Anisotropic flow of charged particles

1. Anisotropic flow of charged particles Ante Bilandzic (Nikhef)for the ALICE collaborationAnnecy,23.05.2011

2. ALICE For anisotropic flow analysis of all charged particles: TPC and ITS See Jurgen’s talk

3. Non-central collisions • Key point: If the created system has interactions the original anisotropies in coordinate space will transfer into the anisotropies in momentum space, which are quantified via anisotropic flow harmonics vn => our observables • Flow harmonics vn are sensitive probe of the properties of the created system • v2 (elliptic flow) is the dominant harmonic in non-central collisions

4. 1st look at the LHC data Phys. Rev. Lett. 105, 252302 (2010) Elliptic flow increases by ~ 30% when compared to RHIC energies

5. 1st look at the LHC data Phys. Rev. Lett. 105, 252302 (2010) pt dependence of elliptic flow at LHC close to the one at RHIC!

6. Analysis outline • Data sample: • ~ 5M PbPb events at 2.76 TeV • Minimum bias trigger • Acceptance -0.8 < eta < 0.8 • Detectors used: • Time Projection Chamber (TPC) • Inner Tracking System (ITS) • Systematic uncertainties: • Non-flow • Multiplicity and flow fluctuations • Centrality determination • Inefficiencies in detectors azimuthal acceptance

7. Q-cumulants (direct cumulants) • For the flow analysis in ALICE the improved version of the original cumulant method (Borghini, Dihn, Ollitrault, PRC 64, 054901 (2001)) was used • Key mathematical result: Analytical expressions for multiparticle azimuthal correlations in terms of Q-vectors evaluated (in general) in different harmonics (M is multiplicity of an event) • All multiparticle cumulants can be expressed analytically in terms of Q-vectors => Q-cumulants (QC) • A.B., R. Snellings, S. Voloshin, “Flow analysis with cumulants: Direct calculations”, PRC 83, 044913 (2011)

8. 1st look at data: Cumulants For centrality determination see talks by Alberica and Constantin Clear nontrivial flow signature (+,-,+,-) in the measured cumulants!

9. Cumulants (closer look at most central) For centralities beyond 5% elliptic flow is already bigger than 3%!

10. Smaller vs wider centrality bins Comparison with results for v2{4} in wider centrality bins => systematic bias due to various fluctuations is negligible at this scale

11. Elliptic flow vs centrality • The difference between 2- and multi-particle estimates is due to fluctuations in the initial geometry • v2{2} might still have some nonflow bias leftover (not in the systematical uncertainty here). With eta gap nonflow is suppressed, not eliminated completely.

12. Charged particle v3 • v3 is not 0 and it develops along its own participant plane • participant plane of v2 is not the participant plane of v3 • v3{4} about two times smaller than v3{2} (predicted by Ollitrault et al in arXiv:1104.4740)

13. Charged particle v3 One of the ways to estimate nonflow

14. Comparison to models (1/2) Comparison to Schenke et al hydro model with Glauber initial conditions (arXiv:1102.0575)

15. Comparison to models (2/2) • More quantitative statement: The magnitude of v2(pt) is described better with eta/s = 0, while for v3(pt) eta/s = 0.08 provides a better description • This model fails to describe well v2 and v3 simultaneously Within this model overall magnitude of v2 and v3 seems to be fine, but the details of pt dependence are not well described

16. Conclusions/Summary • Integrated elliptic flow at LHC energies 30% larger than at RHIC - increase compatible with estimates from hydrodynamic models • pt dependence of elliptic flow at LHC energies close to the one at RHIC energies => increase of 30% in integrated flow is due to increase in radial flow • Elliptic flow for centralities beyond 5% is already larger than 3% (much larger than the value of any other measured harmonic in any other centrality) • v2{4} peaks at ~ 8.5% in midcentral collisions at LHC • Triangular flow is significant and its centrality dependence is in agreement with hydro model predictions • Each harmonic has its own participant plane along which it develops • Participant plane of v2 is not the participant plane of v3 • v3{4} about two times smaller than v3{2} => v3 originates predominantly from event by event fluctuations of the initial spatial geometry (Ollitrault et al, arXiv:1104.4740) • Hydrodynamic prediction with Glauber initial conditions (Schenke et al, arXiv:1102.0575) does not describe well simultaneously the details of pt dependence of v2 and v3

17. Thanks!