1. Francesco Noferini Bologna University Erice, Italy 31 st August 2006 Two-particle correlations: from RHIC to LHC

2. 2 STAR results on two particle correlations Phys.Rev.Lett.91:072304,2003 [STAR Collaboration] arXiv:nucl-ex/0604018 Increasing the value of the p T trigger cut the back-to-back correlation is visible again. In this p T range, only for central AA collisions, the back-to-back correlation is suppressed. 4 < p T trig < 6 GeV/c 2 GeV/c < p T corr < p T trig

3. 3 Geometry of collision L1L1 L2L2 Properties: L 1 ≠L 2 Strong dependence on the impact parameter (b) Quenching (energy loss in the medium) gives a ΔE i increasing with L i Jet pair production

4. 4 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.

5. 5 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 L 2 –dependence.

6. 6 Quenching Mechanism The quenching mechanism proposed by Salgado & Wiedeman (developed in the BDMPS–Z–SW framework) is parameterized as follows (Quenching Weight Model): 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 L 2 = path length squared through the medium.

7. 7 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: If we define: Then: depends on b All information Nuclear Geometry Procedure is described in ref. A.Morsch J.Phys. G31 (2005) s597.

8. 8 Dependence of q from centrality * 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 with q ~ 14 GeV 2 /fm ^ ^

9. 9 Simulation strategy

10. 10 PYTHIA simulation @ 200 GeV eff in central collisions ~ 5 GeV 2 /fm Suppression vs. centrality qualitatively described by the model (factor 5 suppression wrt peripheral collisions, although the away side peak does not disappear completely). ^

11. 11 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 N trig = 2700) confirm this effect. Background doesn’t correspond exactly to RHIC data but the Monte Carlo is not tuned yet.

12. 12 HIJING simulation @ 5.5 TeV Simulation at LHC energy with the same quenching strength tuned on RHIC data for two choices of p T -cut.

13. 13 Associated particles 8 < p T trig < 15 GeV/c The behaviour of the near side and away side correlation as a function of the p T of the associated particles with 8 < p T trig < 15 GeV/c. With this choice of cuts the simulations at LHC energy with the same quenching strength used for √s NN = 200 GeV show a good signal. this kind of selections may be adequate to extend the study of the jet–medium interaction at high–p T.

14. 14 HIJING simulation @ 5.5 TeV 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

15. 15 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 p T.