Two particle correlations: from RHIC to LHC

Two particle correlations: from RHIC to LHC

Francesco NoferiniBologna University

INFN – sez. BolognaALICE-TOF

Tuesday, May 16thVillasimius (Italy)

HOT QUARK 2006

Tuesday, May 16th Francesco Noferini

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OUTLINE

Results from RHIC on two particle correlation studies;

Quenching effect interpretation; Monte Carlo Simulation of quenching

effects (pythia, hijing); Prediction at LHC; Conclusions.


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STAR results on two particle correlationsPhys.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 <

pTtrig


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Geometry of collision

L1

L2

Properties:

• L1≠L2

• Strong dependence on the impact parameter (b)

• ΔEi enhancement with Li

Jet pair production


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Quenching Mechanism

The quenching mechanism proposed by Wiedeman & Salgado is parameterized as follows (Quenching Weight):

2/Lq̂ω 2c

λ/kq̂medium

2t

3Lq̂2

1R

characteristic scale for the radiation

mean squared momentum for unity length

The spectrum emission of gluons depends only on

c and R :

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

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


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Quenching in Aliroot

Quenching Weight (class in Aliroot framework) based on the Wiedemann-Salgado model, takes in 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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Energy loss and radiated gluons

In the code implementation (AliPythia::Quench method) the number of radiated gluons (multiple soft) are = 1 / (1-z*), in this way the energy of radiated gluons is always lower than that of the final leading parton.

*z = fraction of energy loss

ALICE PPR Vol. IIChapter 6


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Some results expected from Jet Quenching

N. Borghini and U. A. Wiedemann, hep-ph/0506218 &ALICE PPR Vol. II, Chapter 6


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Dependence of q from centrality

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

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

^


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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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Simulation strategy


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PYTHIA simulation @ 200 GeV<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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Some parameters in HIJING simulation

Ngluon (emitted gluons) = 1 / (1-z); kT

lead leading parton momentum from the medium = ;

kTrad of the radiated gluons

= kTlead/sqrt(Ngluon);

Max. fraction of energy loss = 0.7,Ngluon

max = 4.

Lq̂

<q>eff in central collisions ~ 14 GeV2/fm ^ z = fraction of energy loss


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Hijing also without background For statistics reasons some simulations are

obtained for events with a single Nucleon-Nucleon collision.However, the quenching effect is simulated assuming the Glauber geometry and the Quenching Weight scheme (as for full simulations).

Results with hijing are consistent with those from PYTHIA. The advantage in using HIJING is that is possible to simulate signal and background together.


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HIJING results @ 200 GeV

HIJING single collisionHIJING full event

Like in PYTHIA+quech. simulations the back side correlation is strongly suppressed.

The full HIJING+quench. simulations (preliminary results Ntrig = 2700) confirm this effect. Background doesn’t correspond exaclty to RHIC data but the Monte Carlo is not tuned yet.


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What happens at higher trigger pT?

Ntrig = 4493

HIJING single collision

Increasing the value of the pT trigger cut the back-to-back correlation is clearly visible again as in RHIC data.


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Radiation effects at low pT

with radiation effects (WR)

without radiation effects (WoR)

Ntrig = 1713(WR)/150(WoR)

In the kinematic region of low pT, for central collisions, the contribution to back-to-back correlations could be due to the radiated gluons.

HIJING single collision


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HIJING simulation @ 5.5 TeV

HIJING single collisionHIJING full event

Simulation at LHC energy with the quenching strength used for the simulation @ 200 GeV shown a clear signal with this choice for pT cut.

It is possible to test the di-hadron correllation for different pT cuts.

5.5 TeV8 < pT

trig < 154 < pT

corr < 6

5.5 TeV8 < pT

trig < 154 < pT

corr < 6|η| < 1 |η| < 1


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Conclusions Quenching Weight implementation in HIJING

seems to work in the kinematical regions investigated @ RHIC and it is more realistic than the standard quenching effect simulated in the HIJING original code;

In this way is possible to study the scenario could happen @ LHC for the observables presented herein;

Implementation of radiated gluons is still not complete but the analysis seems to be sensible at their contribution.


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Backup


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Some results expected from Jet Quenching -II

A.Morsch J.Phys. G31 (2005) s597.

No quenchedQuenched

Energy distribution around a jet axis for a jet of 100 GeV.

Background:

4000d

dNch


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Other plots from STAR

[STAR Collaboration]arXiv:nucl-ex/0604018

8 < pTtrig < 15 GeV/c


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[PHENIX Collaboration]arXiv:nucl-ex/0511044

At lower value of pT some new effects come out.


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Impact Parameter vs. Centrality calculated in Glauber Geometry(class $ALICE/FASTGEN/AliFastGlauber.h)


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Quenching Weights in HIJING Monte Carlo

THijing class

AliQuenchingWeights

AliFastGlauber

classes

HIJING Monte Carlo Fortran

Call to HIJING code:• Generation of partons scattering

Quenching of the hard partons:• call to Quenching Weight class

Call to HIJING code:• Fragmentation


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Pythia without quenching

Rate: 60k events over 10M nucleon-nucleon collisions

In the Transverse Plane (x,y)

GeV200sNN

pp


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Pythia + Quenching L (HIJING)


b = 0 fmGeV200sNN

AuAu

Eloss = 2 GeV/fm


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Pythia + Quenching L2


b = 0 fmGeV200sNN

AuAu

q = 1.5 GeV2/fm^


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Suppression vs. Impact Parameter (b)

Quenching L

Quenching L2

[suppression ΔΦ = π]


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Region of jet production

External Region inCentral Collisions

GeV200sNN

AuAu

b = 0 fm

r

Two particle correlations: from RHIC to LHC

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