Flow phenomena in ATLAS Rencontres de Moriond – La Thuile , 2013

1. Flow phenomena in ATLASRencontres de Moriond – La Thuile, 2013 Attilio Andreazza Università di Milano and INFN on behalf of the ATLAS Collaboration

2. Flow description • Geometry of initial state produces a pressure gradient, driving the expansion of the hadronic system. • Overlapping with non-flow dynamics (resonance decay, Bose-Einstein correlations, jets...) • Observables: • Particle azimuthal distribution:Φn=Event Planes (EP) • Two-particle correlations:a,b=particle classes (pT,η) • For flow: • geometry(Glauber model) • factorization Φ2 Φ4 r Φ3 A. Andreazza, Flow effects in ATLAS 2

3. Observables at detector level • Centrality defined from total FCal ET: • uncorrelated with tracks Particle distribution from tracks in the Inner Detector -2.5 < η < 2.5 A. Andreazza, Flow effects in ATLAS

4. Outline • Previous measurements: • vnmeasured up to v6 • special emphasis on elliptic flow v2 • including pT, centrality, η dependencies • Discussed today: • Cumulant-based v2determination • vn probability distributions • The ridge in p-Pb collisions • Data samples: • 7-8 μb-1Pb-Pb at √sNN=2.76 TeV (2010) • 1 μb-1 p-Pb at √sNN=5.02 TeV (2012) A. Andreazza, Flow effects in ATLAS

5. Elliptic flow: cumulants • Cumulant method • reduce non-flow contribution by computing multi-particle correlations: ⟨ein(ϕ1-ϕ2)⟩ ⇒ vn{2} ⟨ein(ϕ1+ϕ2-ϕ3-ϕ4)⟩ ⇒ vn{4} • v2 with EP method usually between the two cumulants • observed very limited η-dependence v2 from event plane A. Andreazza, Flow effects in ATLAS

6. Event-by-event distributions • The Fourier series for particle anisotropies can be computed for each event: vn distributions contain more information than just ⟨vn⟩ A. Andreazza, Flow effects in ATLAS

7. E-by-E vn distributions • Parameterizations are radial projection of a 2D Gaussian: • probability density distributions compatible with: • vnRP=0 • …and Gaussian fluctuations • except for v2, where a non-zero radial offset v2RP is needed. v3 v2 v4 A. Andreazza, Flow effects in ATLAS

8. E-by-E vn distributions • vnEP from EP methods is equivalent to • If vnRP=0: v3 v2 v4 A. Andreazza, Flow effects in ATLAS

9. E-by-E v2: model comparison • Comparison with models, assuming vn∝εn • Glauber model: arXiv:nucl-ex/0701025 (red line) • MC-KLN, including gluon saturation http://www.aiu.ac.jp/∼ynara/mckln(blue line) v2 A. Andreazza, Flow effects in ATLAS

10. E-by-E v3: model comparison • Agreement is better for higher-order vn(see backup for v4) • Still failing starting from mid-central events • models have a cut-off induced by the assumption vn∝εn<1 v3 A. Andreazza, Flow effects in ATLAS

11. p-Pb: the ridge • Classifying according to ∑ETPb(no self correlation with tracks) • Clearly visible ridge also in p-Pb • …also widening of the away side 2% highest ET 52% smallest ET A. Andreazza, Flow effects in ATLAS

12. Ridge: per-trigger yield • Per-trigger (particle) yield Y • Subtraction of offset from uncorrelated particles • |Δη|>2 gap • ΔY (high-∑ETPb – low-∑ETPb) • Recoil subtraction • Dominant contribution from a cos|2Δϕ| term A. Andreazza, Flow effects in ATLAS

13. Ridge v2-like extraction • Integrated per-trigger yield: • near side: |Δϕ|<π/3 • away side: |Δϕ|>2π/3 • Same high- vs. low-multiplicity difference • Can be translated into vn-equivalent variables. near side away side For comparison: v2 = 0.1-0.2 v3 = 0.04-0.10 in mid-centrality Pb-Pb collisions A. Andreazza, Flow effects in ATLAS

14. p-Pb: v2 from cumulants • Similar technique as for Pb-Pb v2 measurement • Results consistent withtwo-particles correlation measurement • large harmonicv2{4}~0.06 • different suppression of non-flow correlation(visible in low ETPb events) • qualitatively similar behavior to Pb-Pb events ∫ = Theoreticalprediction from hydrodinamics model P. Bożek and W. Broniowski, Phys. Lett. B 718 (2013) 1557 [arXiv:1211.0845] A. Andreazza, Flow effects in ATLAS

15. Conclusions and outlook • Large statistics Pb-Pb samples at LHC allow for detailed study of flow with different analysis techniques. • Many results were not discussed today: event plane correlations, inclusive v2 determination... • Emerging non-trivial observations: • Non-flow subtraction with higher order cumulants • Differential distributions of correlation coefficients: • v3, v4 compatible with pure Gaussian fluctuations • Possible to disentangle fluctuations and average geometrical effects for v2 • Event-by-event distributions cannot be fully described by simple Glauber models • p-Pb data are crucial to the understanding of Pb-Pb • Already in pilot run (1 μb-1) observation and quantitative study of the ridge • The ridge shows flow-like anisotropies, dominated by v2 ≈ 0.06 • More detailed studies will eventually become available on the full 2013 run statistics (~30 nb-1). A. Andreazza, Flow effects in ATLAS

16. Bibliography • Measurement of the pseudorapidity and transverse momentum dependence of the elliptic flow of charged particles in lead-lead collisions at √sNN = 2.76 TeV with the ATLAS detector Phys. Lett. B707 (2012) 330-348, arXiv:1108.6018 • Measurement of the azimuthal anisotropy for charged particle production in √sNN = 2.76 TeV lead-lead collisions with the ATLAS detector Phys. Rev. C86 (2012) 014907, arXiv:1203.3087 • Observation of associated near- and away-side long-range pseudorapidity correlations in √sNN=5.02 TeV proton-lead collisions at the LHC with the ATLAS detector Submitted to Phys. Rev. Lett., arXiv:1212.5198 • Measurement of multi-particle azimuthal correlations in proton-lead collisions at √sNN = 5.02 TeV with the ATLAS detector Submitted to Phys. Lett. B, arXiv:1303.2084 • Correlation of harmonic flow event planes in lead-lead collisions at √sNN =2.76 TeV with the ATLAS detectorATLAS-CONF-2012-049 • Measurement of event by event harmonic flow in 2.76 TeVPb+Pb collisions with ATLAS ATLAS-CONF-2012-114 • Centrality and pseudorapidity dependence of the elliptic flow integrated over pT in lead-lead collisions with 2.76 TeV/nucleons collisions with the ATLAS detector at LHCATLAS-CONF-2012-117 • Collective flow with higher-order cumulants in lead-lead collisions at √sNN=2.76 TeV with the ATLAS detector at the LHCATLAS-CONF-2012-118 https://twiki.cern.ch/twiki/bin/view/AtlasPublic/HeavyIonsPublications =Presented today https://twiki.cern.ch/twiki/bin/view/AtlasPublic/HeavyIonsCONFnotes A. Andreazza, Flow effects in ATLAS

17. Backup

18. Charged particle flow • Particle distribution from tracks in the Inner Detector: • Si pixel • Si strips • TRT straw tubes • 2 T solenoid Efficiency and ϕ-dependent acceptance corrections. A. Andreazza, Flow effects in ATLAS

19. Npart – Centrality conversion A. Andreazza, Flow effects in ATLAS

20. Cumulant method • 2- and 4-particle cumulants • Differential distributions: correlatorsdcorrn computed by correlating one particle of interest (pT, η) bin, with n-1 reference particles A. Andreazza, Flow effects in ATLAS

21. Elliptic flow: cumulants • Cumulant method • reduce non-flow contribution by computing multi-particle correlations • ⟨vn{2}2⟩=⟨ein(ϕ1-ϕ2)⟩ • ⟨vn{4}4⟩=⟨ein(ϕ1+ϕ2-ϕ3-ϕ4)⟩ • v2 with EP method usually between the two cumulants A. Andreazza, Flow effects in ATLAS

22. Elliptic flow: cumulants • Cumulant method • reduce non-flow contribution by computing multi-particle correlations • ⟨vn{2}2⟩=⟨ein(ϕ1-ϕ2)⟩ • ⟨vn{4}4⟩=⟨ein(ϕ1+ϕ2-ϕ3-ϕ4)⟩ • v2 with EP method usually between the two cumulants • observed very limited η-dependence A. Andreazza, Flow effects in ATLAS

23. Elliptic flow: cumulants PHOBOS Phys. Rev. C72 051901 A. Andreazza, Flow effects in ATLAS

24. Event-by-event distributions • The Fourier serie:can be computed for each event. • vn distributions containmore information than just ⟨vn⟩ • Technicalities: • Correct for detector acceptance effects • Unfold resolution effect • Resolution p.d.f. from the two sub-detector method A. Andreazza, Flow effects in ATLAS

25. Event-by-event: v4 A. Andreazza, Flow effects in ATLAS

26. The Ridge – from nucleus to nucleon • The ridge is a well known feature of the 2-particle correlation function in nucleus-nucleus collisions. ATLASPb-Pb collisions √sNN=2.76 TeV • Observation of similar structure in pp collision. • LHC p-Pb pilot run CMSpp collisions √s=7 TeV A. Andreazza, Flow effects in ATLAS

27. Ridge in p-Pb • Clearly visible ridge also in p-Pb • Emphasize evolution of the system: • classifying according to ∑ETPb(no self correlation with tracks) • recoil-subtracted correlations 2% highest ET 52% smallest ET A. Andreazza, Flow effects in ATLAS

28. Event plane correlations Shape of the interaction region • 2-plane correlators: ⟨cosk(Φn-Φm)⟩ (k multiple of n and m) Trivial (but unverified) expectation: no odd-even correlations • 3-plane correlators: ⟨cos[k1(Φn-Φm)±k2(Φn-Φj)]⟩ Φ2 4(Φ2-Φ4) 6(Φ2-Φ3) Φ4 Φ3 6(Φ2-Φ6) 6(Φ3-Φ6) A. Andreazza, Flow effects in ATLAS

29. Elliptic flow: inclusive • Observed v2 is sensitive to pT • Standard ATLAS tracking pT>0.5 GeV • Dedicated tracking (Pixel): pT>0.1 GeV • Inclusive pT:special runs (~1 μb-1) with B-field off 25-50% • LHC data consistent with v2(η) trend observed by PHOBOS at RHIC. A. Andreazza, Flow effects in ATLAS