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Two-particle
correlations: from
RHIC to LHC
Francesco Noferini
Bologna University
Erice, Italy
31st August 2006
STAR results on two particle
correlations
Phys.Rev.Lett.91:072304,2003
4 < pTtrig < 6 GeV/c
2 GeV/c < pTcorr < pTtrig
In this pT range,
only for central
AA collisions, the
back-to-back
correlation is
suppressed.
Increasing the
value of the pT
trigger cut the
back-to-back
correlation is
visible again.
[STAR Collaboration]
arXiv:nucl-ex/0604018
2
Geometry of collision
Jet pair production
Properties:
L1
• L1≠L2
• Strong dependence
on the impact
parameter (b)
L2
• Quenching
(energy loss in the
medium) gives a ΔEi
increasing with Li
3
Standard HIJING results at RHIC
energy
Results for two particle
correlation obtained from
HIJING with the quenching
model implemented in the
original code.
The partial suppression
affects both the peaks
(near correlation, back
correlation) so it is not fine
when compared with RHIC
data.
Energy loss in HIJING
quenching model is
proportional to L = path
length through the
medium.
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The quenching model
The BDMPS–Z quenching model is based on the idea that a
fast parton strongly interacts with the medium formed in
the collision, loosing energy via gluonic bremsstrahlung.
The formation of a deconfined medium (the so called Quark
Gluon Plasma) would produce a very different kind of
quenching compared to the purely hadronic matter case.
The main difference is due to the fact that, in the former case,
also the interaction of the radiated gluons with the medium
has to be considered.
In particular, the probability for a parton to loose a given
energy scales with the square of the path length L instead
of linearly because the strength of the energy loss is
assumed to be proportional to the number of scatterings
( L) and to the formation probability ( L) which makes a
L2–dependence.
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Quenching Mechanism
The quenching mechanism proposed by Salgado & Wiedeman
(developed in the BDMPS–Z–SW framework) is parameterized as
follows (Quenching Weight Model):
ωc  q̂L / 2
2
q̂  k
2
t medium
characteristic scale for the radiation
/λ
The emission spectrum of gluons depends only on
c and R :
1 3
R  q̂L
2
The average energy loss in this
prediction is proportional to
L2 = path length squared through
the medium.
C.A. Salgado and U.A. Wiedemann, Phys. Rev. D 588, 303 (2000)
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Quenching in the Monte Carlo
The Quenching Weight based on the Salgado-Wiedemann model,
takes into account the Nuclear Geometry.
An effective transport coefficient is calculated starting from the

formula:
q̂eff L   k  TATB ( ξ ; b )dξ
If we define:

0
n
I n   ξ q̂( ξ ; b )dξ
depends on b
Nuclear Geometry
0
Then:
ωc  I 1
R  2 I 12 / I 0
L  2 I1 / I0
qˆ  I 02 /( 2 I1 )
All information
Procedure is described in ref. A.Morsch J.Phys. G31 (2005) s597.
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Dependence of q from centrality
^
Dainese-Loizides-Paic
results show* that a good
agreement with RHIC data
is reached with
^
q ~ 14 GeV2/fm
*A. Dainese, C. Loizides and G. Paic, Eur. Phys. J. C 38, 461-474 (2005) 8
Simulation strategy
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PYTHIA simulation @ 200 GeV
^
2
<q>
eff in central collisions ~ 5 GeV /fm
Suppression vs. centrality
qualitatively described by the model
(factor 5 suppression wrt peripheral
collisions, although the away side
peak does not disappear
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completely).
HIJING results @ 200 GeV
HIJING single collision
HIJING full event
Like in PYTHIA+quench. simulations the back side correlation is
strongly suppressed.
The full HIJING+quench. simulations (preliminary results Ntrig = 2700)
confirm this effect. Background doesn’t correspond exactly to RHIC
data but the Monte Carlo is not tuned yet.
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HIJING simulation @ 5.5 TeV
Simulation at LHC energy with the same quenching strength tuned on
RHIC data for two choices of pT-cut.
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Associated particles
8 < pTtrig < 15 GeV/c
With this choice of cuts the
simulations at LHC energy
with the same quenching
strength used for √sNN = 200
GeV show a good signal.
this kind of selections
may be adequate to extend the
study of the jet–medium
interaction at high–pT .
The behaviour of the near
side and away side
correlation as a function of
the pT of the associated
particles with
8 < pTtrig < 15 GeV/c.
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HIJING simulation @ 5.5 TeV
Suppression due
to quenching in
PbPb central
collisions
By comparing the results of pp collisions obtained with the PYTHIA
generator with and without quenching
the back–to–back
correlation is meaningfully suppressed in central PbPb collisions when
quenching effects are taken into account.
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Conclusions
• The present Quenching Weight Model
implementation in PYTHIA/HIJING generators
seems to work in the kinematical regions
investigated @ RHIC and to be more adequate
than the standard quenching simulated in the
HIJING original code;
• With this model it is possible to study the
scenario that could show up @ LHC for the
observables presented herein, extending the
analysis at higher pT.
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