Anisotropic flow of charged Anisotropic flow of charged particlesparticles

Ante Bilandzic (Nikhef)Ante Bilandzic (Nikhef)for the ALICE collaborationfor the ALICE collaboration

Annecy,Annecy, 23.05.201123.05.2011

22

ALICEALICE

For anisotropic flow analysis of all charged particles: TPC and ITS For anisotropic flow analysis of all charged particles: TPC and ITS

SeeJurgen’s talk

33

Non-central collisionsNon-central collisions

x

yz

x

yz

x

yz

x

yz

x

yz

x

yz

Key point: If the created system has Key point: If the created system has interactions the original anisotropies in interactions the original anisotropies in coordinate space will transfer into the coordinate space will transfer into the anisotropies in momentum space, which are anisotropies in momentum space, which are quantified via anisotropic flow harmonics vquantified via anisotropic flow harmonics vnn => our observables=> our observables

Flow harmonics vFlow harmonics vnn are sensitive probe of the are sensitive probe of the properties of the created system properties of the created system

vv22 (elliptic flow) is the dominant harmonic in (elliptic flow) is the dominant harmonic in non-central collisions non-central collisions

44

11stst look at the LHC data look at the LHC data

Elliptic flow increases by ~ 30% when compared to RHIC energies

Phys. Rev. Lett. 105, 252302 (2010)Phys. Rev. Lett. 105, 252302 (2010)

55

pt dependence of elliptic flow at LHC closeto the one at RHIC!

11stst look at the LHC data look at the LHC data

Phys. Rev. Lett. 105, Phys. Rev. Lett. 105, 252302 (2010)252302 (2010)

66

Analysis outlineAnalysis outline Data sample:Data sample:

~ 5M PbPb events at 2.76 TeV~ 5M PbPb events at 2.76 TeV Minimum bias triggerMinimum bias trigger Acceptance -0.8 < eta < 0.8Acceptance -0.8 < eta < 0.8

Detectors used: Detectors used: Time Projection Chamber (TPC)Time Projection Chamber (TPC) Inner Tracking System (ITS)Inner Tracking System (ITS)

Systematic uncertainties:Systematic uncertainties: Non-flowNon-flow Multiplicity and flow fluctuationsMultiplicity and flow fluctuations Centrality determinationCentrality determination Inefficiencies in detectors azimuthal acceptance Inefficiencies in detectors azimuthal acceptance

77

QQ-cumulants (direct cumulants)-cumulants (direct cumulants) For the flow analysis in ALICE the improved version of the original cumulant For the flow analysis in ALICE the improved version of the original cumulant

method (Borghini, Dihn, Ollitrault, PRC 64, 054901 (2001)) was usedmethod (Borghini, Dihn, Ollitrault, PRC 64, 054901 (2001)) was used

Key mathematical result: Analytical expressions for multiparticle azimuthal Key mathematical result: Analytical expressions for multiparticle azimuthal correlations in terms of correlations in terms of QQ-vectors evaluated (in general) in different harmonics (-vectors evaluated (in general) in different harmonics (MM is multiplicity of an event)is multiplicity of an event)

All multiparticle cumulants can be expressed analytically in terms of All multiparticle cumulants can be expressed analytically in terms of QQ-vectors => -vectors => QQ-cumulants -cumulants (QC)(QC) A.B., R. Snellings, S. Voloshin, A.B., R. Snellings, S. Voloshin, “Flow analysis with cumulants: Direct calculations”“Flow analysis with cumulants: Direct calculations”,, PRC 83, 044913 (2011) PRC 83, 044913 (2011)

88

11stst look at data: Cumulants look at data: Cumulants

Clear nontrivial flow signature (+,-,+,-) in the measured cumulants!

For centralitydeterminationsee talks byAlberica and Constantin

99

Cumulants (closer look at most central)Cumulants (closer look at most central)

For centralities beyond 5% elliptic flow is already bigger than 3%!

1010

Smaller Smaller vsvs wider centrality bins wider centrality bins

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

1111

Elliptic flow Elliptic flow vsvs centrality 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.

1212

Charged particle vCharged particle v3 3

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)

1313

Charged particle vCharged particle v3 3

One of the ways to estimate nonflow

1414

Comparison to models (1/2)Comparison to models (1/2)

Comparison to Schenke et al hydro model with Glauber initial conditions (arXiv:1102.0575)

1515

Comparison to models (2/2)Comparison to models (2/2)

More quantitative statement: The magnitude of vMore quantitative statement: The magnitude of v22(p(ptt) is described ) is described better with eta/s = 0, while for vbetter with eta/s = 0, while for v33(pt) eta/s = 0.08 provides a better (pt) eta/s = 0.08 provides a better descriptiondescription

This model fails to describe well vThis model fails to describe well v22 and v and v33 simultaneously simultaneously

Within this model overall magnitude of v2 and v3 seems to be fine, but the details of pt dependence are not well described

1616

Conclusions/SummaryConclusions/Summary Integrated elliptic flow at LHC energies 30% larger than at RHIC - Integrated elliptic flow at LHC energies 30% larger than at RHIC -

increase compatible with estimates from hydrodynamic modelsincrease 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% Elliptic flow for centralities beyond 5% is already larger than 3% (much larger than the value of any other measured harmonic in any (much larger than the value of any other measured harmonic in any other centrality)other centrality) vv22{4} peaks at ~ 8.5% in midcentral collisions at LHC{4} peaks at ~ 8.5% in midcentral collisions at LHC

Triangular flow is significant and its centrality dependence is in Triangular flow is significant and its centrality dependence is in agreement with hydro model predictionsagreement with hydro model predictions Each harmonic has its own participant plane along which it develops Each harmonic has its own participant plane along which it develops Participant plane of vParticipant plane of v22 is not the participant plane of v is not the participant plane of v33 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 Hydrodynamic prediction with Glauber initial conditions (Schenke et et al, al, arXiv:1102.0575) does not describe well simultaneously the ) does not describe well simultaneously the details of pdetails of ptt dependence of v dependence of v22 and v and v3 3

1717

Thanks!Thanks!