucleus (AA, pA, pp, p+pbar)
sqrt(sNN)>50 q+g, qbar+g high
sqrt(sNN)<20 q+g, q+q
low
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Anti-baryon Phase Space Density
STAR preliminary
ARGUS e+esqrt(s)=9.86() ggg
sqrt(s)=10
q+qbar
high
low
ALEPH(LEP) e+esqrt(s)=91(Z) q+qbar
low
AGS, SPS, RHIC, ISR, Tevatron
nucleus+nucleus (AA, pA, pp, p+pbar)
sqrt(sNN)>50 q+g, qbar+g high
sqrt(sNN)<20 q+g, q+q
low
H1(HERA) p
Wp =200
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qqbar+g
high
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Anti-baryon Phase Space Density
STAR preliminary
In e+e-, the density through qqbar
processes is a factor of strong
coupling constant less than that
through ggg processes (s=0.12)
s
(q+qbar->q+qbar+g)
ARGUS e+esqrt(s)=9.86() ggg
sqrt(s)=10
q+qbar
high
low
ALEPH(LEP) e+esqrt(s)=91(Z) q+qbar
low
AGS, SPS, RHIC, ISR, Tevatron
nucleus+nucleus (AA, pA, pp, p+pbar)
sqrt(sNN)>50 q+g, qbar+g high
sqrt(sNN)<20 q+g, q+q
low
H1(HERA) p
Wp =200
qqbar+g
high
H. Liu, Z. Xu nucl-ex/0610035
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Where does (anti-)baryon
come from?
Conclusions:
STAR preliminary
(1) Collisions which contain ggg,
qbar+g or qqbar+g processes have
higher anti-baryon phase space density
(2) Processes q+qbar create few antibaryons
(3) Processes q+g create few antibaryons at low energy – energy too
low?
In short, anti-baryon phase space density from collisions
involving a gluon is much higher than those without gluons
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Conclusions
&
Discussions
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B/M enhancement at intermediate pT
STAR Nucl-ex/0601042
The relative baryon enhancement is clearly observed in the
p/pi ratios at intermediate pT, the similar behavior can also
be seen in the /Ks0 ratios. At the same pT region, the NQ
scaling of v2 has also been observed. This can be explained
by the parton coalescence phenomena.
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Freeze-out volumes
•B2 and B3 have strong centrality dependence, the system
has larger freeze-out volumes in more central collisions.
•B2 and sqrt(B3) have similar values in different centrality
collisions, which indicates that the deuteron and helium-3
have similar freeze-out volume.
•B2 has little beam energy dependence when
sqrt(sNN)>20 GeV, which indicates that the freeze-out
volume won’t change with the beam energy.
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Light nuclei v2
•At intermediate pT, deuteron v2 follows A scaling
within errors while helium-3 v2 seems deviates from
this scaling, we need more statistics to draw further
conclusion.
•At low pT, the dbar v2 is found to be negative. The BW
model, which includes large radial flow scenario, also
shows a negative flow prediction. But the BW model
fails to reproduce our data since there is only mass
input for light nuclei.
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Color charge and flavor dependence
of parton energy loss
High pT Rcp measurements: p, p(pbar), e, , p0
Nucl-ex/0607012
pT
PRL 96 (2006) 202301
Rcp(RAA)~0.2 for all these particles!
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Color charge and flavor dependence
of parton energy loss
pQCD calculations
The partonic source:
•p, , p0 – light quarks
•p(pbar) – glouns
•e – heavy quarks
S. Wicks et al., NPA 784(2007)426
Rcp(RAA)~0.2 for all these particles!
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???
dE/dx(c/b)<dE/dx(uds)< dE/dx(g)
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Physics possible:
g/q jets conversion in the medium
Compton-like scattering:
W. Liu et al., nucl-th/0607047
hard q(qbar)
+
soft g
soft q(qbar)
+
hard g
A much larger cross-section is needed to explain our data
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The future – a good time for discovery
E864 Phys. Rev. Lett. 85 (2000) 2685
Inv. Yield~
Anti-3He : dbar : pbar
1 : 1K : 1M
In the RHIC upcoming high statistics
AuAu runs, with STAR large
acceptance detector TPC/TOF, we
should try to search for anti-, which
has never been observed before.
And, there is also possible to discover
Antihypernucleus
p    n  3 H
STAR Phys. Rev. Lett. 87 (2001) 262301
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3

Haidong Liu
H  3 He  p  Thanks!
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