Measurement of elliptic and higher-order harmonics at 2.76 TeV Pb+Pb - PowerPoint PPT Presentation

Measurement of elliptic and higher-order harmonics at 2.76 TeV Pb+Pb collisions with the ATLAS detector. Dominik Derendarz for the ATLAS Collaboration Institute of Nuclear Physics PAN, Kraków, Poland

Why azimuthal anisotropy in AA is interesting? • Signature of strongly interacting QGP • Sensitive to – Initial shape of the interaction region (v 2 ) – Initial spatial fluctuations of nucleons (higher orders) Related to ridge, Mach cone. • Mechanism of particle production – Low p T (< ~2GeV): hydro expansion (perfect liquid) (Nucl. Phys. A Volume 757) – Medium p T (~2-6 GeV): coalescence models (Nucl. Phys. A Volume 757, D. Molnar and S. Voloshin, nucl-th/0302014) – High p T : constrain on jet quenching models 2/14

Azimuthal anisotropy in heavy ion collisions Reaction plane Ψ RP φ a t e n a K i h s a s a M Pressure gradients lead to azimuthal anisotropy dN =N 0 ( 1 + 2v 1 cos ( ϕ − Ψ 1 ) + 2v 2 cos ( 2 ( ϕ − Ψ 2 ) ) + 2v 3 cos ( 3 ( ϕ − Ψ 3 ) ) + ... ) d ( ϕ − Ψ n ) directed flow elliptic flow triangular flow v n =〈 cos ( n ( Φ − Ψ n ) ) 〉 Fourier harmonics 3/14

ATLAS detector Centrality determination • Energy deposited in entire FCal is used for centrality determination FCal • Event plane is measured based on energy deposition in the first sampling layer of FCal FCal • Fourier harmonics are reconstructed in inner detector from charged particle tracks : • p T > 0.5 GeV • |η|<2.5 4/14

ATLAS detector Inner detector • Energy deposited in entire FCal is used for centrality determination • Event plane is measured based on energy deposition in the first sampling layer of FCal • Fourier harmonics are reconstructed in inner detector Pixel detector from charged particle tracks : SCT detector • p T > 0.5 GeV • |η|<2.5 TRT detector 4/14

Event plane determination Ψ 2 • • Reaction plane (Ψ RP ) is approximated by Reaction plane (Ψ RP ) is approximated by event plane (Ψ n event plane (Ψ n EP ) measured in FCal: EP ) measured in FCal: tower w i sin ( n ϕ i ) n tan − 1 ∑ E T,i EP = 1 i Ψ n tower w i cos ( n ϕ i ) Ψ 3 ∑ E T,i i Ψ 4 ~400 nucleons A T L A S , P h y s . R e v . C 8 6 , 0 1 4 9 0 7 ( 2 0 1 2 ) • The event plane resolution correction factor R is obtained using two-sub event and various tree- subevent method • Significant resolution for harmonics n=2 – 6 • Resolution corrected harmonics: v n =〈 cos ( n ( Φ − Ψ n ) ) 〉/ R 5/14

p T dependence of the v 2 of charged particles ATLAS, Phys.Lett. B707 (2012) 330-348 • All centrality intervals shows: – Rapid rise in v 2 (p T ) up to p T ~ 3 GeV – Decrease out to 7-8 GeV – Weak p T -dependence above 9-10 GeV • The strongest elliptic flow at LHC is observed in centralities 30-50% 6/14

Comparison with ALICE and RHIC experiments ATLAS, Phys.Lett. B707 (2012) 330-348 • All data sets are quite consistent for both low and high p T 7/14

Pseudorapidity dependence of the v 2 ATLAS, Phys.Lett. B707 (2012) 330-348 No substantial η dependence for any p T or centrality interval is observed • Different than PHOBOS measurements at RHIC in which v 2 decreases by • ~30% within the same η range (PHOBOS Phys. Rev. C72 (2005) 051901) 8/14

Higher order flow harmonics ATLAS, Phys. Rev. C 86, 014907 (2012) The p T -dependence of v 2 -v 6 • for several centrality selections • Similar p T -dependence for all harmonics v n generally decreases for • larger n, except in the most central events: – v 3 dominates in p T range ~2-7 GeV – v 4 >v 2 in p T range ~3-5 GeV 9/14

Higher order harmonics scaling ATLAS, Phys. Rev. C 86, 014907 (2012) • Hydrodynamics model suggests scaling v 4 ~v 2 2 (PHENIX PRL 105, 062301 (2010)) • The p T -dependence of the v n 1/n /v 2 1/2 (n=3- 6) ratio for several centrality selections • Weak p T -dependence of the ratio except 5% most central events • Ratio for n=3 systematically lower than for n=4, 5 10/14

Two-particle correlation method )= N s ( Δ ,Δη ϕ ) N s – same event pairs The two-particle correlation function: C ( Δ ,Δη ϕ N m – mixed event pairs N m ( Δ ,Δη ϕ ) 11/14

Two-particle correlation method )= N s ( Δ ,Δη ϕ ) N s – same event pairs The two-particle correlation function: C ( Δ ,Δη ϕ N m – mixed event pairs N m ( Δ ,Δη ϕ ) 2<| Δη |<5 Projected onto Δφ 1D correlation function dN dΔ ϕ ∝ 1 + 2 ∑ v n,n cos ( nΔ ϕ ) n 11/14

Two-particle correlation method )= N s ( Δ ,Δη ϕ ) N s – same event pairs The two-particle correlation function: C ( Δ ,Δη ϕ N m – mixed event pairs N m ( Δ ,Δη ϕ ) 2<| Δη |<5 Projected onto Δφ 1D correlation function dN dΔ ϕ ∝ 1 + 2 ∑ v n,n cos ( nΔ ϕ ) n v n,n are calculated via Discrete Fourier ∑ Transform (DFT) : cos ( nΔ ϕ m ) C ( Δ ϕ m ) m v n,n =<cos ( nΔ ϕ ) >= ∑ C ( Δ ϕ m ) m 11/14

Two-particle correlation method )= N s ( Δ ,Δη ϕ ) N s – same event pairs The two-particle correlation function: C ( Δ ,Δη ϕ N m – mixed event pairs N m ( Δ ,Δη ϕ ) 2<| Δη |<5 Projected onto Δφ 1D correlation function dN dΔ ϕ ∝ 1 + 2 ∑ v n,n cos ( nΔ ϕ ) n v n,n are calculated via Discrete Fourier ∑ Transform (DFT) : cos ( nΔ ϕ m ) C ( Δ ϕ m ) ATLAS, Phys. Rev. C 86, 014907 (2012) m v n,n =<cos ( nΔ ϕ ) >= ∑ C ( Δ ϕ m ) m It is expected that for flow modulations: b ) =v n ( p T a ) v n ( p T b ) a ,p T v n,n ( p T And for ”fixed-pT” correlations: v n = √ v n,n 11/14

Two particle correlation vs EP results ATLAS, Phys. Rev. C 86, 014907 (2012) Good agreement between both methods in the selected kinematical range (p T 1-3 GeV, 2<|η|<5 ) 12/14

Two particle correlation vs EP results 6 2PC cos ΔΦ+ 2 ∑ 2PC ( 1 + 2v 1,1 EP, a v n EP ,b cos n ΔΦ) C (Δ Φ)= b v n n = 2 ATLAS, Phys. Rev. C 86, 014907 (2012) • b 2PC average of the correlation function 2PC first harmonic v 1,1 • from the 2PC analysis More details on v 1 : J. Jia talk 15 Aug 11:20 AM Session: Parallel 4A Other v n components • measured with the event plane method • Correlation function reproduced very well even harmonics contribution odd harmonics contribution 13/14

Summary • ATLAS measured v 2 and higher order flow harmonics up to v 6 in wide p T , η and centrality range • v n (p T ) shows the same trends – rise up to ~3 GeV – decrease within 3-8 GeV – varies weakly out to 20 GeV • v n (η) remains approximately constant • v 3 is dominating in the most central collisions • v n ’s follow approximate scaling relation v n 1/n ∝ v 2 1/2 • Good agreement between event plane and two particle correlation results for v n 14/14