Two particle correlations: from RHIC to LHC

1. Two particle correlations: from RHIC to LHC Francesco Noferini HOT QUARK 2006 Bologna University INFN – sez. Bologna ALICE-TOF Tuesday, May 16th Villasimius (Italy)

2. Francesco Noferini OUTLINE • Results from RHIC on two particle correlation studies; • Quenching effect interpretation; • Monte Carlo Simulation of quenching effects (pythia, hijing); • Prediction at LHC; • Conclusions.

3. Francesco Noferini 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

4. Francesco Noferini Geometry of collision Jet pair production • Properties: • L1≠L2 • Strong dependence on the impact parameter(b) • ΔEi enhancement with Li L1 L2

5. Francesco Noferini Quenching Mechanism The quenching mechanism proposed by Wiedeman & Salgado is parameterized as follows (Quenching Weight): characteristic scale for the radiation mean squared momentum for unity length The spectrum emission of gluons depends only on c and R : The avarage 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)

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

7. Francesco Noferini 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. ALICE PPR Vol. II Chapter 6 *z = fraction of energy loss

8. Francesco Noferini Some results expected from Jet Quenching N. Borghini and U. A. Wiedemann, hep-ph/0506218 & ALICE PPR Vol. II, Chapter 6

9. Francesco Noferini Dependence of q from centrality ^ Dainese, Loizides and Paic results show* that a good agreement with RHIC data is reached with q ~ 14 GeV2/fm for: ^ *A. Dainese, C. Loizides and G. Paic,Eur. Phys. J. C 38, 461-474 (2005)

10. Francesco Noferini 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.

11. Francesco Noferini Simulation strategy

12. Francesco Noferini 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).

13. Francesco Noferini Some parameters in HIJING simulation • Ngluon (emitted gluons) = 1 / (1-z); • kTlead leading parton momentum fromthe medium = ; • kTrad of the radiated gluons = kTlead/sqrt(Ngluon); • Max. fraction of energy loss = 0.7, Ngluonmax = 4. ^ <q>eff in central collisions ~ 14 GeV2/fm z = fraction of energy loss

14. Francesco Noferini 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.

15. Francesco Noferini HIJING results @ 200 GeV HIJING single collision HIJING 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.

16. Francesco Noferini What happens at higher trigger pT? HIJING single collision Increasing the value of the pT trigger cut the back-to-back correlation is clearly visible again as in RHIC data. Ntrig = 4493

17. Francesco Noferini with radiation effects (WR) without radiation effects (WoR) Radiation effects at low pT HIJING single collision In the kinematic region of low pT, for central collisions, the contribution to back-to-back correlations could be due to the radiated gluons. Ntrig = 1713(WR)/150(WoR)

18. Francesco Noferini HIJING simulation @ 5.5 TeV 5.5 TeV 8 < pTtrig < 15 4 < pTcorr < 6 5.5 TeV 8 < pTtrig < 15 4 < pTcorr < 6 |η| < 1 |η| < 1 HIJING full event HIJING single collision 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.

19. Francesco Noferini 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.

20. Francesco Noferini Backup

21. Francesco Noferini Some results expected from Jet Quenching -II Energy distribution around a jet axis for a jet of 100 GeV. Background: Quenched No quenched A.Morsch J.Phys. G31 (2005) s597.

22. Francesco Noferini Other plots from STAR 8 < pTtrig < 15 GeV/c [STAR Collaboration] arXiv:nucl-ex/0604018

23. Francesco Noferini [PHENIX Collaboration] arXiv:nucl-ex/0511044 At lower value of pT some new effects come out.

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

25. Francesco Noferini Quenching Weights in HIJING Monte Carlo • Call to HIJING code: • Generation of partons scattering HIJING Monte Carlo Fortran • Quenching of the hard partons: • call to Quenching Weight class THijing class AliQuenchingWeights AliFastGlauber classes • Call to HIJING code: • Fragmentation

26. Francesco Noferini Pythia without quenching pp In the Transverse Plane (x,y) Rate: 60k events over 10M nucleon-nucleon collisions

27. Francesco Noferini Pythia + Quenching  L (HIJING) Eloss = 2 GeV/fm b = 0 fm AuAu Rate: 5k events over 10M nucleon-nucleon collisions

28. Francesco Noferini Pythia + Quenching  L2 ^ q = 1.5 GeV2/fm b = 0 fm AuAu Rate: 6k events over 10M nucleon-nucleon collisions

29. Francesco Noferini Suppression vs. Impact Parameter (b) Quenching  L Quenching  L2 [suppression ΔΦ = π]

30. Francesco Noferini Region of jet production r External Region in Central Collisions b = 0 fm AuAu