Erice, Italy31st August 2006

Francesco NoferiniBologna University

Two-particle Two-particle correlations: correlations: from RHIC to from RHIC to

LHCLHC

2

STAR results on two STAR results on two particle correlationsparticle correlations

Phys.Rev.Lett.91:072304,2003

[STAR Collaboration]arXiv:nucl-ex/0604018

Increasing the value of the pT trigger cut the back-to-back correlation is visible again.

In this pT range, only for central AA collisions, the back-to-back correlation is suppressed.

4 < pTtrig < 6 GeV/c

2 GeV/c < pTcorr < pT

trig

3

Geometry of collisionGeometry of collision

L1

L2

Properties:

• L1≠L2

• Strong dependence on the impact parameter (b)

• Quenching (energy loss in the medium) gives a ΔEi increasing with Li

Jet pair production

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Standard HIJING results at Standard HIJING results at RHIC energyRHIC 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 modelThe 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 MechanismQuenching MechanismThe quenching mechanism proposed by Salgado & Wiedeman (developed in the BDMPS–Z–SW framework) is parameterized as follows (Quenching Weight Model):

2/Lq̂ω 2c

λ/kq̂medium

2t

3Lq̂2

1R

characteristic scale for the radiation

The emission spectrum of gluons depends only on

c and R :

C.A. Salgado and U.A. Wiedemann, Phys. Rev. D 588, 303 (2000)

The average energy loss in this prediction is proportional to L2 = path length squared through the medium.

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Quenching in the Monte Quenching in the Monte CarloCarlo

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:

0 BAeff ξd)b;ξ(TTkLq̂

0

nn ξd)b;ξ(q̂ξIIf we define:

Then: 1c Iω 021 I/I2R

01 I/I2L )2/(ˆ 120 IIq

depends on b

All information

Nuclear Geometry

Procedure is described in ref. A.Morsch J.Phys. G31 (2005) s597.

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Dependence of q from Dependence of q from centralitycentrality

*A. Dainese, C. Loizides and G. Paic, Eur. Phys. J. C 38, 461-474 (2005)

Dainese-Loizides-Paic results show* that a good agreement with RHIC data is reached withq ~ 14 GeV2/fm^

^

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Simulation strategySimulation strategy

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PYTHIA simulation @ 200 PYTHIA simulation @ 200 GeVGeV

<q>eff in central collisions ~ 5 GeV2/fm

Suppression vs. centrality qualitatively described by the model (factor 5 suppression wrt peripheral collisions, although the away side peak does not disappear completely).

^

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HIJING results @ 200 GeVHIJING results @ 200 GeV

HIJING single collisionHIJING 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 HIJING simulation @ 5.5 TeVTeV

Simulation at LHC energy with the same quenching strength tuned on RHIC data for two choices of pT-cut.

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Associated particlesAssociated particles8 < pT

trig < 15 GeV/c

The behaviour of the near side and away side

correlation as a function of the pT of the associated

particles with8 < pT

trig < 15 GeV/c.

With this choice of cuts the simulations at LHC energywith 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 .

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HIJING simulation @ 5.5 HIJING simulation @ 5.5 TeVTeV

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.

Suppression due to quenching in PbPb central

collisions

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ConclusionsConclusions

• 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.